



CoRE Working Group                                             M. Tiloca
Internet-Draft                                                   RISE AB
Intended status: Standards Track                             G. Selander
Expires: 26 June 2026                                       F. Palombini
                                                       J. Preuß Mattsson
                                                             Ericsson AB
                                                              R. Höglund
                                                                 RISE AB
                                                        23 December 2025


   Group Object Security for Constrained RESTful Environments (Group
                                OSCORE)
                  draft-ietf-core-oscore-groupcomm-28

Abstract

   This document defines the security protocol Group Object Security for
   Constrained RESTful Environments (Group OSCORE), providing end-to-end
   security of messages exchanged with the Constrained Application
   Protocol (CoAP) between members of a group, e.g., sent over IP
   multicast.  In particular, the described protocol defines how OSCORE
   is used in a group communication setting to provide source
   authentication for CoAP group requests, sent by a client to multiple
   servers, and for protection of the corresponding CoAP responses.
   Group OSCORE also defines a pairwise mode where each member of the
   group can efficiently derive a symmetric pairwise key with each other
   member of the group for pairwise OSCORE communication.  Group OSCORE
   can be used between endpoints communicating with CoAP or CoAP-
   mappable HTTP.

About This Document

   This note is to be removed before publishing as an RFC.

   Status information for this document may be found at
   https://datatracker.ietf.org/doc/draft-ietf-core-oscore-groupcomm/.

   Discussion of this document takes place on the Constrained RESTful
   Environments (core) Working Group mailing list
   (mailto:core@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/core/.  Subscribe at
   https://www.ietf.org/mailman/listinfo/core/.

   Source for this draft and an issue tracker can be found at
   https://github.com/core-wg/oscore-groupcomm.





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Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 26 June 2026.

Copyright Notice

   Copyright (c) 2025 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   8
   2.  Security Context  . . . . . . . . . . . . . . . . . . . . . .  10
     2.1.  Common Context  . . . . . . . . . . . . . . . . . . . . .  12
       2.1.1.  AEAD Algorithm  . . . . . . . . . . . . . . . . . . .  12
       2.1.2.  HKDF Algorithm  . . . . . . . . . . . . . . . . . . .  12
       2.1.3.  ID Context  . . . . . . . . . . . . . . . . . . . . .  13
       2.1.4.  Common IV . . . . . . . . . . . . . . . . . . . . . .  13
       2.1.5.  Authentication Credential Format  . . . . . . . . . .  13
       2.1.6.  Group Manager Authentication Credential . . . . . . .  13
       2.1.7.  Group Encryption Algorithm  . . . . . . . . . . . . .  14
       2.1.8.  Signature Algorithm . . . . . . . . . . . . . . . . .  15
       2.1.9.  Signature Encryption Key  . . . . . . . . . . . . . .  15
       2.1.10. Pairwise Key Agreement Algorithm  . . . . . . . . . .  15
     2.2.  Sender Context and Recipient Context  . . . . . . . . . .  16



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     2.3.  Establishment of Security Context Parameters  . . . . . .  17
     2.4.  Authentication Credentials  . . . . . . . . . . . . . . .  18
     2.5.  Pairwise Keys . . . . . . . . . . . . . . . . . . . . . .  20
       2.5.1.  Derivation of Pairwise Keys . . . . . . . . . . . . .  20
       2.5.2.  ECDH with Montgomery Coordinates  . . . . . . . . . .  22
       2.5.3.  Usage of Sequence Numbers . . . . . . . . . . . . . .  23
       2.5.4.  Security Context for Pairwise Mode  . . . . . . . . .  23
     2.6.  Update of Security Context  . . . . . . . . . . . . . . .  24
       2.6.1.  Loss of the Varying Part of the Security Context  . .  24
       2.6.2.  Exhaustion of Sender Sequence Number Space  . . . . .  26
       2.6.3.  Retrieving New Security Context Parameters  . . . . .  27
   3.  The COSE Object . . . . . . . . . . . . . . . . . . . . . . .  29
     3.1.  Countersignature  . . . . . . . . . . . . . . . . . . . .  30
       3.1.1.  Clarifications on Using a Countersignature  . . . . .  30
     3.2.  The 'kid' and 'kid context' parameters  . . . . . . . . .  30
     3.3.  Nonce Computation . . . . . . . . . . . . . . . . . . . .  31
     3.4.  Additional Authenticated Data . . . . . . . . . . . . . .  31
   4.  OSCORE Header Compression . . . . . . . . . . . . . . . . . .  34
     4.1.  Encoding of the OSCORE Option Value and Group OSCORE
           Payload . . . . . . . . . . . . . . . . . . . . . . . . .  34
     4.2.  Keystream Derivation for Countersignature Encryption  . .  35
     4.3.  Examples of Compressed COSE Objects . . . . . . . . . . .  36
       4.3.1.  Examples in Group Mode  . . . . . . . . . . . . . . .  36
       4.3.2.  Examples in Pairwise Mode . . . . . . . . . . . . . .  38
   5.  Message Binding, Sequence Numbers, Freshness, and Replay
           Protection  . . . . . . . . . . . . . . . . . . . . . . .  39
     5.1.  Supporting Multiple Responses in Long Exchanges . . . . .  39
     5.2.  Freshness . . . . . . . . . . . . . . . . . . . . . . . .  39
     5.3.  Replay Protection . . . . . . . . . . . . . . . . . . . .  40
       5.3.1.  Replay Protection of Responses  . . . . . . . . . . .  41
   6.  Message Reception . . . . . . . . . . . . . . . . . . . . . .  43
   7.  Message Processing in Group Mode  . . . . . . . . . . . . . .  44
     7.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  45
     7.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  46
     7.3.  Protecting the Response . . . . . . . . . . . . . . . . .  49
     7.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  51
     7.5.  External Signature Checkers . . . . . . . . . . . . . . .  55
   8.  Message Processing in Pairwise Mode . . . . . . . . . . . . .  56
     8.1.  Pre-Conditions  . . . . . . . . . . . . . . . . . . . . .  57
     8.2.  Main Differences from OSCORE  . . . . . . . . . . . . . .  57
     8.3.  Protecting the Request  . . . . . . . . . . . . . . . . .  58
     8.4.  Verifying the Request . . . . . . . . . . . . . . . . . .  58
     8.5.  Protecting the Response . . . . . . . . . . . . . . . . .  59
     8.6.  Verifying the Response  . . . . . . . . . . . . . . . . .  60
   9.  Challenge-Response Based Freshness and Replay Window
           Recovery  . . . . . . . . . . . . . . . . . . . . . . . .  61
   10. Implementation Compliance . . . . . . . . . . . . . . . . . .  63
   11. Web Linking . . . . . . . . . . . . . . . . . . . . . . . . .  65



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   12. The Group Manager . . . . . . . . . . . . . . . . . . . . . .  66
     12.1.  Set-up of New Endpoints  . . . . . . . . . . . . . . . .  67
     12.2.  Management of Group Keying Material  . . . . . . . . . .  68
       12.2.1.  Recycling of Identifiers . . . . . . . . . . . . . .  72
     12.3.  Support for Signature Checkers . . . . . . . . . . . . .  74
   13. Implementation Status . . . . . . . . . . . . . . . . . . . .  75
     13.1.  Implementation #1  . . . . . . . . . . . . . . . . . . .  76
     13.2.  Implementation #2  . . . . . . . . . . . . . . . . . . .  77
     13.3.  Interoperability . . . . . . . . . . . . . . . . . . . .  78
   14. Security Considerations . . . . . . . . . . . . . . . . . . .  79
     14.1.  Security of the Group Mode . . . . . . . . . . . . . . .  81
       14.1.1.  Example of Need for Proof of Group Membership  . . .  82
     14.2.  Security of the Pairwise Mode  . . . . . . . . . . . . .  83
     14.3.  Uniqueness of (key, nonce) . . . . . . . . . . . . . . .  84
     14.4.  Management of Group Keying Material  . . . . . . . . . .  85
       14.4.1.  Denial of Service  . . . . . . . . . . . . . . . . .  85
     14.5.  Update of Security Context and Key Rotation  . . . . . .  85
       14.5.1.  Late Update on the Sender  . . . . . . . . . . . . .  86
       14.5.2.  Late Update on the Recipient . . . . . . . . . . . .  87
     14.6.  Collision of Group Identifiers . . . . . . . . . . . . .  87
     14.7.  Cross-group Message Injection  . . . . . . . . . . . . .  88
       14.7.1.  Attack Description . . . . . . . . . . . . . . . . .  88
       14.7.2.  Attack Prevention in Group Mode  . . . . . . . . . .  89
     14.8.  Prevention of Group Cloning Attack . . . . . . . . . . .  89
     14.9.  Group OSCORE for Unicast Requests  . . . . . . . . . . .  90
     14.10. End-to-end Protection  . . . . . . . . . . . . . . . . .  92
     14.11. Master Secret  . . . . . . . . . . . . . . . . . . . . .  92
     14.12. Replay Protection  . . . . . . . . . . . . . . . . . . .  93
     14.13. Message Ordering . . . . . . . . . . . . . . . . . . . .  93
     14.14. Message Freshness  . . . . . . . . . . . . . . . . . . .  93
     14.15. Client Aliveness . . . . . . . . . . . . . . . . . . . .  94
     14.16. Cryptographic Considerations . . . . . . . . . . . . . .  95
     14.17. Message Segmentation . . . . . . . . . . . . . . . . . .  96
     14.18. Privacy Considerations . . . . . . . . . . . . . . . . .  97
   15. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  98
     15.1.  OSCORE Flag Bits Registry  . . . . . . . . . . . . . . .  98
     15.2.  Target Attributes Registry . . . . . . . . . . . . . . .  98
   16. References  . . . . . . . . . . . . . . . . . . . . . . . . .  99
     16.1.  Normative References . . . . . . . . . . . . . . . . . .  99
     16.2.  Informative References . . . . . . . . . . . . . . . . . 101
   Appendix A.  Assumptions and Security Objectives  . . . . . . . . 104
     A.1.  Assumptions . . . . . . . . . . . . . . . . . . . . . . . 105
     A.2.  Security Objectives . . . . . . . . . . . . . . . . . . . 107
   Appendix B.  List of Use Cases  . . . . . . . . . . . . . . . . . 108
   Appendix C.  Example of Group Identifier Format . . . . . . . . . 110
   Appendix D.  Responsibilities of the Group Manager  . . . . . . . 111
   Appendix E.  Document Updates . . . . . . . . . . . . . . . . . . 112
     E.1.  Version -27 to -28  . . . . . . . . . . . . . . . . . . . 112



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     E.2.  Version -26 to -27  . . . . . . . . . . . . . . . . . . . 113
     E.3.  Version -25 to -26  . . . . . . . . . . . . . . . . . . . 114
     E.4.  Version -24 to -25  . . . . . . . . . . . . . . . . . . . 115
     E.5.  Version -23 to -24  . . . . . . . . . . . . . . . . . . . 115
     E.6.  Version -22 to -23  . . . . . . . . . . . . . . . . . . . 115
     E.7.  Version -21 to -22  . . . . . . . . . . . . . . . . . . . 116
     E.8.  Version -20 to -21  . . . . . . . . . . . . . . . . . . . 116
     E.9.  Version -19 to -20  . . . . . . . . . . . . . . . . . . . 117
     E.10. Version -18 to -19  . . . . . . . . . . . . . . . . . . . 117
     E.11. Version -17 to -18  . . . . . . . . . . . . . . . . . . . 117
     E.12. Version -16 to -17  . . . . . . . . . . . . . . . . . . . 118
     E.13. Version -15 to -16  . . . . . . . . . . . . . . . . . . . 118
     E.14. Version -14 to -15  . . . . . . . . . . . . . . . . . . . 118
     E.15. Version -13 to -14  . . . . . . . . . . . . . . . . . . . 118
     E.16. Version -12 to -13  . . . . . . . . . . . . . . . . . . . 119
     E.17. Version -11 to -12  . . . . . . . . . . . . . . . . . . . 119
     E.18. Version -10 to -11  . . . . . . . . . . . . . . . . . . . 120
     E.19. Version -09 to -10  . . . . . . . . . . . . . . . . . . . 121
     E.20. Version -08 to -09  . . . . . . . . . . . . . . . . . . . 121
     E.21. Version -07 to -08  . . . . . . . . . . . . . . . . . . . 122
     E.22. Version -06 to -07  . . . . . . . . . . . . . . . . . . . 124
     E.23. Version -05 to -06  . . . . . . . . . . . . . . . . . . . 124
     E.24. Version -04 to -05  . . . . . . . . . . . . . . . . . . . 125
     E.25. Version -03 to -04  . . . . . . . . . . . . . . . . . . . 125
     E.26. Version -02 to -03  . . . . . . . . . . . . . . . . . . . 126
     E.27. Version -01 to -02  . . . . . . . . . . . . . . . . . . . 127
     E.28. Version -00 to -01  . . . . . . . . . . . . . . . . . . . 128
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 128
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . . 129

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] is a web
   transfer protocol specifically designed for constrained devices and
   networks [RFC7228].  Group communication for CoAP
   [I-D.ietf-core-groupcomm-bis] addresses use cases where deployed
   devices benefit from a group communication model, for example to
   reduce latencies, improve performance, and reduce bandwidth
   utilization.  Use cases include lighting control, integrated building
   control, software and firmware updates, parameter and configuration
   updates, commissioning of constrained networks, and emergency
   multicast (see Appendix B).  Group communication for CoAP
   [I-D.ietf-core-groupcomm-bis] mainly uses UDP/IP multicast as the
   underlying data transport.

   Object Security for Constrained RESTful Environments (OSCORE)
   [RFC8613] describes a security protocol based on the exchange of
   protected CoAP messages.  OSCORE builds on CBOR Object Signing and



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   Encryption (COSE) [RFC9052][RFC9053] and provides end-to-end
   encryption, integrity, replay protection, and binding of response to
   request between a sender and a recipient, independent of the
   transport layer also in the presence of intermediaries.  To this end,
   a CoAP message is protected by including its payload (if any),
   certain options, and header fields into a COSE object, which is
   conveyed within the CoAP payload and the CoAP OSCORE Option of the
   protected message, thereby replacing those message fields with an
   authenticated and encrypted object.

   This document defines Group OSCORE, a security protocol for group
   communication with CoAP [I-D.ietf-core-groupcomm-bis] and for CoAP-
   mappable HTTP requests and responses, providing the same end-to-end
   security properties as OSCORE also in the case where requests have
   multiple recipients.  In particular, the described protocol defines
   how OSCORE is used in a group communication setting to provide source
   authentication for group requests sent by a client to multiple
   servers, and for protection of the corresponding responses.  Group
   OSCORE also defines a pairwise mode where each member of the group
   can efficiently derive a symmetric pairwise key with each other
   member of the group for pairwise-protected OSCORE communication.
   Just like OSCORE, Group OSCORE is independent of the transport layer
   and works wherever CoAP does.

   As with OSCORE, it is possible to combine Group OSCORE with
   communication security on other layers.  One example is the use of
   transport layer security, such as DTLS [RFC9147], between one client
   and one proxy, or between one proxy and one server.  This prevents
   observers from accessing addressing information conveyed in CoAP
   options that would not be protected by Group OSCORE, but would be
   protected by DTLS.  These options include Uri-Host, Uri-Port, and
   Proxy-Uri. Note that DTLS does not define how to secure messages sent
   over IP multicast and cannot be used for end-to-end protection over a
   proxy.  Group OSCORE is also intended to work with OSCORE-capable
   proxies [I-D.ietf-core-oscore-capable-proxies] thereby enabling, for
   example, nested OSCORE operations with OSCORE-protected communication
   between a CoAP client and a proxy, carrying messages that are
   additionally protected with Group OSCORE between the CoAP client and
   the target CoAP servers.

   Group OSCORE defines two modes of operation that can be used
   independently or together:









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   *  In the group mode, Group OSCORE requests and responses are
      digitally signed with the private key of the sender and the
      signature is embedded in the protected CoAP message.  The group
      mode supports all COSE signature algorithms as well as signature
      verification by intermediaries.  This mode is defined in
      Section 7.

   *  In the pairwise mode, two group members exchange OSCORE requests
      and responses (typically) over unicast, and the messages are
      protected with symmetric keys not known by the other group
      members.  These symmetric keys are derived from Diffie-Hellman
      shared secrets, calculated with the asymmetric keys of the sender
      and recipient, allowing for shorter integrity tags and therefore
      lower message overhead.  This mode is defined in Section 8.

   Both modes provide source authentication of CoAP messages.  The
   application decides what mode to use, potentially on a per-message
   basis.  Such decisions can be based, for instance, on pre-configured
   policies or dynamic assessing of the target recipient and/or
   resource, among other things.  One important case is when requests
   are protected in group mode, and responses in pairwise mode.  Since
   such responses convey shorter integrity tags instead of bigger, full-
   fledged signatures, this significantly reduces the message overhead
   in case of many responses to one request.

   A special deployment of Group OSCORE consists in using the pairwise
   mode only.  For example, consider the case of a constrained-node
   network [RFC7228] with a large number of CoAP endpoints and the
   objective to establish secure communication between any pair of
   endpoints with a small provisioning effort and message overhead.
   Since the total number of security associations that needs to be
   established grows with the square of the number of endpoints, it is
   desirable to restrict the amount of secret keying material provided
   to each endpoint.  Moreover, a key establishment protocol would need
   to be executed for each security association.  One solution to this
   issue is to deploy Group OSCORE, with the endpoints being part of a
   group, and to use the pairwise mode.  This solution has the benefit
   of providing a single shared secret, while distributing only the
   public keys of group members or a subset of those.  After that, a
   CoAP endpoint can locally derive the OSCORE Security Context for the
   other endpoint in the group, and protect CoAP communications with
   very low overhead [I-D.ietf-iotops-security-protocol-comparison].









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   In some circumstances, Group OSCORE messages may be transported in
   HTTP, e.g., when they are protected with the pairwise mode and target
   a single recipient, or when they are protected with the group mode
   and target multiple CoAP recipients through cross-protocol
   translators such as HTTP-to-CoAP proxies
   [RFC8075][I-D.ietf-core-groupcomm-proxy].  The use of Group OSCORE
   with HTTP is as defined for OSCORE in Section 11 of [RFC8613].

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Readers are expected to be familiar with the terms and concepts
   described in CoAP [RFC7252], including "endpoint", "client",
   "server", "sender", and "recipient"; group communication for CoAP
   [I-D.ietf-core-groupcomm-bis]; Observe [RFC7641]; Concise Binary
   Object Representation (CBOR) [RFC8949]; Concise Data Definition
   Language (CDDL) [RFC8610]; COSE [RFC9052][RFC9053] and related
   countersignatures [RFC9338].

   Readers are also expected to be familiar with the terms and concepts
   for protection and processing of CoAP messages through OSCORE, such
   as "Security Context" and "Master Secret", defined in [RFC8613].

   Terminology for constrained environments, such as "constrained
   device" and "constrained-node network", is defined in [RFC7228].

   This document refers also to the following terminology.

   *  Keying material: data that is necessary to establish and maintain
      secure communication among endpoints.  This includes, for
      instance, keys and IVs [RFC4949].

   *  Authentication credential: information associated with an entity,
      including that entity's public key and parameters associated with
      the public key.  Examples of formats of authentication credentials
      are CBOR Web Tokens (CWTs) and CWT Claims Sets (CCSs) [RFC8392],
      X.509 certificates [RFC5280], and C509 certificates
      [I-D.ietf-cose-cbor-encoded-cert].  Further details about
      authentication credentials are provided in Section 2.4.

   *  Group: a set of endpoints that share group keying material and
      security parameters (Common Context, see Section 2).  That is,
      unless otherwise specified, the term group used in this document



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      refers to a "security group" (see Section 2.1 of
      [I-D.ietf-core-groupcomm-bis]), not to be confused with "CoAP
      group" or "application group".

   *  Group Manager: an entity responsible for a group, required neither
      to be an actual group member nor to take part in the group
      communication.  The operations of the Group Manager are defined in
      Section 12 and its responsibilities are listed in Appendix D.

   *  Silent server: a member of a group that performs only group mode
      processing on incoming requests and never sends responses
      protected with Group OSCORE.  For CoAP group communications,
      requests are normally sent without necessarily expecting a
      response.  A silent server may send unprotected responses, as
      error responses reporting a Group OSCORE error.

   *  Group Identifier (Gid): identifier assigned to the group, unique
      within the set of groups of a given Group Manager.  The Gid value
      changes every time the group is rekeyed (see Section 12.2).

   *  Birth Gid: with respect to a group member, the Gid obtained by
      that group member upon (re-)joining the group.

   *  Key Generation Number: an integer value identifying the current
      version of the keying material used in a group.

   *  Source authentication: evidence that a received message in the
      group originated from a specific identified group member.  This
      also provides assurance that the message was not tampered with by
      anyone, be it a different legitimate group member or an endpoint
      which is not a group member.

   *  Group request: a CoAP request message sent by a client in the
      group to servers in that group.

   *  Long exchange: an exchange of messages associated with a request
      that is a group request and/or an Observe request [RFC7641].

      In either case, multiple responses can follow from the same server
      to the request associated with the long exchange, even if the
      request is not an Observe request (see Section 3.1.6 of
      [I-D.ietf-core-groupcomm-bis]).  The client terminates a long
      exchange when freeing up the CoAP Token value used for the
      associated request, for which no further responses will be
      accepted afterwards.






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2.  Security Context

   This document refers to a group as a set of endpoints sharing keying
   material and security parameters for executing the Group OSCORE
   protocol, see Section 1.1.  All members of a group maintain a
   Security Context as defined in this section.

   How the Security Context is established by the group members is out
   of scope for this document, but if there is more than one Security
   Context applicable to a message, then the endpoints MUST be able to
   determine which Security Context was latest established.  The
   management of information about the group (i.e., identifiers, OSCORE
   input parameters, and keying material) is described in terms of a
   Group Manager (see Section 12).

   An endpoint of the group may use the group mode (see Section 7), the
   pairwise mode (see Section 8), or both, depending on the modes it
   supports and on the parameters of the Security Context.  The Security
   Context of Group OSCORE extends the OSCORE Security Context defined
   in Section 3 of [RFC8613] as follows (see Figure 1).

   *  One Common Context, shared by all the endpoints in the group and
      extended as defined below.

      -  The new parameter Authentication Credential Format (see
         Section 2.1.5), specifying the format of authentication
         credentials used in the group (see Section 2.4).

      -  The new parameter Group Manager Authentication Credential,
         specifying the authentication credential of the Group Manager
         responsible for the group (see Section 2.1.6).

      -  For the group mode, the Common Context is extended with the
         following new parameters.

         o  Group Encryption Algorithm, specifying the algorithm used
            for encrypting and decrypting messages protected in group
            mode (see Section 2.1.7).

         o  Signature Algorithm, specifying the algorithm used for
            computing and verifying the countersignature of messages
            protected in group mode (see Section 2.1.8).

         o  Signature Encryption Key, specifying the symmetric key used
            for deriving a keystream, which is in turn used for
            encrypting and decrypting the countersignature of messages
            protected in group mode (see Section 2.1.9).




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      -  For the pairwise mode, the Common Context is extended with a
         Pairwise Key Agreement Algorithm (see Section 2.1.10) used for
         the agreement on a static-static Diffie-Hellman shared secret,
         from which pairwise keys are derived (see Section 2.5.1).

      The content of the Common Context is long-term, as it is meant to
      be stable once the Common Context is established.

   *  One Sender Context, extended with the following new parameters.

      -  The endpoint's own private key used to sign messages protected
         in group mode (see Section 7), or for deriving pairwise keys
         used with the pairwise mode (see Section 2.5).

      -  The endpoint's own authentication credential containing its
         public key (see Section 2.4).

      -  For the pairwise mode, the Sender Context is extended with the
         Pairwise Sender Keys associated with the other endpoints (see
         Section 2.5).

      Except for the Sender Sequence Number defined in Section 3.1 of
      [RFC8613], the content of the Sender Context is long-term, as it
      is meant to be stable once the Sender Context is established.

      If the endpoint is configured exclusively as a silent server (see
      Section 1.1), then the Sender Context is omitted.

   *  One Recipient Context for each other endpoint from which messages
      are received.  It is not necessary to maintain Recipient Contexts
      associated with endpoints from which messages are not (expected to
      be) received.

      -  Each Recipient Context is extended with the authentication
         credential of the other endpoint, used to verify the signature
         of messages protected in group mode, or for deriving the
         pairwise keys used with the pairwise mode (see Section 2.5).

      -  For the pairwise mode, each Recipient Context is extended with
         the Pairwise Recipient Key associated with the other endpoint
         (see Section 2.5).

      Except for the Replay Window defined in Section 3.1 of [RFC8613],
      the content of each Recipient Context is long-term, as it is meant
      to be stable once the Recipient Context is established.






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   The varying part of the Group OSCORE Security Context is composed of
   the Sender Sequence Number in the Sender Context and the Replay
   Windows in the different Recipient Contexts.

 +-------------------+-------------------------------------------------+
 | Context Component | New Information Elements                        |
 +-------------------+-------------------------------------------------+
 | Common Context    |   Authentication Credential Format              |
 |                   |   Group Manager Authentication Credential       |
 |                   | * Group Encryption Algorithm                    |
 |                   | * Signature Algorithm                           |
 |                   | * Signature Encryption Key                      |
 |                   | ^ Pairwise Key Agreement Algorithm              |
 +-------------------+-------------------------------------------------+
 | Sender Context    |   Endpoint's own private key                    |
 |                   |   Endpoint's own authentication credential      |
 |                   | ^ Pairwise Sender Keys for the other endpoints  |
 +-------------------+-------------------------------------------------+
 | Each              |   Other endpoint's authentication credential    |
 | Recipient Context | ^ Pairwise Recipient Key for the other endpoint |
 +-------------------+-------------------------------------------------+

   Figure 1: Additions to the OSCORE Security Context.  The elements
     labeled with * and with ^ are relevant only for the group mode
             and only for the pairwise mode, respectively.

2.1.  Common Context

   The following sections specify how the Common Context is used and
   extended compared to [RFC8613].  The Common Context may be acquired
   from the Group Manager (see Section 12).

2.1.1.  AEAD Algorithm

   The AEAD Algorithm (see Section 3.1 of [RFC8613]) identifies the COSE
   AEAD algorithm to use for encryption and decryption when messages are
   protected using the pairwise mode (see Section 8).  This algorithm
   MUST provide integrity protection.  If this parameter is not set, the
   pairwise mode is not used in the group.

2.1.2.  HKDF Algorithm

   The HKDF Algorithm (see Section 3.1 of [RFC8613]) identifies the used
   key derivation function, which MUST be one of the HMAC-based HKDF
   [RFC5869] algorithms defined for COSE (see Section 5.1 of [RFC9053])
   and registered at [COSE.Algorithms].





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2.1.3.  ID Context

   The ID Context parameter (see Sections 3.1 and 3.3 of [RFC8613])
   contains the Group Identifier (Gid) of the group.  The choice of the
   Gid format is application specific.  An example of specific
   formatting of the Gid is given in Appendix C.  The application needs
   to specify how to handle potential collisions between Gids (see
   Section 14.6).

2.1.4.  Common IV

   The Common IV parameter (see Section 3.1 of [RFC8613]) identifies the
   Common IV used in the group.  Differently from OSCORE, the length of
   the Common IV is determined as follows.

   *  If only one among the AEAD Algorithm and the Group Encryption
      Algorithm is set (see Section 2.1.1 and Section 2.1.7), the length
      of the Common IV is the nonce length for the set algorithm.

   *  If both the AEAD Algorithm and the Group Encryption Algorithm are
      set, the length of the Common IV is the greatest nonce length
      among those of the two algorithms.

   If the Group Encryption Algorithm is A128CTR, A192CTR, or A256CTR
   (see Section 4 of [RFC9459]), then the length of the nonce used by
   that algorithm is 12 bytes (see Section 2.1.7).

2.1.5.  Authentication Credential Format

   The new parameter Authentication Credential Format specifies the
   format of authentication credentials used in the group.

2.1.6.  Group Manager Authentication Credential

   The new parameter Group Manager Authentication Credential specifies
   the authentication credential of the Group Manager, including the
   Group Manager's public key.  The endpoint MUST achieve proof of
   possession of the corresponding private key.  As an example, such
   proof of possession is possible to achieve during the join process
   provided by the realization of Group Manager specified in
   [I-D.ietf-ace-key-groupcomm-oscore].  Further details on the
   provisioning of the Group Manager's authentication credential to the
   group members are out of the scope of this document.








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2.1.7.  Group Encryption Algorithm

   The new parameter Group Encryption Algorithm identifies the algorithm
   to use for encryption and decryption, when messages are protected in
   group mode (see Section 7).  This algorithm MAY provide integrity
   protection.  If it does not, integrity protection is still provided
   by the countersignature added to the message due to the use of the
   group mode.  If this parameter is not set, the group mode is not used
   in the group.

   In order to be eligible to use as Group Encryption Algorithm, a non-
   authenticated algorithm MUST ensure that the same key is not reused
   with the same IV or intermediate values used in the algorithm, e.g.,
   for algorithms that increment the IV internally.  If a non-
   authenticated algorithm does not fulfill the requirement above, that
   algorithm MUST NOT be used as Group Encryption Algorithm.

   Examples of non-authenticated algorithms that can be used as Group
   Encryption Algorithm are A128CTR, A192CTR, and A256CTR (see Section 4
   of [RFC9459]).  When either of those three algorithms is used, the
   following applies:

   *  A 12-byte nonce MUST be computed as defined in Section 3.3 of this
      document.

   *  The Initialization Vector (IV) used in Section 4 of [RFC9459] is
      equivalent to the nonce above (12 bytes) concatenated with
      0x00000000 (4 bytes), in this order.

   *  The algorithm MUST NOT be used to encrypt a plaintext or decrypt a
      ciphertext whose length is larger than 64 GB (i.e., 2^36 bytes).

   The non-authenticated algorithms A128CBC, A192CBC, and A256CBC (see
   Section 5 of [RFC9459]) MUST NOT be used as Group Encryption
   Algorithm.

   Future specifications can admit alternative non-authenticated
   algorithms that can be used as Group Encryption Algorithm.  When
   doing so, it MUST be defined how to securely compose the IV and
   possible intermediate values used in the algorithm, building on the
   nonce computed as defined in Section 3.3 of this document.  Absent
   such a specification, alternative non-authenticated algorithms MUST
   NOT be used as Group Encryption Algorithm.








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2.1.8.  Signature Algorithm

   The new parameter Signature Algorithm identifies the digital
   signature algorithm used for computing and verifying the
   countersignature on the COSE object (see Sections 3.2 and 3.3 of
   [RFC9338]), when messages are protected in group mode (see
   Section 7).  If this parameter is not set, the group mode is not used
   in the group.

2.1.9.  Signature Encryption Key

   The new parameter Signature Encryption Key specifies the symmetric
   key for deriving a keystream to encrypt/decrypt a countersignature
   (see Section 4.2) when a message is protected in group mode (see
   Section 7).

   The Signature Encryption Key is derived as defined for Sender/
   Recipient Keys in Section 3.2.1 of [RFC8613], with the following
   differences.

   *  The 'id' element of the 'info' array is the empty byte string.

   *  The 'alg_aead' element of the 'info' array specifies the Group
      Encryption Algorithm from the Common Context (see Section 2.1.7),
      encoded as a CBOR integer or text string, consistently with the
      "Value" field in the "COSE Algorithms" Registry for this
      algorithm.

   *  The 'type' element of the 'info' array is "SEKey".  The label is
      an ASCII string and does not include a trailing NUL byte.

   *  L and the 'L' element of the 'info' array are the size of the key
      for the Group Encryption Algorithm specified in the Common Context
      (see Section 2.1.7), in bytes.  While the obtained Signature
      Encryption Key is never used with the Group Encryption Algorithm,
      its length was chosen to obtain a matching level of security.

2.1.10.  Pairwise Key Agreement Algorithm

   The new parameter Pairwise Key Agreement Algorithm identifies the
   elliptic curve Diffie-Hellman algorithm used to derive a static-
   static Diffie-Hellman shared secret, from which pairwise keys are
   derived (see Section 2.5.1) to protect messages with the pairwise
   mode (see Section 8).  If this parameter is not set, the pairwise
   mode is not used in the group.






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   When two endpoints compute their Diffie-Hellman shared secret, the
   Pairwise Key Agreement Algorithm takes as input the static-static
   Diffie-Hellman keys of the two endpoints.  The lifetime of those keys
   is the same as the lifetime of the authentication credentials that
   the two endpoints use in the group.  As detailed in Section 2.5.1,
   the derivation of the pairwise keys takes as input not only the
   Diffie-Hellman shared secret, but also group keying material from the
   latest established Security Context.

   If the HKDF Algorithm specified in the Common Context is "HKDF SHA-
   256" (identified as "HMAC 256/256"), then the Pairwise Key Agreement
   Algorithm is "ECDH-SS + HKDF-256" (COSE algorithm encoding: -27).

   If the HKDF Algorithm specified in the Common Context is "HKDF SHA-
   512" (identified as "HMAC 512/512"), then the Pairwise Key Agreement
   Algorithm is "ECDH-SS + HKDF-512" (COSE algorithm encoding: -28).

   Note that the HKDF Algorithm in the Common Context is denoted by the
   corresponding COSE HMAC Algorithm.  For example, the HKDF Algorithm
   "HKDF SHA-256" is specified as the HMAC Algorithm "HMAC 256/256".

   More generally, if Pairwise Key Agreement Algorithm is set, it MUST
   identify a COSE algorithm such that: i) it performs a direct ECDH
   Static-Static key agreement; and ii) it indicates the use of the same
   HKDF Algorithm used in the group as specified in the Common Context.

2.2.  Sender Context and Recipient Context

   The Sender ID SHALL be unique for each endpoint in a group with a
   certain triplet (Master Secret, Master Salt, Group Identifier), see
   Section 3.3 of [RFC8613].

   The maximum length of a Sender ID in bytes equals L minus 6, where L
   is determined as follows.

   *  If only one among the AEAD Algorithm and the Group Encryption
      Algorithm is set (see Section 2.1.1 and Section 2.1.7), then L is
      the nonce length for the set algorithm.

   *  If both the AEAD Algorithm and the Group Encryption Algorithm are
      set, then L is the smallest nonce length among those of the two
      algorithms.

   With the exception of the authentication credential of the sender
   endpoint, a receiver endpoint can derive a complete Security Context
   from a received Group OSCORE message and the Common Context (see
   Section 2.3).




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   The authentication credentials in the Recipient Contexts can be
   retrieved from the Group Manager (see Section 12) upon joining the
   group.  An authentication credential can alternatively be acquired
   from the Group Manager at a later time, for example the first time a
   message is received from a particular endpoint in the group (see
   Section 7.2 and Section 7.4).

   For severely constrained devices, it may be infeasible to
   simultaneously handle the ongoing processing of a recently received
   message in parallel with the retrieval of the sender endpoint's
   authentication credential.  Such devices can be configured to drop a
   received message for which there is no (complete) Recipient Context,
   and retrieve the sender endpoint's authentication credential in order
   to have it available to verify subsequent messages from that
   endpoint.

   An endpoint may admit a maximum number of Recipient Contexts for a
   same Security Context, e.g., due to memory limitations.  After
   reaching that limit, the endpoint has to delete a current Recipient
   Context to install a new one (see Section 2.6.1.2).  It is up to the
   application to define the maximum number of Recipient Contexts for a
   same Security Context as well as policies for deleting Recipient
   Contexts.

2.3.  Establishment of Security Context Parameters

   OSCORE defines the derivation of Sender Context and Recipient Context
   (specifically, of Sender/Recipient Keys) and of the Common IV, from a
   set of input parameters (see Section 3.2 of [RFC8613]).

   The derivation of Sender/Recipient Keys and of the Common IV defined
   in OSCORE applies also to Group OSCORE, with the following
   modifications compared to Section 3.2.1 of [RFC8613].

   *  If Group Encryption Algorithm in the Common Context is set (see
      Section 2.1.7), then the 'alg_aead' element of the 'info' array
      MUST specify Group Encryption Algorithm from the Common Context as
      a CBOR integer or text string, consistently with the "Value" field
      in the "COSE Algorithms" Registry for this algorithm.

   *  If Group Encryption Algorithm in the Common Context is not set,
      then the 'alg_aead' element of the 'info' array MUST specify AEAD
      Algorithm from the Common Context (see Section 2.1.1), as per
      Section 5.4 of [RFC8613].

   *  When deriving the Common IV, the 'L' element of the 'info' array
      MUST specify the length of the Common IV in bytes, which is
      determined as defined in Section 2.1.4.



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2.4.  Authentication Credentials

   The authentication credentials of the endpoints in a group MUST be
   encoded according to the format used in the group, as indicated by
   the Authentication Credential Format parameter in the Common Context
   (see Section 2.1.5).  The authentication credential of the Group
   Manager SHOULD be encoded according to that same format, in order to
   limit the number of formats that the group members have to support
   and handle, unless it is infeasible or impractical for the particular
   realization or instance of the Group Manager to have an own
   authentication credential encoded in that same format.

   The format of authentication credentials MUST provide the public key
   and a comprehensive set of information related to the public key
   algorithm, including, e.g., the used elliptic curve (when
   applicable).  If Group Encryption Algorithm in the Common Context is
   not set (see Section 2.1.7), then the public key algorithm is the
   Pairwise Key Agreement Algorithm used in the group (see
   Section 2.1.10), else the Signature Algorithm used in the group (see
   Section 2.1.8).

   Examples of formats of authentication credentials are CBOR Web Tokens
   (CWTs) and CWT Claims Sets (CCSs) [RFC8392], X.509 certificates
   [RFC5280], and C509 certificates [I-D.ietf-cose-cbor-encoded-cert].

   If the authentication credentials are X.509 certificates or C509
   certificates, the public key algorithm is identified by the
   "algorithm" field of the "SubjectPublicKeyInfo" structure, and by the
   "subjectPublicKeyAlgorithm" element, respectively.

   If authentication credentials are CBOR Web Tokens (CWTs) or CWT
   Claims Sets (CCSs), then a COSE Key structure and its "kty" and "crv"
   parameters identify the types of pertinent public key algorithms.
   For example: the pair ("crv" = X25519, "kty" = OKP) indicates that
   the public key is meant to be used with X25519 ECDH key agreement;
   the pair ("crv" = Ed25519, "kty" = OKP) indicates that the public key
   is meant to be used with the signature algorithm EdDSA; the pair
   ("crv" = P-256, "kty" = EC2) indicates that the public key is meant
   to be used with the signature algorithm ECDSA and/or with P-256 ECDH
   key agreement.











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   Authentication credentials are used to derive pairwise keys (see
   Section 2.5.1) and are included in the external additional
   authenticated data when processing messages (see Section 3.4).  In
   both these cases, an endpoint in a group MUST treat authentication
   credentials as opaque data, i.e., by considering the same binary
   representation made available to other endpoints in the group,
   possibly through a designated trusted source (e.g., the Group
   Manager).

   For example, an X.509 certificate is provided as its direct binary
   serialization.  If C509 certificates or CWTs are used as
   authentication credentials, each is provided as the binary
   serialization of a (possibly tagged) CBOR array.  If CCSs are used as
   authentication credentials, each is provided as the binary
   serialization of a (possibly tagged) CBOR map.

   If authentication credentials are CWTs or CCSs, then the untagged CWT
   or CCS associated with an entity is stored in the Security Context
   and used as authentication credential for that entity.

   If authentication credentials are X.509 / C509 certificates, CWTs, or
   CCSs and the authentication credential associated with an entity is
   provided within a chain or a bag, then only the end-entity
   certificate or end-entity untagged CWT / CCS is stored in the
   Security Context and used as authentication credential for that
   entity.

   Storing whole authentication credentials rather than only a subset of
   those may result in a non-negligible storage overhead.  On the other
   hand, it also ensures that authentication credentials are correctly
   used in a simple, flexible and non-error-prone way, also taking into
   account future credential formats as entirely new or extending
   existing ones.  In particular, it is ensured that:

   *  When used to derive pairwise keys and when included in the
      external additional authenticated data, authentication credentials
      can also specify possible metadata and parameters related to the
      included public key.  Besides the public key algorithm, these
      comprise other relevant pieces of information such as key usage,
      expiration time, issuer, and subject.

   *  All endpoints using another endpoint's authentication credential
      use exactly the same binary serialization, as obtained and
      distributed by the credential provider (e.g., the Group Manager),
      and as originally crafted by the credential issuer.  In turn, this
      does not require to define and maintain canonical subsets of
      authentication credentials and their corresponding encoding, and
      spares endpoints from error-prone re-encoding operations.



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   Depending on the particular deployment and the intended group size,
   limiting the storage overhead of endpoints in a group can be an
   incentive for system/network administrators to prefer using a compact
   format of authentication credentials in the first place.

2.5.  Pairwise Keys

   In certain Elliptic Curve Cryptographic schemes, it is possible to
   use public/private key pairs with both signature operations (ECDSA or
   EdDSA) and key agreement operations (ECDH).  This section specifies
   the derivation of "pairwise keys" for use in the pairwise mode
   defined in Section 8.

   Group OSCORE keys used for both signature operations and key
   agreement operations MUST be used only for purposes related to Group
   OSCORE.  These include the processing of messages with Group OSCORE,
   as well as performing proof of possession of private keys, e.g., upon
   joining a group through the Group Manager (see Section 12).

2.5.1.  Derivation of Pairwise Keys

   Using the Group OSCORE Security Context (see Section 2), a group
   member can derive AEAD keys, to protect point-to-point communication
   between itself and each other endpoint X in the group by means of the
   AEAD Algorithm from the Common Context (see Section 2.1.1).

   Analogous to the construction used by OSCORE in Section 3.2.1 of
   [RFC8613], the key derivation of these so-called pairwise keys relies
   on an HKDF algorithm and is as defined below:

  Pairwise Sender Key    = HKDF(Sender Key, IKM-Sender, info, L)
  Pairwise Recipient Key = HKDF(Recipient Key, IKM-Recipient, info, L)

  with

  IKM-Sender    = Sender Auth Cred | Recipient Auth Cred | Shared Secret
  IKM-Recipient = Recipient Auth Cred | Sender Auth Cred | Shared Secret

   where:

   *  The Pairwise Sender Key is the AEAD key for processing outgoing
      messages addressed to endpoint X.

   *  The Pairwise Recipient Key is the AEAD key for processing incoming
      messages from endpoint X.

   *  HKDF is the OSCORE HKDF algorithm [RFC8613] from the Common
      Context.



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   *  The Sender Key from the Sender Context is used as salt in the
      HKDF, when deriving the Pairwise Sender Key.

   *  The Recipient Key from the Recipient Context associated with
      endpoint X is used as salt in the HKDF, when deriving the Pairwise
      Recipient Key.

   *  Sender Auth Cred is the endpoint's own authentication credential
      from the Sender Context.

   *  Recipient Auth Cred is the endpoint X's authentication credential
      from the Recipient Context associated with the endpoint X.

   *  The Shared Secret is computed as a cofactor Diffie-Hellman shared
      secret, see Section 5.7.1.2 of [NIST-800-56A], using the Pairwise
      Key Agreement Algorithm.  The endpoint uses its private key from
      the Sender Context and the other endpoint's public key included in
      Recipient Auth Cred.  Note the requirement of validation of public
      keys in Section 14.16.

      In case the other endpoint's public key has COSE Key Type "EC2"
      [RFC9053] (e.g., for the curves P-256, P-384, and P-521), then the
      public key is used as is.  In case the other endpoint's public key
      has COSE Key Type "OKP" [RFC9053], the procedure is described in
      Section 5 of [RFC7748].  In particular, if the public key is for
      X25519 or X448, it is used as is.  Otherwise, if the public key is
      for the curve Ed25519 or Ed448, it is first mapped to Montgomery
      coordinates (see Section 2.5.2).

   *  IKM-Sender is the Input Keying Material (IKM) used in the HKDF for
      the derivation of the Pairwise Sender Key. IKM-Sender is the byte
      string concatenation of Sender Auth Cred, Recipient Auth Cred, and
      the Shared Secret.  The authentication credentials Sender Auth
      Cred and Recipient Auth Cred are binary encoded as defined in
      Section 2.4.

   *  IKM-Recipient is the Input Keying Material (IKM) used in the HKDF
      for the derivation of the Pairwise Recipient Key. IKM-Recipient is
      the byte string concatenation of Recipient Auth Cred, Sender Auth
      Cred, and the Shared Secret.  The authentication credentials
      Recipient Auth Cred and Sender Auth Cred are binary encoded as
      defined in Section 2.4.

   *  info and L are as defined in Section 3.2.1 of [RFC8613].  That is:

      -  The 'alg_aead' element of the 'info' array takes the value of
         AEAD Algorithm from the Common Context (see Section 2.1.1).




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      -  L and the 'L' element of the 'info' array are the size of the
         key for the AEAD Algorithm from the Common Context (see
         Section 2.1.1), in bytes.

   If EdDSA asymmetric keys are used, the Edward coordinates are mapped
   to Montgomery coordinates using the maps defined in Sections 4.1 and
   4.2 of [RFC7748], before using the X25519 or X448 function defined in
   Section 5 of [RFC7748].  For further details, see Section 2.5.2.  ECC
   asymmetric keys in Montgomery or Weierstrass form are used directly
   in the key agreement algorithm, without coordinate mapping.

   As long as any two group members preserve the same asymmetric keys,
   their Diffie-Hellman shared secret does not change across updates of
   the group keying material.  The lifetime of those keys is the same as
   the lifetime of the authentication credentials Sender Auth Cred and
   Recipient Auth Cred.

   After establishing a partially or completely new Security Context
   (see Section 2.6 and Section 12.2), the old pairwise keys MUST be
   deleted.  Since new Sender/Recipient Keys are derived from the new
   group keying material (see Section 2.2), every group member MUST use
   the new Sender/Recipient Keys when deriving new pairwise keys.

2.5.2.  ECDH with Montgomery Coordinates

2.5.2.1.  Curve25519

   The y-coordinate of the other endpoint's Ed25519 public key is
   decoded as specified in Section 5.1.3 of [RFC8032].  The Curve25519
   u-coordinate is recovered as u = (1 + y) / (1 - y) (mod p) following
   the map in Section 4.1 of [RFC7748].  Note that the mapping is not
   defined for y = 1, and that y = -1 maps to u = 0 which corresponds to
   the neutral group element and thus will result in a degenerate shared
   secret.  Therefore, implementations MUST abort if the y-coordinate of
   the other endpoint's Ed25519 public key is 1 or -1 (mod p).

   The private signing key byte strings (i.e., the lower 32 bytes used
   for generating the public key, see Step 1 of Section 5.1.5 of
   [RFC8032]) are decoded the same way for signing in Ed25519 and scalar
   multiplication in X25519.  Hence, in order to compute the shared
   secret, the endpoint applies the X25519 function to the Ed25519
   private signing key byte string and the encoded u-coordinate byte
   string as specified in Section 5 of [RFC7748].








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2.5.2.2.  Curve448

   The y-coordinate of the other endpoint's Ed448 public key is decoded
   as specified in Section 5.2.3. of [RFC8032].  The Curve448
   u-coordinate is recovered as u = y^2 * (d * y^2 - 1) / (y^2 - 1) (mod
   p) following the map from "edwards448" in Section 4.2 of [RFC7748],
   and also using the relation x^2 = (y^2 - 1)/(d * y^2 - 1) from the
   curve equation.  Note that the mapping is not defined for y = 1 or
   -1.  Therefore, implementations MUST abort if the y-coordinate of the
   peer endpoint's Ed448 public key is 1 or -1 (mod p).

   The private signing key byte strings (i.e., the lower 57 bytes used
   for generating the public key, see Step 1 of Section 5.2.5 of
   [RFC8032]) are decoded the same way for signing in Ed448 and scalar
   multiplication in X448.  Hence, in order to compute the shared
   secret, the endpoint applies the X448 function to the Ed448 private
   signing key byte string and the encoded u-coordinate byte string as
   specified in Section 5 of [RFC7748].

2.5.3.  Usage of Sequence Numbers

   When using any of its Pairwise Sender Keys, a sender endpoint
   including the 'Partial IV' parameter in the protected message MUST
   use the current fresh value of the Sender Sequence Number from its
   Sender Context (see Section 2.2).  That is, the same Sender Sequence
   Number space is used for all outgoing messages protected with Group
   OSCORE, thus limiting both storage and complexity.

   When combining communications with the group mode and the pairwise
   mode, this may result in the Partial IV values moving forward more
   often than when using OSCORE [RFC8613].  This can happen when a
   client engages in frequent or long sequences of one-to-one exchanges
   with servers in the group, by sending requests over unicast.  In
   turn, this contributes to a sooner exhaustion of the Sender Sequence
   Number space of the client, which would then require to take actions
   for deriving a new Sender Context before resuming communications in
   the group (see Section 2.6.2).

2.5.4.  Security Context for Pairwise Mode

   If the pairwise mode is supported, the Security Context additionally
   includes the Pairwise Key Agreement Algorithm and the pairwise keys,
   as described at the beginning of Section 2.

   The pairwise keys as well as the shared secrets used in their
   derivation (see Section 2.5.1) may be stored in memory or recomputed
   every time they are needed.  The shared secret changes only when a
   public/private key pair used for its derivation changes, which



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   results in the pairwise keys also changing.  Additionally, the
   pairwise keys change if the Sender ID changes or if a new Security
   Context is established for the group (see Section 2.6.3).  In order
   to optimize protocol performance, an endpoint may store the derived
   pairwise keys for easy retrieval.

   In the pairwise mode, the Sender Context includes the Pairwise Sender
   Keys to use with the other endpoints (see Figure 1).  In order to
   identify the right key to use, the Pairwise Sender Key for endpoint X
   may be associated with the Recipient ID of endpoint X, as defined in
   the Recipient Context (i.e., the Sender ID from the point of view of
   endpoint X).  In this way, the Recipient ID can be used to lookup for
   the right Pairwise Sender Key. This association may be implemented in
   different ways, e.g., by storing the pair (Recipient ID, Pairwise
   Sender Key) or linking a Pairwise Sender Key to a Recipient Context.

2.6.  Update of Security Context

   It is RECOMMENDED that the long-term part of the Security Context is
   stored in non-volatile memory, or that it can otherwise be reliably
   accessed throughout the operation of the group, e.g., after device
   reboots.  However, also data in the long-term part of the Security
   Context may need to be updated, for example due to scheduled key
   renewal, new or re-joining members in the group, or the fact that the
   endpoint changes Sender ID (see Section 2.6.3).

   The data in the varying part of the Security Context are updated by
   the endpoint when executing the security protocol, but may be lost
   (see Section 2.6.1) or become outdated by exhaustion of Sender
   Sequence Numbers (see Section 2.6.2).

2.6.1.  Loss of the Varying Part of the Security Context

   An endpoint may lose the varying part of its Security Context due to
   accidental events, e.g., if a reboot occurred in an unprepared way
   (see Section 2.6.1.1) or due to a deliberately deleted Recipient
   Context (see Section 2.6.1.2).

   If it is not feasible or practically possible to store and maintain
   up-to-date the varying part in non-volatile memory (e.g., due to
   limited number of write operations), the endpoint MUST be able to
   detect a loss of the varying part of the Security Context, to prevent
   the re-use of a nonce with the same key and to handle incoming
   replayed messages.







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2.6.1.1.  Accidental Loss of Sender Context and/or Recipient Contexts

   In case a loss of the Sender Context and/or of the Recipient Contexts
   is detected (e.g., if a reboot occurred in an unprepared way), the
   endpoint MUST NOT protect further messages using this Security
   Context, to avoid reusing a nonce with the same key.

   Before resuming its operations in the group, the endpoint MUST
   retrieve new Security Context parameters from the Group Manager (see
   Section 2.6.3) and use them to derive a new Sender Context and
   Recipient Contexts (see Section 2.2).  Since the new Sender Context
   includes newly derived encryption keys, an endpoint will not reuse
   the same pair (key, nonce), even when it is a server using the
   Partial IV of (old re-injected) requests to build the nonce for
   protecting the responses.

   From then on, the endpoint MUST use the latest installed Sender
   Context to protect outgoing messages.  Newly derived Recipient
   Contexts will have a Replay Window which is initialized as valid.

   If an endpoint is not configured as a silent server and is not able
   to establish an updated Sender Context, e.g., because of lack of
   connectivity with the Group Manager, then the endpoint MUST NOT
   protect further messages using the current Security Context and MUST
   NOT accept incoming messages from other group members, as it is
   currently unable to detect possible replays.

   If an endpoint is configured as a silent server and is not able to
   establish an updated Security Context, e.g., because of lack of
   connectivity with the Group Manager, then the endpoint MUST NOT
   accept incoming messages from other group members, as it is currently
   unable to detect possible replays.

2.6.1.2.  Deliberately Deleted Recipient Contexts

   The Security Context may contain a large and variable number of
   Recipient Contexts.  While Group OSCORE in itself does not establish
   a maximum number of Recipient Contexts, there are circumstances by
   which implementations might choose to discard Recipient Contexts or
   have to do so in accordance with enforced application policies.  Such
   circumstances include the need to reclaim memory or other resources
   on the node hosting the endpoint, for example because the predefined
   maximum number of Recipient Contexts has been reached in the Security
   Context (see Section 2.2).  Implementations can provide means for the
   application to gain knowledge about the deliberate deletion of
   Recipient Contexts, e.g., through notifications sent to the
   application and/or logs made available to the application.




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   When a Recipient Context is deleted, this not only results in losing
   information about previously received messages from the corresponding
   other endpoint.  It also results in the inability to be aware of the
   Security Contexts from which information has been lost.

   Therefore, if the Recipient Context is derived again from the same
   Security Context, there is a risk that a replayed message is not
   detected.  If any Recipient Context associated with any peer has ever
   been deleted from the current Security Context, then the Replay
   Window of any new Recipient Context in this Security Context MUST be
   initialized as invalid.  An exception applies when the deleted
   Recipient Context was created upon receiving a message and that
   message was not verified successfully (see Section 7.2, Section 7.4,
   Section 8.4, and Section 8.6).  Messages associated with a Recipient
   Context that has an invalid Replay Window MUST NOT be delivered to
   the application.

   If the endpoint receives a message to be processed with any such new
   Recipient Context whose Replay Window is invalid, then the endpoint
   MUST take one of the following courses of action.

   *  The endpoint discards the message.

   *  The endpoint follows the procedure based on the CoAP Echo Option
      [RFC9175] and specified in Section 9, in order to establish a
      valid Replay Window.  In particular, the endpoint MUST use its
      Partial IV when generating the nonce and MUST include the Partial
      IV in the response message conveying the Echo Option.  If the
      endpoint supports the CoAP Echo Option, then it is RECOMMENDED to
      take this course of action.

   *  The endpoint retrieves or waits for new Security Context
      parameters from the Group Manager and derives new Sender and
      Recipient Contexts, as defined in Section 2.6.1.1.  In this case
      the Replay Windows of all Recipient Contexts become valid if they
      are not already.  In particular, any invalid Replay Window is re-
      initialized as valid and with 0 as its current lower limit.

2.6.2.  Exhaustion of Sender Sequence Number Space

   Since an endpoint increments its Sender Sequence Number for each new
   outgoing message including a Partial IV, an endpoint can eventually
   exhaust the Sender Sequence Number space.

   Implementations MUST be able to detect an exhaustion of Sender
   Sequence Number space, after the endpoint has consumed the largest
   usable value.  This may be influenced by additional limitations
   besides the mere 40-bit size limit of the Partial IV.



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   Upon exhausting the Sender Sequence Number space, the endpoint MUST
   NOT use this Security Context to protect further messages including a
   Partial IV.

   When approaching the exhaustion of the Sender Sequence Number space,
   the endpoint SHOULD inform the Group Manager, retrieve new Security
   Context parameters from the Group Manager (see Section 2.6.3), and
   use them to derive a new Sender Context (see Section 2.2).  It is
   RECOMMENDED that the endpoint takes this course of action with some
   margin, i.e., well before exhausting the Sender Sequence Number
   space, in order to avoid a period of inability to protect messages
   including a Partial IV.

   From then on, the endpoint MUST use its latest installed Sender
   Context to protect outgoing messages.

2.6.3.  Retrieving New Security Context Parameters

   The Group Manager can assist an endpoint with an incomplete Sender
   Context to retrieve missing data of the Security Context and thereby
   become fully operational in the group again.  The two main options
   for the Group Manager are: i) assignment of a new Sender ID to the
   endpoint (see Section 2.6.3.1); and ii) establishment of a new
   Security Context for the group (see Section 2.6.3.2).  The update of
   the Replay Window in each of the Recipient Contexts is discussed in
   Section 2.6.1.

   As group membership changes, or as group members get new Sender IDs
   (see Section 2.6.3.1), so do the relevant Recipient IDs that the
   other endpoints need to keep track of.  As a consequence, group
   members may end up retaining stale Recipient Contexts that are no
   longer useful to verify incoming secure messages.

   The Recipient ID ('kid') SHOULD NOT be considered as a persistent and
   reliable identifier of a group member.  Such an indication can be
   achieved only by using that member's public key, when verifying
   countersignatures of received messages (in group mode), or when
   verifying messages integrity-protected with pairwise keying material
   derived from authentication credentials and associated asymmetric
   keys (in pairwise mode).

   Furthermore, applications MAY define policies to: i) delete
   (long-)unused Recipient Contexts and reduce the impact on storage
   space; as well as ii) check with the Group Manager that an
   authentication credential with the public key included therein is
   currently the one associated with a 'kid' value, after a number of
   consecutive failed verifications.




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2.6.3.1.  New Sender ID for the Endpoint

   The Group Manager may assign a new Sender ID to an endpoint, while
   leaving the Gid, Master Secret, and Master Salt unchanged in the
   group.  In this case, the Group Manager assigns a Sender ID that has
   not been used in the group since the latest time when the current Gid
   value was assigned to the group (see Section 12.2).

   Having retrieved the new Sender ID, and potentially other missing
   data for the long-term part of the Security Context, the endpoint can
   derive a new Sender Context (see Section 2.2).  When doing so, the
   endpoint resets the Sender Sequence Number in its Sender Context to
   0, and derives a new Sender Key. This is in turn used to possibly
   derive new Pairwise Sender Keys.

   From then on, the endpoint MUST use its latest installed Sender
   Context to protect outgoing messages.

   The assignment of a new Sender ID may be the result of different
   processes.  The endpoint may request a new Sender ID, e.g., because
   of the impending exhaustion of the Sender Sequence Number space (see
   Section 2.6.2).  An endpoint may request to re-join the group, e.g.,
   because of losing the varying part of its Security Context (see
   Section 2.6.1), and is provided with a new Sender ID together with
   the latest data for the long-term part of the Security Context.

   For the other group members, the Recipient Context corresponding to
   the old Sender ID becomes stale (see Section 12.2).

2.6.3.2.  New Security Context for the Group

   The Group Manager may establish a new Security Context for the group
   (see Section 12.2).  The Group Manager does not necessarily establish
   a new Security Context for the group if one member has an outdated
   Security Context (see Section 2.6.3.1), unless that was already
   planned or required for other reasons.

   All the group members need to acquire new Security Context parameters
   from the Group Manager.  Once having acquired new Security Context
   parameters, each group member performs the following actions.

   *  From then on, it MUST NOT use the current Security Context to
      start processing new messages for the considered group.

   *  It completes any ongoing message processing for the considered
      group.

   *  It derives and installs a new Security Context.  In particular:



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      -  It re-derives the keying material stored in its Sender Context
         and Recipient Contexts (see Section 2.2).  The Master Salt used
         for the re-derivations is the updated Master Salt parameter if
         provided by the Group Manager, or the empty byte string
         otherwise.

      -  It resets its Sender Sequence Number in its Sender Context to
         0.

      -  It re-initializes the Replay Window of each Recipient Context
         as valid and with 0 as its current lower limit.

      -  For each long exchange where it is a client and that it wants
         to keep active, it sets the Response Number of each associated
         server as not initialized (see Section 5.1).

   From then on, it can resume processing new messages for the
   considered group.  In particular:

   *  It MUST use its latest installed Sender Context to protect
      outgoing messages.

   *  It SHOULD use only its latest installed Recipient Contexts to
      process incoming messages, unless application policies admit to
      temporarily retain and use the old, recent, Security Context (see
      Section 14.5.1).

   The distribution of a new Gid and Master Secret may result in
   temporarily misaligned Security Contexts among group members.  In
   particular, this may result in a group member not being able to
   process messages received right after a new Gid and Master Secret
   have been distributed.  A discussion on practical consequences and
   possible ways to address them, as well as on how to handle the old
   Security Context, is provided in Section 14.5.

3.  The COSE Object

   Building on Section 5 of [RFC8613], this section defines how to use
   COSE [RFC9052] to wrap and protect data in the original message.
   Like OSCORE, Group OSCORE uses the untagged COSE_Encrypt0 structure
   with an Authenticated Encryption with Associated Data (AEAD)
   algorithm.  Unless otherwise specified, the following modifications
   to what is defined for OSCORE apply for both the group mode and the
   pairwise mode of Group OSCORE.







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3.1.  Countersignature

   When protecting a message in group mode, the 'unprotected' field MUST
   additionally include the following parameter:

   *  Countersignature0 version 2: its value is set to the
      countersignature of the COSE object.

      The countersignature is computed by the sender as described in
      Sections 3.2 and 3.3 of [RFC9338], by using its private key and
      according to the Signature Algorithm in the Security Context.

      In particular, the Countersign_structure contains the context text
      string "CounterSignature0", the external_aad as defined in
      Section 3.4 of this document, and the ciphertext of the COSE
      object as payload.

3.1.1.  Clarifications on Using a Countersignature

   The literature commonly refers to a countersignature as a signature
   computed by an entity A over a document already protected by a
   different entity B.

   However, the COSE_Countersignature0 structure belongs to the set of
   abbreviated countersignatures defined in Sections 3.2 and 3.3 of
   [RFC9338], which were designed primarily to deal with the problem of
   encrypted group messaging, but where it is required to know who
   originated the message.

   Since the parameters for computing or verifying the abbreviated
   countersignature generated by A are provided by the same context used
   to describe the security processing performed by B and to be
   countersigned, these structures are applicable also when the two
   entities A and B are actually the same one, like the sender of a
   Group OSCORE message protected in group mode.

3.2.  The 'kid' and 'kid context' parameters

   The value of the 'kid' parameter in the 'unprotected' field of
   response messages MUST be set to the Sender ID of the endpoint
   transmitting the message, if the request was protected in group mode.
   That is, unlike in [RFC8613], the 'kid' parameter is always present
   in responses to a request that was protected in group mode.

   The value of the 'kid context' parameter in the 'unprotected' field
   of request messages MUST be set to the ID Context, i.e., the Group
   Identifier value (Gid) of the group.  That is, unlike in [RFC8613],
   the 'kid context' parameter is always present in requests.



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3.3.  Nonce Computation

   The nonce is constructed like in OSCORE, with the difference that
   Step 4 in Section 5.2 of [RFC8613] is replaced with:

   4.  and then XOR with X bytes from the Common IV's start, where X is
       the length in bytes of the nonce.

   For example, if X = 7 and the Common IV is
   0x00112233445566778899aabbcc (13 bytes), then the bytes to XOR are
   0x00112233445566 (7 bytes).

   The constructed nonce is used both by the AEAD Algorithm (see
   Section 2.1.1) and by the Group Encryption Algorithm (see
   Section 2.1.7), independent of whether they are AEAD or plain
   encryption algorithms.  Algorithms that do not use a nonce are not
   supported, as per Section 2.1.7.

3.4.  Additional Authenticated Data

   The external_aad of the Additional Authenticated Data (AAD) is
   different compared to OSCORE [RFC8613] and is defined in this
   section.

   The same external_aad structure is used in group mode and pairwise
   mode for encryption/decryption (see Section 5.3 of [RFC9052]), as
   well as in group mode for computing and verifying the
   countersignature (see Sections 3.2 and 3.3 of [RFC9338]).

   In particular, the external_aad includes also the Signature
   Algorithm, the Group Encryption Algorithm, the Pairwise Key Agreement
   Algorithm, the value of the 'kid context' in the COSE object of the
   request, the OSCORE Option of the protected message, the sender's
   authentication credential, and the Group Manager's authentication
   credential.

   The external_aad SHALL be a CBOR array wrapped in a bstr object as
   defined below, following the notation of [RFC8610]:













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     external_aad = bstr .cbor aad_array

     aad_array = [
        oscore_version : uint,
        algorithms : [alg_aead : int / tstr / null,
                      alg_group_enc : int / tstr / null,
                      alg_signature : int / tstr / null,
                      alg_pairwise_key_agreement : int / tstr / null],
        request_kid : bstr,
        request_piv : bstr,
        options : bstr,
        request_kid_context : bstr,
        OSCORE_option : bstr,
        sender_cred : bstr,
        gm_cred : bstr
     ]

                           Figure 2: external_aad

   Compared with Section 5.4 of [RFC8613], the aad_array has the
   following differences.

   *  The 'algorithms' array is extended as follows.

      The parameter 'alg_aead' MUST be set to the CBOR simple value null
      (0xf6) if the parameter AEAD Algorithm is not set in the Common
      Context of the Security Context used (see Section 2.1.1).
      Otherwise, regardless of whether the endpoint supports the
      pairwise mode or not, this parameter MUST specify AEAD Algorithm
      from the Common Context (see Section 2.1.1) as per Section 5.4 of
      [RFC8613].

      Furthermore, the 'algorithms' array additionally includes:

      -  'alg_group_enc', which specifies Group Encryption Algorithm
         from the Common Context of the Security Context used (see
         Section 2.1.7).  This parameter MUST be set to the CBOR simple
         value null (0xf6) if the parameter Group Encryption Algorithm
         in the Common Context is not set.  Otherwise, regardless of
         whether the endpoint supports the group mode or not, this
         parameter MUST specify Group Encryption Algorithm as a CBOR
         integer or text string, consistently with the "Value" field in
         the "COSE Algorithms" Registry for this algorithm.

      -  'alg_signature', which specifies Signature Algorithm from the
         Common Context of the Security Context used (see
         Section 2.1.8).  This parameter MUST be set to the CBOR simple
         value null (0xf6) if the parameter Signature Algorithm in the



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         Common Context is not set.  Otherwise, regardless of whether
         the endpoint supports the group mode or not, this parameter
         MUST specify Signature Algorithm as a CBOR integer or text
         string, consistently with the "Value" field in the "COSE
         Algorithms" Registry for this algorithm.

      -  'alg_pairwise_key_agreement', which specifies Pairwise Key
         Agreement Algorithm from the Common Context of the Security
         Context used (see Section 2.1.10).  This parameter MUST be set
         to the CBOR simple value null (0xf6) if Pairwise Key Agreement
         Algorithm in the Common Context is not set.  Otherwise,
         regardless of whether the endpoint supports the pairwise mode
         or not, this parameter MUST specify Pairwise Key Agreement
         Algorithm as a CBOR integer or text string, consistently with
         the "Value" field in the "COSE Algorithms" Registry for this
         algorithm.

   *  The new element 'request_kid_context' contains the value of the
      'kid context' in the COSE object of the request (see Section 3.2).

      This enables endpoints to safely keep a long exchange active
      beyond a possible change of Gid (i.e., of ID Context), following a
      group rekeying (see Section 12.2).  In fact, it ensures that every
      response within a long exchange cryptographically matches with
      only one request (i.e., the request associated with that long
      exchange), rather than with multiple requests that were protected
      with different keying material but share the same 'request_kid'
      and 'request_piv' values.

   *  The new element 'OSCORE_option', containing the value of the
      OSCORE Option present in the protected message, encoded as a
      binary string.  This prevents the attack described in Section 14.7
      when using the group mode, as further explained in Section 14.7.2.

      Note for implementation: this construction requires the OSCORE
      Option of the message to be generated and finalized before
      computing the ciphertext of the COSE_Encrypt0 object (when using
      the group mode or the pairwise mode) and before calculating the
      countersignature (when using the group mode).  Also, the aad_array
      needs to be large enough to contain the largest possible OSCORE
      Option.

   *  The new element 'sender_cred', containing the sender's
      authentication credential.  This parameter MUST be set to a CBOR
      byte string, which encodes the sender's authentication credential
      in its original binary representation made available to other
      endpoints in the group (see Section 2.4).




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   *  The new element 'gm_cred', containing the Group Manager's
      authentication credential.  This parameter MUST be set to a CBOR
      byte string, which encodes the Group Manager's authentication
      credential in its original binary representation made available to
      other endpoints in the group (see Section 2.4).  This prevents the
      attack described in Section 14.8.

4.  OSCORE Header Compression

   Group OSCORE relies on a header compression mechanism similar to the
   one used by OSCORE and specified in Section 4.1.  Examples are
   provided in Section 4.3.

4.1.  Encoding of the OSCORE Option Value and Group OSCORE Payload

   The OSCORE header compression defined in Section 6 of [RFC8613] is
   used for compactly encoding the COSE_Encrypt0 object specified in
   Section 3 of this document, with the following differences.

   *  When the Group OSCORE message is protected in group mode, the
      message payload SHALL encode the ciphertext of the COSE object,
      concatenated with the encrypted countersignature of the COSE
      object.  That is:

      -  The plain, original countersignature of the COSE object, namely
         SIGNATURE, is specified in the "Countersignature0 version 2"
         parameter within the 'unprotected' field of the COSE object
         (see Section 3.1).

      -  The encrypted countersignature, namely ENC_SIGNATURE, is
         computed as

         ENC_SIGNATURE = SIGNATURE XOR KEYSTREAM

         where KEYSTREAM is derived as per Section 4.2.

   *  When the Group OSCORE message is protected in pairwise mode, the
      message payload SHALL encode the ciphertext of the COSE object.

   *  This document defines the usage of the sixth least significant
      bit, called "Group Flag", in the first byte of the OSCORE Option
      containing the OSCORE flag bits.  This flag bit is specified in
      Section 15.1.

   *  The Group Flag MUST be set to 1 if the Group OSCORE message is
      protected using the group mode (see Section 7).





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   *  The Group Flag MUST be set to 0 if the Group OSCORE message is
      protected using the pairwise mode (see Section 8).  The Group Flag
      MUST also be set to 0 for ordinary OSCORE messages processed
      according to [RFC8613].

4.2.  Keystream Derivation for Countersignature Encryption

   The following defines how an endpoint derives the keystream
   KEYSTREAM, used to encrypt/decrypt the countersignature of an
   outgoing/incoming message M protected in group mode.

   The keystream SHALL be derived as follows, by using the HKDF
   Algorithm from the Common Context (see Section 3.2 of [RFC8613]),
   which consists of composing the HKDF-Extract and HKDF-Expand steps
   [RFC5869].

   KEYSTREAM = HKDF(salt, IKM, info, L)

   The input parameters of HKDF are as follows.

   *  salt takes as value the Partial IV (PIV) used to protect M.  Note
      that, if M is a response, salt takes as value either: i) the fresh
      Partial IV generated by the server and included in the response;
      or ii) the same Partial IV of the request generated by the client
      and not included in the response.

   *  IKM is the Signature Encryption Key from the Common Context (see
      Section 2.1.9).

   *  info is the serialization of a CBOR array with the structure
      defined below, following the notation of [RFC8610]:

      info = [
        id : bstr,
        id_context : bstr,
        type : bool,
        L : uint
      ]

   where:

   *  id is the Sender ID of the endpoint that generated PIV.

   *  id_context is the ID Context (Gid) used when protecting M.







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      Note that, in the case of group rekeying, a server might use a
      different Gid when protecting a response, compared to the Gid that
      it used to verify (that the client used to protect) the request,
      see Section 7.3.

   *  type is the CBOR simple value true (0xf5) if M is a request, or
      the CBOR simple value false (0xf4) otherwise.

   *  L is the size of the countersignature, as per Signature Algorithm
      from the Common Context (see Section 2.1.8), in bytes.

4.3.  Examples of Compressed COSE Objects

   This section covers a list of OSCORE Header Compression examples of
   Group OSCORE used in group mode (see Section 4.3.1) or in pairwise
   mode (see Section 4.3.2).

   The examples assume that the COSE_Encrypt0 object is set (which means
   the CoAP message and cryptographic material is known).  Note that the
   examples do not include the full CoAP unprotected message or the full
   Security Context, but only the input necessary to the compression
   mechanism, i.e., the COSE_Encrypt0 object.  The output is the
   compressed COSE object as defined in Section 4.1 and divided into two
   parts, since the object is transported in two CoAP fields: OSCORE
   Option and payload.

   The examples assume that the plaintext (see Section 5.3 of [RFC8613])
   is 6 bytes long, and that the AEAD tag is 8 bytes long, hence
   resulting in a ciphertext which is 14 bytes long.  When using the
   group mode, the COSE_Countersignature0 byte string as described in
   Section 3 is assumed to be 64 bytes long.

4.3.1.  Examples in Group Mode

   Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid = 0x25,
   Partial IV = 5 and kid context = 0x44616c.

   *  Before compression (96 bytes):













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      [
        / protected / h'',
        / unprotected / {
          / kid /                           4 : h'25',
          / Partial IV /                    6 : h'05',
          / kid context /                  10 : h'44616c',
          / Countersignature0 version 2 /  12 : h'66e6d9b0
          db009f3e105a673f8855611726caed57f530f8cae9d0b168
          513ab949fedc3e80a96ebe94ba08d3f8d3bf83487458e2ab
          4c2f936ff78b50e33c885e35'
        },
        / ciphertext / h'aea0155667924dff8a24e4cb35b9'
      ]

   *  After compression (85 bytes):

      Flag byte: 0b00111001 = 0x39 (1 byte)

      Option Value: 0x39 05 03 44 61 6c 25 (7 bytes)

      Payload: 0xaea0155667924dff8a24e4cb35b9 de9e ... f1
      (14 bytes + size of the encrypted countersignature)


   Response with ciphertext = 0x60b035059d9ef5667c5a0710823b, kid = 0x52
   and no Partial IV.

   *  Before compression (88 bytes):

      [
        / protected / h'',
        / unprotected / {
          / kid /                           4 : h'52',
          / Countersignature0 version 2 /  12 : h'f5b659b8
          24487eb349c5c5c8a3fe401784cade2892725438e8be0fab
          daa2867ee6d29f68edb0818e50ebf98c28b923d0205f5162
          e73662e27c1a3ec562a49b80'
        },
        / ciphertext / h'60b035059d9ef5667c5a0710823b'
      ]

   *  After compression (80 bytes):









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      Flag byte: 0b00101000 = 0x28 (1 byte)

      Option Value: 0x28 52 (2 bytes)

      Payload: 0x60b035059d9ef5667c5a0710823b ca1e ... b3
      (14 bytes + size of the encrypted countersignature)

4.3.2.  Examples in Pairwise Mode

   Request with ciphertext = 0xaea0155667924dff8a24e4cb35b9, kid = 0x25,
   Partial IV = 5 and kid context = 0x44616c.

   *  Before compression (29 bytes):

      [
        / protected / h'',
        / unprotected / {
          / kid /           4 : h'25',
          / Partial IV /    6 : h'05',
          / kid context /  10 : h'44616c'
        },
        / ciphertext / h'aea0155667924dff8a24e4cb35b9'
      ]

   *  After compression (21 bytes):

      Flag byte: 0b00011001 = 0x19 (1 byte)

      Option Value: 0x19 05 03 44 61 6c 25 (7 bytes)

      Payload: 0xaea0155667924dff8a24e4cb35b9 (14 bytes)


   Response with ciphertext = 0x60b035059d9ef5667c5a0710823b and no
   Partial IV.

   *  Before compression (18 bytes):

      [
        / protected / h'',
        / unprotected / {},
        / ciphertext / h'60b035059d9ef5667c5a0710823b'
      ]

   *  After compression (14 bytes):






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      Flag byte: 0b00000000 = 0x00 (1 byte)

      Option Value: 0x (0 bytes)

      Payload: 0x60b035059d9ef5667c5a0710823b (14 bytes)

5.  Message Binding, Sequence Numbers, Freshness, and Replay Protection

   Like OSCORE, Group OSCORE provides message binding of responses to
   requests, as well as uniqueness of (key, nonce) pairs (see Sections
   7.1 and 7.2 of [RFC8613], respectively).

5.1.  Supporting Multiple Responses in Long Exchanges

   For each of its ongoing long exchanges, a client maintains one
   Response Number for each different server.  Then, separately for each
   server, the client uses the associated Response Number to perform
   ordering and replay protection of responses received from that server
   within that long exchange (see Section 5.3.1).

   That is, the Response Number has the same purpose that the
   Notification Number has in OSCORE (see Section 4.1.3.5.2 of
   [RFC8613]), but a client uses it for handling any response from the
   associated server within a long exchange.

   Group OSCORE allows a long exchange to remain active, even if the
   group is rekeyed (thus changing the ID Context) or the client obtains
   a new Sender ID.

   As defined in Section 7, this is achieved by the client and server(s)
   storing the 'kid' and 'kid context' used in the original request,
   throughout the whole duration of the long exchange.

   Upon leaving the group or before re-joining the group, a group member
   MUST terminate all the ongoing long exchanges that it has started in
   the group as a client.  This frees up the CoAP Token associated with
   the corresponding request.

5.2.  Freshness

   If the application requires freshness, e.g., according to time- or
   event-based policies (see Section 2.5.1 of [RFC9175]), a server can
   use the approach in Section 9 as a variant of the Challenge-Response
   procedure based on the Echo Option [RFC9175] before delivering
   request messages from a client to the application.






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   Like in OSCORE [RFC8613], assuming an honest server, the message
   binding guarantees that a response is not older than the request it
   replies to.  Therefore, building on Section 7.3 of [RFC8613], the
   following properties hold for Group OSCORE.

   *  The freshness of a response can be assessed if it is received soon
      after the request.

      For responses within a long exchange, this assessment gets weaker
      with time.  If such responses are Observe notifications [RFC7641],
      it is RECOMMENDED that the client regularly re-register the
      observation.

      If the request was neither a group request nor an Observe request,
      there is at most a single valid response and only from one,
      individually targeted server in the group.  Thus, freshness can be
      assessed depending on when the request was sent.

   *  It is not guaranteed that a misbehaving server did not create the
      response before receiving the request, i.e., Group OSCORE does not
      verify server aliveness.

   *  For requests and responses, the received Partial IV allows a
      recipient to determine the relative order of requests or
      responses.

5.3.  Replay Protection

   Like in OSCORE [RFC8613], the replay protection relies on the Partial
   IV of incoming messages.  A server updates the Replay Window of its
   Recipient Contexts based on the Partial IV values in received request
   messages, which correspond to the Sender Sequence Numbers of the
   clients.  Note that there can be large jumps in these Sender Sequence
   Number values, for example when a client exchanges unicast messages
   with other servers.  The operation of validating the Partial IV and
   performing replay protection MUST be atomic.  Section 2.6.1 and
   Section 2.6.3.2 describe the update of Replay Windows after the loss
   of data from the Security Context and the retrieving of new Security
   Context parameters.

   The protection from replay of requests is performed as per
   Section 7.4 of [RFC8613], separately for each client and by
   leveraging the Replay Window in the corresponding Recipient Context.
   The protection from replay of responses in a long exchange is
   performed as defined in Section 5.3.1 of this document.






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5.3.1.  Replay Protection of Responses

   A client uses the method defined in this section in order to check
   whether a received response is a replay.

   This especially applies to responses received within a long exchange,
   during which multiple such responses can be received from the same
   server to the corresponding request.  These include Observe
   notifications [RFC7641]; and non-notification responses as a reply to
   a group request, which the client can receive until the CoAP Token
   value associated with the group request is freed up (see
   Section 3.1.6 of [I-D.ietf-core-groupcomm-bis]).

   When sending a response (both successful and error), a server MUST
   include its Sender Sequence Number as Partial IV in the response,
   except when sending the first response to the corresponding request,
   in which case the Partial IV in the response MAY be omitted.

   In order to protect against replay, the client SHALL maintain for
   each ongoing long exchange one Response Number for each different
   server.  The Response Number is a non-negative integer containing the
   largest Partial IV of the responses received from that server during
   the long exchange, while using the same Security Context.

   Then, separately for each server, the client uses the associated
   Response Number to perform ordering and replay protection of the
   responses from that server during the long exchange, by comparing
   their Partial IVs with one another and against the Response Number.

   For every long exchange, the Response Number associated with a server
   is initialized to the Partial IV of the response from that server
   such that, within the long exchange, it is the first response from
   that server to include a Partial IV and to be successfully verified
   with the used Security Context.  Note that, when a new Security
   Context is established in the group, the client sets the Response
   Number of each associated server as not initialized (see
   Section 2.6.3.2), hence later responses within the same long exchange
   and protected with the new Security Context will result in a new
   initialization of Response Numbers.  Furthermore, for every long
   exchange, a client MUST only accept at most one response without
   Partial IV from each server, and treat it as the oldest response from
   that server within that long exchange.









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   During a long exchange, a client receiving a response containing a
   Partial IV SHALL compare the Partial IV with the Response Number
   associated with the replying server within that long exchange.  The
   client MUST stop processing a response from a server, if that
   response has a Partial IV that has been previously received from that
   server during that long exchange, while using the same Security
   Context.

   Applications MAY decide that a client only processes responses within
   a long exchange if those have a greater Partial IV than the Response
   Number associated with the replying server within that long exchange.
   This limits the storage overhead on the client to maintaining one
   Response Number per replying server within the long exchange.
   Conversely, more permissive applications can allow clients to also
   process responses that have a smaller Partial IV than the Response
   Number associated with the replying server.  For a client, the
   ability to detect previously received Partial IVs while admitting the
   processing of such responses comes at the cost of additional storage
   overhead, for which a reasonable bound has to be defined by the
   application.  A possible way to achieve that relies on using a
   sliding Replay Window uniquely paired with the replying server within
   the long exchange, similarly to that used by a server for detecting
   replayed requests.

   If the verification of the response succeeds, and the received
   Partial IV (when included) was greater than the Response Number
   associated with the replying server, then the client SHALL overwrite
   that Response Number with the received Partial IV.

   As long as a server uses the same Security Context to protect its
   responses to the same request, the client MUST consider the response
   with the highest Partial IV as the freshest response from that server
   among those protected with that Security Context, regardless of the
   order of arrival.  Within a long exchange, implementations need to
   make sure that a response without Partial IV is considered the oldest
   response from the replying server within that long exchange.

   The method defined in this section is not relevant for responses to
   requests that are neither group requests nor Observe requests.  In
   fact, for each of such requests, there is at most one response and
   only from one individually targeted server in the group.










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6.  Message Reception

   Upon receiving a protected message, a recipient endpoint retrieves a
   Security Context as in [RFC8613].  An endpoint MUST be able to
   distinguish between a Security Context to process OSCORE messages as
   in [RFC8613] and a Group OSCORE Security Context to process Group
   OSCORE messages as defined in this document.

   The way to accomplish this distinction is implementation specific.
   For example, an endpoint can take into account the different
   structure of the Security Context defined in Section 2, e.g., based
   on the presence of Signature Algorithm and Pairwise Key Agreement
   Algorithm in the Common Context.  Alternatively, at the cost of
   increasing storage, implementations can use an additional parameter
   in the Security Context, to explicitly mark that it is intended for
   processing Group OSCORE messages.

   If either of the following conditions holds, a recipient endpoint
   MUST discard the incoming protected message:

   *  The Group Flag is set to 0 and the retrieved Security Context is
      associated with an OSCORE group, but the endpoint does not support
      the pairwise mode or any of the following parameters is not set in
      the Security Context: the AEAD Algorithm and the Pairwise Key
      Agreement Algorithm.

   *  The Group Flag is set to 1 and the retrieved Security Context is
      associated with an OSCORE group, but the endpoint does not support
      the group mode or any of the following parameters is not set in
      the Security Context: the Group Encryption Algorithm and the
      Signature Algorithm.

   *  The Group Flag is set to 1 but there is no Security Context
      associated with an OSCORE group.

      Future specifications may define how to process incoming messages
      protected with Security Contexts as in [RFC8613], when the Group
      Flag bit is set to 1.

   Otherwise, if a Security Context associated with an OSCORE group is
   retrieved, the recipient endpoint processes the message with Group
   OSCORE, using the group mode (see Section 7) if the Group Flag is set
   to 1, or the pairwise mode (see Section 8) if the Group Flag is set
   to 0.







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   Note that if the Group Flag is set to 0 and the recipient endpoint
   retrieves a Security Context which is valid to process the message
   but is not associated with an OSCORE group, then the message is
   processed according to [RFC8613].

7.  Message Processing in Group Mode

   When using the group mode, messages are protected and processed as
   specified in [RFC8613] with the modifications described in this
   section.  The security objectives of the group mode are discussed in
   Appendix A.2.

   The possible use of the group mode is indicated by the Group Manager
   as part of the group data provided to new group members when joining
   the group, according to which the parameters Group Encryption
   Algorithm and Signature Algorithm in the Security Context are set
   (see Section 2).

   During all the steps of the message processing, an endpoint MUST use
   the same Security Context for the considered group.  That is, an
   endpoint MUST NOT install a new Security Context for that group (see
   Section 2.6.3.2) until the message processing is completed.

   The group mode SHOULD be used to protect group requests intended for
   multiple recipients or for the whole group.  This applies to both
   requests directly addressed to multiple recipients, e.g., sent by the
   client over multicast, as well as requests sent by the client over
   unicast to a proxy that forwards them to the intended recipients over
   multicast [I-D.ietf-core-groupcomm-bis].  Exceptions where the
   requirement above is not fulfilled and the pairwise mode is used to
   protect group requests include: the efficient discovery of a server's
   address in the group (see Section 8.1); or the enabling of simple
   constructions where a variation of the pairwise mode protects
   requests possibly intended to multiple servers, in such a way that
   the corresponding responses are effectively cacheable by
   intermediaries (e.g., see [I-D.ietf-core-cacheable-oscore]).

   As per [RFC7252][I-D.ietf-core-groupcomm-bis], group requests sent
   over multicast are always Non-confirmable, and thus are not
   retransmitted by the CoAP messaging layer.  Instead, applications
   should store such outgoing messages for a predefined, sufficient
   amount of time, in order to correctly perform potential
   retransmissions at the application layer.  If performed, these
   retransmissions are repetitions of previous protected messages, which
   the sender endpoint does not protect again with Group OSCORE.






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   According to Section 5.2.3 of [RFC7252], "[i]f the request message is
   Non-confirmable, then the response SHOULD be returned in a Non-
   confirmable message as well.  However, an endpoint MUST be prepared
   to receive (...) a Confirmable response in reply to a Non-confirmable
   request."  Confirmable group requests are acknowledged when sent over
   non-multicast transports, as specified in [RFC7252].

   Furthermore, endpoints in the group locally perform error handling
   and processing of invalid messages according to the same principles
   adopted in [RFC8613].  In addition, a recipient MUST stop processing
   and reject any message that is malformed and that does not follow the
   format specified in Section 3 of this document, or that is not
   cryptographically validated in a successful way as per the processing
   defined in Section 7.2 and Section 7.4 of this document.

   In either case, it is RECOMMENDED that a server does not send back
   any error message in reply to a received request if either of the
   following conditions holds:

   *  The server is not able to identify whether the received request is
      a group request, i.e., as sent to all servers in the group.

   *  The server identifies the received request as a group request.

   This prevents servers from replying with multiple error messages to a
   client sending a group request, so avoiding the risk of flooding and
   possibly congesting the network.

7.1.  Protecting the Request

   When using the group mode to protect a request, a client proceeds as
   described in Section 8.1 of [RFC8613] with the following
   modifications.

   *  In Step 2, the Additional Authenticated Data is modified as
      described in Section 3 of this document.

   *  In Step 4, the encryption of the COSE object is modified as
      described in Section 3 of this document.  The encoding of the
      compressed COSE object is modified as described in Section 4 of
      this document.  In particular, the Group Flag MUST be set to 1.
      The Group Encryption Algorithm from the Common Context MUST be
      used.

   *  In Step 5, the countersignature is computed and the format of the
      OSCORE message is modified as described in Section 3 and Section 4
      of this document.  In particular, the payload of the Group OSCORE
      message includes also the encrypted countersignature.



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   In addition, the following applies when sending a request that
   establishes a long exchange.

   *  If the client intends to keep the long exchange active beyond a
      possible change of Sender ID, the client MUST store the value of
      the 'kid' parameter from the request, and retain it until the long
      exchange is terminated.  Even in case the client is individually
      rekeyed and receives a new Sender ID from the Group Manager (see
      Section 2.6.3.1), the client MUST NOT update the stored 'kid'
      parameter value associated with the long exchange and the
      corresponding request.

   *  If the client intends to keep the long exchange active beyond a
      possible change of ID Context following a group rekeying (see
      Section 12.2), then the following applies.

      -  The client MUST store the value of the 'kid context' parameter
         from the request, and retain it until the long exchange is
         terminated.  Upon establishing a new Security Context with a
         new Gid as ID Context (see Section 2.6.3.2), the client MUST
         NOT update the stored 'kid context' parameter value associated
         with the long exchange and the corresponding request.

      -  The client MUST store an invariant identifier of the group,
         which is immutable even if the Security Context of the group is
         re-established.  For example, this invariant identifier can be
         the "group name" in [I-D.ietf-ace-key-groupcomm-oscore], where
         it is used for joining the group and retrieving the current
         group keying material from the Group Manager.

         After a group rekeying, the client might have missed both the
         rekeying messages and the servers' first responses that are
         protected with the new Security Context and include the new ID
         Context (Gid) in the 'kid context' parameter (see Section 7.3).
         In such a case, while still not knowing the new ID Context
         (Gid) used in the group, the client is able to retrieve the
         current group keying material from the Group Manager, using the
         invariant identifier to unambiguously refer to the group.

7.2.  Verifying the Request

   Upon receiving a protected request with the Group Flag set to 1,
   following the procedure in Section 6, a server proceeds as described
   in Section 8.2 of [RFC8613] with the following modifications.

   *  In Step 2, the decoding of the compressed COSE object follows
      Section 4 of this document.  In particular:




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      -  If the server discards the request due to not retrieving a
         Security Context associated with the OSCORE group, the server
         MAY respond with a 4.01 (Unauthorized) error message.  When
         doing so, the server MAY set an Outer Max-Age Option with value
         zero, and MAY include a descriptive string as diagnostic
         payload.

      -  If the received 'kid context' matches an existing ID Context
         (Gid) but the received 'kid' does not match any Recipient ID in
         this Security Context, then the server MAY create a new
         Recipient Context for this Recipient ID and initialize it
         according to Section 3 of [RFC8613], and also retrieve the
         authentication credential associated with the Recipient ID to
         be stored in the new Recipient Context.  Such a configuration
         is application specific.  If the application does not specify
         dynamic derivation of new Recipient Contexts, then the server
         SHALL stop processing the request.

   *  In Step 4, the Additional Authenticated Data is modified as
      described in Section 3 of this document.

   *  In Step 6, the server also verifies the countersignature by using
      the public key from the client's authentication credential stored
      in the associated Recipient Context.  In particular:

      -  If the server does not have the public key of the client yet,
         the server MUST stop processing the request and MAY respond
         with a 5.03 (Service Unavailable) response.  The response MAY
         include a Max-Age Option, indicating to the client the number
         of seconds after which to retry.  If the Max-Age Option is not
         present, Section 5.10.5 of [RFC7252] specifies a default retry
         time of 60 seconds.

      -  The signature verification as defined below SHOULD be performed
         before decrypting the COSE object.  An exception applies to
         implementations that cannot perform the two steps in this
         order.  Those implementations MUST ensure that no access to the
         plaintext is possible before a successful signature
         verification and MUST prevent any possible leak of time-related
         information that can yield side-channel attacks.

      -  The server retrieves the encrypted countersignature
         ENC_SIGNATURE from the message payload, and computes the
         original countersignature SIGNATURE as

         SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM

         where KEYSTREAM is derived as per Section 4.2.



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      -  The server verifies the original countersignature SIGNATURE as
         described in Sections 3.2 and 3.3 of [RFC9338] by using the
         client's public key and according to the Signature Algorithm in
         the Security Context.

         In particular, the Countersign_structure contains the context
         text string "CounterSignature0", the external_aad as defined in
         Section 3.4 of this document, and the ciphertext of the COSE
         object as payload.

      -  If the signature verification fails, the server SHALL stop
         processing the request, SHALL NOT update the Replay Window, and
         MAY respond with a 4.00 (Bad Request) response.  Such a
         response MAY include an Outer Max-Age Option with value zero,
         and its diagnostic payload MAY contain a string, which, if
         present, MUST be "Decryption failed" as if the decryption of
         the COSE object had failed.

      -  When decrypting the COSE object using the Recipient Key, the
         Group Encryption Algorithm from the Common Context MUST be
         used.

   *  Additionally, if the used Recipient Context was created upon
      receiving this request and the message is not verified
      successfully, the server MAY delete that Recipient Context.  When
      this behavior is specified by the application, it mitigates
      attacks that aim at overloading the server's storage.

      If the server deletes the used Recipient Context in this
      particular circumstance, then this deletion does not require the
      server to initialize as invalid the Replay Window of any new
      Recipient Context created later within the Security Context (see
      Section 2.6.1.2).

   A server SHOULD NOT process a request if the received Recipient ID
   ('kid') is equal to its own Sender ID in its own Sender Context.
   However, in some applications the server can prepare a request to be
   sent to itself (e.g., see
   [I-D.ietf-core-observe-multicast-notifications]), in which case such
   requests would be expected.

   In addition, the following applies if the request establishes a long
   exchange and the server intends to reply with multiple responses.








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   *  The server MUST store the value of the 'kid' parameter from the
      request, and retain it until the last response has been sent.  The
      server MUST NOT update the stored value of the 'kid' parameter
      associated with the request, even if the client is individually
      rekeyed and starts using a new Sender ID received from the Group
      Manager (see Section 2.6.3.1).

   *  The server MUST store the value of the 'kid context' parameter
      from the request, and retain it until the last response has been
      sent, i.e., beyond a possible change of ID Context following a
      group rekeying (see Section 12.2).  That is, upon establishing a
      new Security Context with a new Gid as ID Context (see
      Section 2.6.3.2), the server MUST NOT update the stored value of a
      'kid context' parameter associated with the request.

7.3.  Protecting the Response

   When using the group mode to protect a response, a server proceeds as
   described in Section 8.3 of [RFC8613] with the following
   modifications.

   Note that the server always protects a response with the Sender
   Context from its latest Security Context, and that establishing a new
   Security Context resets the Sender Sequence Number to 0 (see
   Section 2.6.3.1 and Section 2.6.3.2).

   *  In Step 2, the Additional Authenticated Data is modified as
      described in Section 3 of this document.

      In addition, the following applies if the server intends to reply
      with multiple responses within the long exchange established by
      the corresponding request.

      -  The server MUST use the stored value of the 'kid' parameter
         from the request (see Section 7.2), as value for the
         'request_kid' parameter in the external_aad (see Section 3.4).

      -  The server MUST use the stored value of the 'kid context'
         parameter from the request (see Section 7.2), as value for the
         'request_kid_context' parameter in the external_aad (see
         Section 3.4).

   *  In Step 3, if either of the following conditions holds, the server
      MUST include its Sender Sequence Number as Partial IV in the
      response and use it to build the nonce to protect the response.
      This prevents the server from reusing the nonce from the request
      together with the same encryption key.




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      -  The response is not the first response that the server sends to
         the request.

      -  The server is using a different Security Context for the
         response than the one that was used to verify the request (see
         Section 12.2).

   *  In Step 4, the encryption of the COSE object is modified as
      described in Section 3 of this document.  The encoding of the
      compressed COSE object is modified as described in Section 4 of
      this document.  In particular, the Group Flag MUST be set to 1.
      The Group Encryption Algorithm from the Common Context MUST be
      used.

      In addition, the following applies.

      -  If the server is using a different ID Context (Gid) for the
         response than the one that was used to verify the request (see
         Section 12.2) and this is the first response from the server to
         that request, then the new ID Context MUST be included in the
         'kid context' parameter of the response.

      -  The server may be replying to a request that was protected with
         an old Security Context.  After completing the establishment of
         a new Security Context, the server MUST protect all the
         responses to that request with the Sender Context of the new
         Security Context.

         For each ongoing long exchange, the server can help the client
         to synchronize, by including also the 'kid context' parameter
         in responses following a group rekeying, with value set to the
         ID Context (Gid) of the new Security Context.

         If there is a known upper limit to the duration of a group
         rekeying, the server SHOULD include the 'kid context' parameter
         during that time.  Otherwise, the server SHOULD include it
         until the Max-Age has expired for the last response sent before
         the installation of the new Security Context.

      -  The server can obtain a new Sender ID from the Group Manager
         when individually rekeyed (see Section 2.6.3.1) or when re-
         joining the group.  In such a case, the server can help the
         client to synchronize by including the 'kid' parameter in a
         response protected in group mode even when the request was
         protected in pairwise mode (see Section 8.3).






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         That is, when responding to a request protected in pairwise
         mode, the server SHOULD include the 'kid' parameter in a
         response protected in group mode, if it is replying to that
         client for the first time since the assignment of its new
         Sender ID.

   *  In Step 5, the countersignature is computed and the format of the
      OSCORE message is modified as described in Section 3 and Section 4
      of this document.  In particular the payload of the Group OSCORE
      message includes also the encrypted countersignature (see
      Section 4.1).

7.4.  Verifying the Response

   Upon receiving a protected response with the Group Flag set to 1,
   following the procedure in Section 6, a client proceeds as described
   in Section 8.4 of [RFC8613] with the modifications described in this
   section.

   Note that a client may receive a response protected with a Security
   Context different from the one used to protect the corresponding
   request, and that, upon the establishment of a new Security Context,
   the client re-initializes its Replay Windows in its Recipient
   Contexts (see Section 12.2).

   *  In Step 2, the decoding of the compressed COSE object is modified
      as described in Section 4 of this document.  In particular, a
      'kid' may not be present if the response is a reply to a request
      protected in pairwise mode.  In such a case, the client assumes
      the response 'kid' to be the Recipient ID for the server for which
      the request protected in pairwise mode was intended.

      If the response 'kid context' matches an existing ID Context (Gid)
      but the received/assumed 'kid' does not match any Recipient ID in
      this Security Context, then the client MAY create a new Recipient
      Context for this Recipient ID and initialize it according to
      Section 3 of [RFC8613], and also retrieve the authentication
      credential associated with the Recipient ID to be stored in the
      new Recipient Context.  If the application does not specify
      dynamic derivation of new Recipient Contexts, then the client
      SHALL stop processing the response.

   *  In Step 3, the Additional Authenticated Data is modified as
      described in Section 3 of this document.

      In addition, the following applies if the client processes a
      response to a request within a long exchange.




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      -  The client MUST use the stored value of the 'kid' parameter
         from the request (see Section 7.1), as value for the
         'request_kid' parameter in the external_aad (see Section 3.4).

      -  The client MUST use the stored value of the 'kid context'
         parameter from the request (see Section 7.1), as value for the
         'request_kid_context' parameter in the external_aad (see
         Section 3.4).

      This ensures that, throughout a long exchange, the client can
      correctly verify the received responses, even if the client is
      individually rekeyed and starts using a new Sender ID received
      from the Group Manager (see Section 2.6.3.1), as well as when it
      installs a new Security Context with a new ID Context (Gid)
      following a group rekeying (see Section 12.2).

   *  In Step 5, the client also verifies the countersignature by using
      the public key from the server's authentication credential stored
      in the associated Recipient Context.  In particular:

      -  The signature verification as defined below SHOULD be performed
         before decrypting the COSE object.  An exception applies to
         implementations that cannot perform the two steps in this
         order.  Those implementations MUST ensure that no access to the
         plaintext is possible before a successful signature
         verification and MUST prevent any possible leak of time-related
         information that can yield side-channel attacks.

      -  The client retrieves the encrypted countersignature
         ENC_SIGNATURE from the message payload, and computes the
         original countersignature SIGNATURE as

         SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM

         where KEYSTREAM is derived as per Section 4.2.

         The client verifies the original countersignature SIGNATURE.

      -  If the verification of the countersignature fails, the client:
         i) SHALL stop processing the response; and ii) SHALL NOT update
         the Response Number associated with the server.

      -  After a successful verification of the countersignature, the
         client performs also the following actions in case the request
         was protected in pairwise mode (see Section 8.3).






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         o  If the 'kid' parameter is present in the response, the
            client checks whether this received 'kid' is equal to the
            expected 'kid', i.e., the known Recipient ID for the server
            for which the request was intended.

         o  If the 'kid' parameter is not present in the response, the
            client checks whether the server that has sent the response
            is the same one for which the request was intended.  This
            can be done by checking that the public key used to verify
            the countersignature of the response is equal to the public
            key included in the authentication credential Recipient Auth
            Cred, which was taken as input to derive the Pairwise Sender
            Key used for protecting the request (see Section 2.5.1).

         In either case, if the client determines that the response has
         come from a different server than the expected one, then the
         client: i) SHALL discard the response and SHALL NOT deliver it
         to the application; ii) SHALL NOT update the Response Number
         associated with the server.

         Otherwise, the client hereafter considers the received 'kid' as
         the current Recipient ID for the server.

   *  In Step 5, when decrypting the COSE object using the Recipient
      Key, the Group Encryption Algorithm from the Common Context MUST
      be used.

      In addition, the client performs the following actions if the
      response is received within a long exchange.

      -  The ordering and the replay protection of responses received
         from the server during the long exchange are performed as per
         Section 5.3.1 of this document, by using the Response Number
         associated with that server within that long exchange.  In case
         of unsuccessful decryption and verification of a response, the
         client SHALL NOT update the Response Number associated with the
         server.

      -  When receiving the first valid response from the server within
         the long exchange, the client MUST store the kid "kid1" of that
         server for that long exchange.  If the 'kid' field is included
         in the OSCORE Option of the response, its value specifies
         "kid1".  If the request was protected in pairwise mode (see
         Section 8.3), the 'kid' field may not be present in the OSCORE
         Option of the response (see Section 3.2).  In this case, the
         client assumes "kid1" to be the Recipient ID for the server for
         which the request was intended.




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      -  When receiving another valid response to the same request from
         the same server - which can be identified and recognized
         through the same public key used to verify the countersignature
         and included in the server's authentication credential - the
         client determines the kid "kid2" of the server as above for
         "kid1", and MUST check whether "kid2" is equal to the stored
         "kid1".

         If "kid1" and "kid2" are different, the client SHOULD NOT
         accept the response as valid to be delivered to the
         application, and SHOULD NOT update the Response Number
         associated with the server.  Exceptions can apply as the client
         can retain the information required to order the responses, or
         if the client application does not require response ordering
         altogether.  Servers MUST NOT rely on clients tolerating this,
         unless it was explicitly agreed on (e.g., as part of the
         group's setup).

      Note that, if "kid2" is different from "kid1" and the 'kid' field
      is omitted from the response - which is possible if the request
      was protected in pairwise mode - then the client will compute a
      wrong keystream to decrypt the countersignature (i.e., by using
      "kid1" rather than "kid2" in the 'id' field of the 'info' array in
      Section 4.2), thus subsequently failing to verify the
      countersignature and discarding the response.

      This ensures that the client remains able to correctly perform the
      ordering and replay protection of responses within a long
      exchange, even if the server legitimately starts using a new
      Sender ID, as received from the Group Manager when individually
      rekeyed (see Section 2.6.3.1) or when re-joining the group.

   *  In Step 8, if the used Recipient Context was created upon
      receiving this response and the message is not verified
      successfully, the client MAY delete that Recipient Context.  When
      this behavior is specified by the application, it mitigates
      attacks that aim at overloading the client's storage.

      If the client deletes the used Recipient Context in this
      particular circumstance, then this deletion does not require the
      client to initialize as invalid the Replay Window of any new
      Recipient Context created later within the Security Context (see
      Section 2.6.1.2).








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7.5.  External Signature Checkers

   When a message is protected in group mode, it is possible for
   designated external signature checkers to verify the countersignature
   of the message.  For example, an external signature checker can be an
   intermediary gateway that intercepts messages protected in group mode
   and ensures that they reach the intended recipients only if it
   successfully verifies their countersignatures.

   Since they do not join a group as members, external signature
   checkers need to retrieve from the Group Manager the authentication
   credentials of group members and other selected group data, such as
   the current Signature Encryption Key (see Section 2.1.9).
   Section 12.3 describes how the Group Manager supports signature
   checkers.

   When receiving a message protected in group mode, a signature checker
   proceeds as follows.

   *  The signature checker retrieves the encrypted countersignature
      ENC_SIGNATURE from the message payload, and computes the original
      countersignature SIGNATURE as

      SIGNATURE = ENC_SIGNATURE XOR KEYSTREAM

      where KEYSTREAM is derived as per Section 4.2.

   *  The signature checker verifies the original countersignature
      SIGNATURE, by using the public key of the sender endpoint as
      included in that endpoint's authentication credential.  The
      signature checker determines the right authentication credential
      based on the ID Context (Gid) and the Sender ID of the sender
      endpoint.

   Note that the following applies when attempting to verify the
   countersignature of a response message.

   *  The response may not include a Partial IV and/or an ID Context.
      In such a case, the signature checker considers the same values
      from the corresponding request, i.e., the request matching with
      the response by CoAP Token value.










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   *  The response may not include a Sender ID.  This can happen when
      the response protected in group mode matches a request protected
      in pairwise mode (see Section 8.1), with a case in point provided
      by [I-D.ietf-core-cacheable-oscore].  In such a case, the
      signature checker needs to use other means (e.g., source
      addressing information of the server endpoint) to identify the
      correct authentication credential including the public key to use
      for verifying the countersignature of the response.

   The particular actions following a successful or unsuccessful
   verification of the countersignature are application specific and out
   of the scope of this document.

8.  Message Processing in Pairwise Mode

   When using the pairwise mode of Group OSCORE, messages are protected
   and processed as in [RFC8613] with the modifications described in
   this section.  The security objectives of the pairwise mode are
   discussed in Appendix A.2.

   The possible use of the pairwise mode is indicated by the Group
   Manager as part of the group data provided to new group members when
   joining the group, according to which the parameters AEAD Algorithm
   and Pairwise Key Agreement Algorithm in the Security Context are set
   (see Section 2).

   The pairwise mode takes advantage of an existing Security Context to
   establish keying material shared exclusively with each other member.
   For encryption and decryption operations in pairwise mode, the AEAD
   Algorithm from the Common Context is used (see Section 2.1.1).

   In order to use the pairwise mode in a group where the group mode is
   also used (i.e., Group Encryption Algorithm and Signature Algorithm
   in the Security Context are set), the public/private key pairs used
   for signature operations of the group mode MUST be possible to also
   use for key agreement operations.  For example, this can rely on
   signing operations using ECDSA, and encryption operations using AES-
   CCM with keying material derived through ECDH.

   The pairwise mode does not support external verifiers of source
   authentication and message integrity like the group mode does, e.g.,
   for external signature checkers (see Section 7.5).

   An endpoint implementing only a silent server does not support the
   pairwise mode.






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   Endpoints using the CoAP Echo Option [RFC9175] in a group where the
   AEAD Algorithm and Pairwise Key Agreement Algorithm are set MUST
   support the pairwise mode.  When using the challenge-response method
   defined in Section 9, this prevents the attack described in
   Section 14.9, which leverages requests sent over unicast to a single
   group member and protected in group mode.

   The pairwise mode cannot be used to protect messages intended for
   multiple recipients, as the keying material used for the pairwise
   mode is shared only between two endpoints.

   However, a sender can use the pairwise mode to protect a message sent
   to (but not intended for) multiple recipients, if interested in a
   response from only one of them.  For instance, this is useful to
   support the address discovery service defined in Section 8.1, when a
   single 'kid' value is indicated in the payload of a request sent to
   multiple recipients, e.g., over multicast.

8.1.  Pre-Conditions

   In order to protect an outgoing message in pairwise mode, the sender
   needs to know the authentication credential and the Recipient ID for
   the recipient endpoint, as stored in the Recipient Context associated
   with that endpoint (see Section 2.5.4).

   Typically, the sender endpoint sends the message protected in
   pairwise mode over unicast, so that the message is delivered only to
   the intended recipient endpoint for which it is protected.  This
   requires the sender to know the individual address of that recipient
   endpoint, which the sender may not know at any given point in time.
   For instance, right after having joined the group, a client may know
   the authentication credential and Recipient ID for a given server,
   but not the addressing information required to reach it with an
   individual, one-to-one request.

   In order to make addressing information of individual endpoints
   available, servers in the group MAY expose a resource to which a
   client can send a request targeting a set of servers, identified by
   their 'kid' values specified in the request payload, or implicitly if
   the request is sent in pairwise mode.  Further details of such an
   interface are out of scope for this document.

8.2.  Main Differences from OSCORE

   The pairwise mode protects messages between two members of a group,
   essentially following [RFC8613] but with the following notable
   differences.




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   *  The 'kid' and 'kid context' parameters of the COSE object are used
      as defined in Section 3.2 of this document.

   *  The external_aad defined in Section 3.4 of this document is used
      for the encryption and decryption process.

   *  The Pairwise Sender/Recipient Keys used as Sender/Recipient keys
      are derived as defined in Section 2.5 of this document.

8.3.  Protecting the Request

   When using the pairwise mode to protect a request, a client SHALL
   proceed as described in Section 8.1 of [RFC8613] with the differences
   summarized in Section 8.2 of this document.

   Furthermore, when sending a request that establishes a long exchange,
   what is specified in Section 7.1 of this document holds, with respect
   to storing the value of the 'kid' and 'kid context' parameters, and
   to storing an invariant identifier of the group.

8.4.  Verifying the Request

   Upon receiving a protected request with the Group Flag set to 0,
   following the procedure in Section 6, a server SHALL proceed as
   described in Section 8.2 of [RFC8613] with the differences summarized
   in Section 8.2 of this document.  The following differences also
   apply.

   *  If the server discards the request due to not retrieving a
      Security Context associated with the OSCORE group or to not
      supporting the pairwise mode, the server MAY respond with a 4.01
      (Unauthorized) error message or a 4.02 (Bad Option) error message,
      respectively.  When doing so, the server MAY set an Outer Max-Age
      Option with value zero, and MAY include a descriptive string as
      diagnostic payload.

   *  If a new Recipient Context is created for this Recipient ID, new
      Pairwise Sender/Recipient Keys are also derived (see
      Section 2.5.1).  The new Pairwise Sender/Recipient Keys are
      deleted if the Recipient Context is deleted as a result of the
      message not being successfully verified.

   *  What is specified in Section 7.2 of this document holds with
      respect to the following points.

      -  The possible, dynamic creation and configuration of a Recipient
         Context upon receiving the request.




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      -  The possible deletion of a Recipient Context created upon
         receiving the request, in case the request is not verified
         successfully.

      -  The rule about processing the request where the received
         Recipient ID ('kid') is equal to the server's Sender ID.

      -  The storing of the value of the 'kid' and 'kid context'
         parameters from the request, if the server intends to reply
         with multiple responses within the long exchange established by
         the request.

8.5.  Protecting the Response

   When using the pairwise mode to protect a response, a server SHALL
   proceed as described in Section 8.3 of [RFC8613] with the differences
   summarized in Section 8.2 of this document.  The following
   differences also apply.

   *  What is specified in Section 7.3 of this document holds with
      respect to the following points.

      -  The protection of a response when using a different Security
         Context than the one used to verify the corresponding request
         (see Section 12.2).  That is, the server always protects a
         response with the Sender Context from its latest Security
         Context, and establishing a new Security Context resets the
         Sender Sequence Number to 0 (see Section 2.6.3.1 and
         Section 2.6.3.2).

      -  The use of the stored value of the 'kid' and 'kid context'
         parameters, if the server intends to reply with multiple
         responses within the long exchange established by the request.

      -  The rules for the inclusion of the server's Sender Sequence
         Number as Partial IV in a response, as used to build the nonce
         to protect the response.

      -  The rules for the inclusion of the ID Context (Gid) in the 'kid
         context' parameter of a response, if the ID Context used for
         the response differs from the one used to verify the request
         (see Section 12.2), also for helping the client to synchronize.

      -  The rules for the inclusion of the Sender ID in the 'kid'
         parameter of a response to a request that was protected in
         pairwise mode, if the server has obtained a new Sender ID from
         the Group Manager when individually rekeyed (see
         Section 2.6.3.1), thus helping the client to synchronize.



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8.6.  Verifying the Response

   Upon receiving a protected response with the Group Flag set to 0,
   following the procedure in Section 6, a client SHALL proceed as
   described in Section 8.4 of [RFC8613] with the differences summarized
   in Section 8.2 of this document.  The following differences also
   apply.

   *  The client may receive a response protected with a Security
      Context different from the one used to protect the corresponding
      request.  Also, upon the establishment of a new Security Context,
      the client re-initializes its Replay Windows in its Recipient
      Contexts (see Section 2.2).

   *  The same as described in Section 7.4 holds with respect to
      handling the 'kid' parameter of the response, when received as a
      reply to a request protected in pairwise mode.  The client can
      also in this case check whether the replying server is the
      expected one, by relying on the server's public key.  However,
      since the response is protected in pairwise mode, the public key
      is not used for verifying a countersignature as in Section 7.4.
      Instead, the expected server's authentication credential - namely
      Recipient Auth Cred and including the server's public key - was
      taken as input to derive the Pairwise Recipient Key used to
      decrypt and verify the response (see Section 2.5.1).

   *  If a new Recipient Context is created for this Recipient ID, new
      Pairwise Sender/Recipient Keys are also derived (see
      Section 2.5.1).  The new Pairwise Sender/Recipient Keys are
      deleted if the Recipient Context is deleted as a result of the
      message not being successfully verified.

   *  What is specified in Section 7.4 of this document holds with
      respect to the following points.

      -  The possible, dynamic creation and configuration of a Recipient
         Context upon receiving the response.

      -  The use of the stored value of the 'kid' and 'kid context'
         parameters, when processing a response received within a long
         exchange.

      -  The performing of ordering and replay protection for responses
         received within a long exchange.

      -  The possible deletion of a Recipient Context created upon
         receiving the response, in case the response is not verified
         successfully.



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9.  Challenge-Response Based Freshness and Replay Window Recovery

   This section describes how a server endpoint can verify freshness of
   a request by means of a challenge-response exchange with a client
   using the Echo Option for CoAP specified in Section 2 of [RFC9175].
   The same mechanism, with small alterations, is also used by the
   server when first processing a request using a Recipient Context
   whose Replay Window was initialized as invalid.

   If the application requires freshness, e.g., according to time- or
   event-based policies (see Section 2.5.1 of [RFC9175]), a server
   proceeds as described below upon receiving a request from a
   particular client for the first time.

   The server processes the message as described in this document, but,
   even if valid, does not deliver it to the application.  Instead, the
   server replies to the client with a Group OSCORE protected 4.01
   (Unauthorized) response message, including only the Echo Option and
   no diagnostic payload.  The server MUST use its Partial IV when
   generating the nonce for protecting the response conveying the Echo
   Option, and MUST include the Partial IV in the response.

   The Echo Option value SHOULD NOT be reused; if it is reused, it MUST
   be highly unlikely to have been recently used with this client.
   Since this response is protected with the Security Context used in
   the group, the client will consider the response valid upon
   successfully decrypting and verifying it.

   The server stores the Echo Option value included in the response
   together with the pair (gid,kid), where 'gid' is the Group Identifier
   of the OSCORE group and 'kid' is the Sender ID of the client in the
   group.  These are specified in the 'kid context' and 'kid' fields of
   the OSCORE Option of the request, respectively.  After a group
   rekeying has been completed and a new Security Context has been
   established in the group, which results also in a new Group
   Identifier (see Section 12.2), the server MUST delete all the stored
   Echo values associated with members of the group.

   After receiving a 4.01 (Unauthorized) response that includes an Echo
   Option and originates from a verified group member, a subsequent
   client request sent to that server and echoing the Echo Option value
   MUST be a message sent unicast to that server.

   If in the group the AEAD Algorithm and Pairwise Key Agreement
   Algorithm are set in the Security Context, the client MUST use the
   pairwise mode to protect the request, as per Section 8.3.  Note that,
   as defined in Section 8, endpoints that are members of such a group
   and that use the Echo Option support the pairwise mode.  In a group



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   where the AEAD Algorithm and Pairwise Key Agreement Algorithm are not
   set, only the group mode can be used.  Hence, requests including the
   Echo Option can be protected only with the Group Mode, with the
   caveat due to the risk for those requests to be redirected to a
   different server than the intended one, as discussed in Section 14.9.

   The client does not necessarily resend the same request, but can
   instead send a more recent one if the application permits it.  This
   allows the client to not retain previously sent requests for full
   retransmission, unless the application explicitly requires otherwise.
   In either case, the client uses a fresh Sender Sequence Number value
   from its own Sender Context.  If the client stores requests for
   possible retransmission with the Echo Option, it should not store a
   given request for longer than a preconfigured time interval.  Note
   that the unicast request echoing the Echo Option is correctly treated
   and processed, since the 'kid context' field including the Group
   Identifier of the OSCORE group is still present in the OSCORE Option
   as part of the COSE object (see Section 3).

   Upon receiving the unicast request including the Echo Option, the
   server performs the following verifications.

   *  If the server does not store an Echo Option value for the pair
      (gid,kid), it considers: i) the time t1 when it has established
      the Security Context used to protect the received request; and ii)
      the time t2 when the request has been received.  Since a valid
      request cannot be older than the Security Context used to protect
      it, the server verifies that (t2 - t1) is less than the largest
      amount of time acceptable to consider the request fresh.

   *  If the server stores an Echo Option value for the pair (gid,kid)
      associated with that same client in the same group, the server
      verifies that the option value equals that same stored value
      previously sent to that client.

   If the verifications above fail, the server MUST NOT process the
   request further and MAY send a 4.01 (Unauthorized) response including
   an Echo Option, hence performing a new challenge-response exchange.

   If the verifications above are successful, the server considers the
   Recipient Context associated with the sender client and proceeds as
   follows.

   *  If the Replay Window is invalid, the steps below occur.







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      1.  The server updates the Replay Window by marking as received
          the Sender Sequence Number from the latest received request.
          This becomes the lower limit of the Replay Window, while all
          the greater Sender Sequence Number values within the Replay
          Window are marked as not received.

      2.  The server makes the Replay Window valid, and accepts the
          request as fresh.

   *  If the Replay Window is already valid, the server discards the
      verification result and accepts the request as fresh or treats it
      as a replay, according to the existing Replay Window.

   A server should not deliver requests from a given client to the
   application until one valid request from that same client has been
   verified as fresh via an echoed Echo Option included therein.  A
   server may perform the challenge-response described above at any
   time, e.g., after a device reboot occurred in an unprepared way.  A
   client has to be ready to perform the challenge-response based on the
   Echo Option if a server starts it.

   Message freshness is further discussed in Section 14.14.

10.  Implementation Compliance

   Like in [RFC8613], HKDF SHA-256 is the mandatory-to-implement HKDF.

   An endpoint may support only the group mode, or only the pairwise
   mode, or both.

   For endpoints that support the group mode, the following applies.

   *  For endpoints that use authenticated encryption, the AEAD
      algorithm AES-CCM-16-64-128 defined in Section 4.2 of [RFC9053] is
      mandatory to implement as Group Encryption Algorithm (see
      Section 2.1.7).

   *  For endpoints that use non-authenticated encryption, the algorithm
      A128CTR defined in Section 4 of [RFC9459] is mandatory to
      implement as Group Encryption Algorithm (see Section 2.1.7).

   *  Section 6 of [RFC9459] mandates that COSE libraries supporting the
      AES-CTR algorithm and accepting Additional Authenticated Data
      (AAD) as input must return an error if AAD is provided when such a
      non-AEAD content encryption algorithm is selected.

      In case the used Group Encryption Algorithm (see Section 2.1.7)
      does not provide integrity protection, the following applies.



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      When invoking the execution of the Group Encryption Algorithm, the
      Group OSCORE implementation MUST NOT provide any AAD to the COSE
      library, unless AAD is always expected as input.  In the latter
      case, the AAD will not be protected by the Group Encryption
      Algorithm, which is unable to do so.

      If the used COSE library adheres to the mandate in Section 6 of
      [RFC9459], then a Group OSCORE implementation requires that the
      COSE library supports using the Group Encryption Algorithm without
      taking AAD as input.

   *  For many constrained IoT devices, it is problematic to support
      more than one signature algorithm.  The following applies with
      respect to the Signature Algorithm (see Section 2.1.8).

      Less constrained endpoints MUST implement at least one of the
      following and SHOULD implement both: the EdDSA signature algorithm
      together with the elliptic curve Ed25519 [RFC8032]; the ECDSA
      signature algorithm together with the elliptic curve P-256.

      Constrained endpoints MUST implement at least one of the following
      and, if affordable, SHOULD implement both: the EdDSA signature
      algorithm together with the elliptic curve Ed25519 [RFC8032]; the
      ECDSA signature algorithm together with the elliptic curve P-256.

   *  Endpoints that implement the ECDSA signature algorithm MAY use
      "deterministic ECDSA" as specified in [RFC6979].  Pure
      deterministic elliptic-curve signature algorithms such as
      deterministic ECDSA and EdDSA have the advantage of not requiring
      access to a source of high-quality randomness.  However, these
      signature algorithms have been shown vulnerable to some side-
      channel and fault injection attacks due to their determinism,
      which can result in extracting a device's private key.  As
      suggested in Section 2.1.1 of [RFC9053], this can be addressed by
      combining both randomness and determinism
      [I-D.irtf-cfrg-det-sigs-with-noise].

   For endpoints that support the pairwise mode, the following applies.

   *  The AEAD algorithm AES-CCM-16-64-128 defined in Section 4.2 of
      [RFC9053] is mandatory to implement as AEAD Algorithm (see
      Section 2.1.1).

   *  The ECDH-SS + HKDF-256 algorithm specified in Section 6.3.1 of
      [RFC9053] is mandatory to implement as Pairwise Key Agreement
      Algorithm (see Section 2.1.10).

   *  The following applies with respect to ECDH curves.



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      Less constrained endpoints MUST implement at least one of the
      following ECDH curves and SHOULD implement both: the X25519 curve
      [RFC7748]; the P-256 curve.

      Constrained endpoints MUST implement at least one of the following
      ECDH curves and, if affordable, SHOULD implement both: the X25519
      curve [RFC7748]; the P-256 curve.

   Constrained IoT devices may alternatively represent Montgomery curves
   and (twisted) Edwards curves [RFC7748] in the short-Weierstrass form
   Wei25519, with which the algorithms ECDSA25519 and ECDH25519 can be
   used for signature operations and Diffie-Hellman secret calculation,
   respectively [I-D.ietf-lwig-curve-representations].

11.  Web Linking

   The use of Group OSCORE or OSCORE [RFC8613] MAY be indicated by a
   target "gosc" attribute in a web link [RFC8288] to a resource, e.g.,
   using a link-format document [RFC6690] if the resource is accessible
   over CoAP.

   The "gosc" attribute is a hint indicating that the destination of
   that link is only accessible using Group OSCORE or OSCORE, and
   unprotected access to it is not supported.  Note that this is simply
   a hint, it does not include any security context material or any
   other information required to run Group OSCORE or OSCORE.

   A value MUST NOT be given for the "gosc" attribute; any present value
   MUST be ignored by parsers.  The "gosc" attribute MUST NOT appear
   more than once in a given link-value; occurrences after the first
   MUST be ignored by parsers.

   When a link-value includes the "gosc" attribute, the link-value MUST
   also include the "osc" attribute defined in Section 9 of [RFC8613].
   If the endpoint parsing the link-value supports Group OSCORE and
   understands the "gosc" attribute, then the parser MUST ignore the
   "osc" attribute, which is overshadowed by the "gosc" attribute.

   The example in Figure 3 shows a use of the "gosc" attribute: the
   client does resource discovery on a server and gets back a list of
   resources, one of which includes the "gosc" attribute indicating that
   the resource is protected with Group OSCORE or OSCORE.  The link-
   format notation (see Section 5 of [RFC6690]) is used.








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                     REQ: GET /.well-known/core

                     RES: 2.05 Content
                         </sensors/temp>;gosc;osc,
                         </sensors/light>;if="sensor"

       Figure 3: Example of using the "gosc" attribute in a web link.

12.  The Group Manager

   As with OSCORE, endpoints communicating with Group OSCORE need to
   establish the relevant Security Context.  Group OSCORE endpoints need
   to acquire OSCORE input parameters, information about the group(s)
   and about other endpoints in the group(s).

   Every group is associated with a Group Manager that is responsible
   for distributing security parameters and keying material within the
   group, among other tasks.  The details of how the Group Manager
   interacts with (candidate) group members or with external entities
   like signature checkers, as well as the protocols used for those
   interactions, are out of scope.

   The Group Manager assigns unique Group Identifiers (Gids) to the
   groups under its control.  Within each of such groups, the Group
   Manager assigns unique Sender IDs (and thus Recipient IDs) to the
   respective group members.  The maximum length of Sender IDs depends
   on the length of the nonce for the algorithms used in the group (see
   Section 2.2).

   The Gid value assigned to a group is associated with a dedicated
   space for the values of Sender ID and Recipient ID of the members of
   that group.  When an endpoint (re-)joins a group, it is provided with
   the current Gid to use in the group.  The Group Manager also assigns
   an integer Key Generation Number counter to each of its groups,
   identifying the current version of the keying material used in that
   group.  Further details about identifiers and keys are provided in
   Section 12.2.

   The Group Manager maintains records of the authentication credentials
   of endpoints in a group, and provides information about the group and
   its members to other group members (see Section 12.1).  Optionally,
   the Group Manager provides information about the group and its
   members to external entities with a specific role, such as signature
   checkers (see Section 12.3).

   The list of responsibilities of the Group Manager is compiled in
   Appendix D.




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   One realization of a Group Manager is specified in
   [I-D.ietf-ace-key-groupcomm-oscore], where the process by which an
   endpoint (re-)joins a group is based on the ACE framework for
   authentication and authorization in constrained environments
   [RFC9200].

12.1.  Set-up of New Endpoints

   From the Group Manager, an endpoint acquires group data such as the
   Gid and OSCORE input parameters including its own Sender ID, with
   which it can derive the Sender Context.

   When joining the group or later on as a group member, an endpoint can
   also retrieve from the Group Manager the authentication credential of
   the Group Manager as well as the authentication credential and other
   information associated with other members of the group, with which it
   can derive the corresponding Recipient Context.  An application can
   configure a group member to asynchronously retrieve information about
   Recipient Contexts, e.g., by Observing [RFC7641] a resource at the
   Group Manager to get updates on the group membership.

   Upon endpoints' joining, the Group Manager collects their
   authentication credentials and MUST verify proof of possession of the
   respective private key.  As an example, such proof of possession is
   possible to achieve during the join process provided by the
   realization of Group Manager specified in
   [I-D.ietf-ace-key-groupcomm-oscore].  Together with the requested
   authentication credentials of other group members, the Group Manager
   MUST provide the joining endpoints with the Sender ID of the
   associated group members and the current Key Generation Number in the
   group (see Section 12.2).

   An endpoint may join a group, for example, by explicitly interacting
   with the responsible Group Manager, or by being configured with some
   tool performing the tasks of the Group Manager.  When becoming
   members of a group, endpoints are not required to know how many and
   what endpoints are in the same group.

   Communications that the Group Manager has with joining endpoints and
   group members MUST be secured.  Specific details on how to secure
   such communications are out of the scope of this document.

   The Group Manager MUST verify that the joining endpoint is authorized
   to join the group.  To this end, the Group Manager can directly
   authorize the joining endpoint, or expect it to provide authorization
   evidence previously obtained from a trusted entity.  Further details
   about the authorization of joining endpoints are out of the scope of
   this document.



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   In case of successful authorization check, the Group Manager provides
   the joining endpoint with the keying material to initialize the
   Security Context.  The actual provisioning of keying material and
   parameters to the joining endpoint is out of the scope of this
   document.

12.2.  Management of Group Keying Material

   In order to establish a new Security Context for a group, the Group
   Manager MUST generate and assign to the group a new Group Identifier
   (Gid) and a new value for the Master Secret parameter.  When doing
   so, a new value for the Master Salt parameter MAY also be generated
   and assigned to the group.  When establishing the new Security
   Context, the Group Manager SHOULD preserve the current value of the
   Sender ID of each group member in order to ensure an efficient key
   rollover.  Exceptions can apply if there are compelling reasons for
   making available again some of the Sender ID values currently used.

   The specific group key management scheme used to distribute new
   keying material is out of the scope of this document.  A simple group
   key management scheme is defined in
   [I-D.ietf-ace-key-groupcomm-oscore].  Different group key management
   schemes rely on different approaches to compose and deliver rekeying
   messages, i.e., individually targeting single recipients, or
   targeting multiple recipients at once (e.g., over UDP/IP multicast),
   or a combination of the two approaches.  As long as it is viable for
   the specific rekeying message to be delivered and it is supported by
   the intended message recipient(s), using a reliable transport to
   deliver a rekeying message should be preferred, as it reduces chances
   of group members missing a rekeying instance.

   Irrespective of the transport used being reliable or unreliable,
   appropriate congestion control MUST be enforced.  If the key
   distribution traffic uses CoAP over UDP or over other unreliable
   transports, mechanisms for enforcing congestion control are specified
   in Section 4.7 of [RFC7252] and in Section 3.6 of
   [I-D.ietf-core-groupcomm-bis] for the case of group communication
   (e.g., over UDP/IP multicast).  If, irrespective of using CoAP, the
   key distribution traffic relies on alternative setups with unreliable
   transports, one can rely on general congestion-control mechanisms
   such as DCCP [RFC4340], or on dedicated congestion control mechanisms
   for the transport specifically used (e.g., those defined in [RFC9002]
   for QUIC [RFC9000]).

   The set of group members should not be assumed as fixed, i.e., the
   group membership is subject to changes, possibly on a frequent basis.





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   The Group Manager MUST rekey the group without undue delay when one
   or more endpoints leave the group.  An endpoint may leave the group
   at own initiative, or may be evicted from the group by the Group
   Manager, e.g., in case the endpoint is compromised, or is suspected
   to be compromised (as determined by the Group Manager through its own
   means or based on information that it obtains from a trusted source
   such as an Intrusion Detection System or an issuer of authentication
   credentials).  In either case, rekeying the group excludes such
   endpoints from future communications in the group, and thus preserves
   forward security.  If a network node is compromised or suspected to
   be compromised, the Group Manager MUST evict from the group all the
   endpoints hosted by that node that are members of the group and rekey
   the group accordingly.

   If required by the application, the Group Manager MUST also rekey the
   group when one or more new joining endpoints are added to the group,
   thus preserving backward security.

   The Group Manager MAY also rekey the group for other reasons, e.g.,
   according to an application-specific rekeying period or scheduling.

   Separately for each group, the value of the Key Generation Number
   increases by one each time the Group Manager distributes new keying
   material to that group (see below).

   The establishment of the new Security Context for the group takes the
   following steps.

   1.  The Group Manager MUST increment the Key Generation Number for
       the group by 1.  It is up to the Group Manager what actions to
       take when a wrap-around of the Key Generation Number is detected.

   2.  The Group Manager MUST build a set of stale Sender IDs including:

       *  The Sender IDs that, during the current Gid, were both
          assigned to an endpoint and subsequently relinquished (see
          Section 2.6.3.1).

       *  The current Sender IDs of the group members that the upcoming
          group rekeying aims to exclude from future group
          communications, if any.

   3.  The Group Manager rekeys the group, by distributing:

       *  The new keying material, i.e., the new Master Secret, the new
          Gid and (optionally) the new Master Salt.

       *  The new Key Generation Number from Step 1.



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       *  The set of stale Sender IDs from Step 2.

       Further information may be distributed, depending on the specific
       group key management scheme used in the group.

   When receiving the new group keying material, a group member
   considers the received stale Sender IDs and performs the following
   actions.

   *  The group member MUST remove every authentication credential
      associated with a stale Sender ID from its list of group members'
      authentication credentials used in the group.

   *  The group member MUST delete each of its Recipient Contexts used
      in the group whose corresponding Recipient ID is a stale Sender
      ID.

   After that, the group member installs the new keying material and
   derives the corresponding new Security Context.

   A group member might miss one or more consecutive instances of group
   rekeying.  As a result, the group member will retain old group keying
   material with Key Generation Number GEN_OLD.  Eventually, the group
   member can notice the discrepancy, e.g., by repeatedly failing to
   verify incoming messages, or by explicitly querying the Group Manager
   for the current Key Generation Number.  Once the group member gains
   knowledge of having missed a group rekeying, it MUST delete the old
   keying material it stores.

   Then, the group member proceeds according to the following steps.

   1.  The group member retrieves from the Group Manager the current
       group keying material, together with the current Key Generation
       Number GEN_NEW.  The group member MUST NOT install the obtained
       group keying material yet.

   2.  The group member asks the Group Manager for the set of stale
       Sender IDs between GEN_OLD and GEN_NEW.

   3.  If no exact and complete indication can be obtained from the
       Group Manager, the group member MUST remove all the
       authentication credentials from its list of group members'
       authentication credentials used in the group and MUST delete all
       its Recipient Contexts used in the group.







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       Otherwise, the group member MUST remove every authentication
       credential associated with a stale Sender ID from its list of
       group members' authentication credentials used in the group, and
       MUST delete each of its Recipient Contexts used in the group
       whose corresponding Recipient ID is a stale Sender ID.

   4.  The group member installs the current group keying material, and
       derives the corresponding new Security Context.

   Alternatively, the group member can re-join the group.  In such a
   case, the group member MUST take one of the following two actions.

   *  First, the group member performs Steps 2 and 3 above.  Then, the
      group member re-joins the group.

   *  The group member re-joins the group with the same roles it
      currently has in the group, and, during the re-join process, it
      asks the Group Manager for the authentication credentials of all
      the current group members.

      Then, given Z the set of authentication credentials received from
      the Group Manager, the group member removes every authentication
      credential which is not in Z from its list of group members'
      authentication credentials used in the group, and deletes each of
      its Recipient Contexts used in the group that does not include any
      of the authentication credentials in Z.

   By removing authentication credentials and deleting Recipient
   Contexts associated with stale Sender IDs, it is ensured that a
   recipient endpoint storing the latest group keying material does not
   store the authentication credentials of sender endpoints that are not
   current group members.  This in turn allows group members to rely on
   stored authentication credentials to confidently verify the group
   membership of sender endpoints, when receiving incoming messages
   protected in group mode (see Section 7).

   Strictness in managing the authentication credentials and Recipient
   Contexts associated with other group members is required for two
   reasons.  First, as further discussed in Section 14.1, it ensures
   that the group mode can be used securely, even in a group where the
   Group Encryption Algorithm does not provide integrity protection (see
   Section 2.1.7) and external signature checkers are used (see
   Section 7.5).  Second, it ensures that the wrong (old) authentication
   credential associated with a group member A is never used with a
   Sender ID that used to be associated with A and has been later issued
   to a different group member B (see Section 12.2.1.2), thus preventing
   the need to recover from an identity mix-up.




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12.2.1.  Recycling of Identifiers

   This section specifies how the Group Manager handles and possibly
   reassigns Gid values and Sender ID values in a group.

12.2.1.1.  Recycling of Group Identifiers

   Since the Gid value changes every time a group is rekeyed, it can
   happen that, after several rekeying instances, the whole space of Gid
   values has been used for the group in question.  When this happens,
   the Group Manager has no available Gid values to use that have never
   been assigned to the group during the group's lifetime.

   The occurrence of such an event and how long it would take to occur
   depend on the format and encoding of Gid values used in the group
   (see, e.g., Appendix C), as well as on the frequency of rekeying
   instances yielding a change of Gid value.  Independently for each
   group under its control, the Group Manager can take one of the two
   following approaches.

   *  The Group Manager does not reassign Gid values.  That is, once the
      whole space of Gid values has been used for a group, the Group
      Manager terminates the group and may re-establish a new group.

   *  While the Gid value changes every time a group is rekeyed, the
      Group Manager can reassign Gid values previously used during a
      group's lifetime.  By doing so, the group can continue to exist
      even once the whole space of Gid values has been used.

      The Group Manager MAY support and use this approach, according to
      what is specified in Section 12.2.1.1.1.

12.2.1.1.1.  Reassignment of Group Identifiers

   If the Group Manager performs the reassignment of Gid values
   previously used during a group's lifetime, the Group Manager MUST
   take additional actions when handling Gid values and rekeying the
   group, as specified below.

   When a node (re-)joins the group and it is provided with the current
   Gid to use in the group, the Group Manager considers such a Gid as
   the Birth Gid of that endpoint for that group.  For each group
   member, the Group Manager MUST store the latest corresponding Birth
   Gid until that member leaves the group.  In case the endpoint has in
   fact re-joined the group, the newly determined Birth Gid overwrites
   the one currently stored.





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   When establishing a new Security Context for the group, the Group
   Manager takes the additional following step between Steps 1 and 2 of
   Section 12.2.

   A.  The Group Manager MUST check if the new Gid to be distributed is
   equal to the Birth Gid of any of the current group members.  If any
   of such "elder members" is found in the group, then:

   *  The Group Manager MUST evict the elder members from the group.
      That is, the Group Manager MUST terminate their membership and, in
      the following steps, it MUST rekey the group in such a way that
      the new keying material is not provided to those evicted elder
      members.

      This ensures that any response from the same server to the request
      of a long exchange can never successfully match against the
      request of two different long exchanges.

      The excluded elder members could eventually re-join the group,
      thus terminating any of their ongoing long exchanges (see
      Section 5.1).

      Therefore, it is ensured by construction that no client can have
      with the same server two ongoing long exchanges, such that the two
      respective requests were protected using the same Partial IV, Gid,
      and Sender ID.

12.2.1.2.  Recycling of Sender IDs

   From the moment T_start when a Gid is assigned to a group until the
   moment when a new Gid is assigned to that same group, the following
   restrictions apply within the group.

   *  The Group Manager MUST NOT assign a Sender ID that was already the
      Sender ID of an endpoint in the group at T_start.

   *  The Group Manager MUST NOT assign a given Sender ID more than
      once.

   That is, under the ongoing use of the current Gid, a given Sender ID
   is not reassigned to the same or a different endpoint.  This prevents
   from reusing a Sender ID ('kid') with the same triple (Gid, Master
   Secret, Master Salt).  Within these restrictions, the Group Manager
   can assign a Sender ID used under an old Gid value (including under a
   same, recycled Gid value), thus avoiding Sender ID values to
   irrecoverably grow in size.





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   Even when an endpoint joining a group is recognized as a current
   member of that group, e.g., through the ongoing secure communication
   association, the Group Manager MUST assign a new Sender ID different
   than the one currently used by the endpoint in the group, unless the
   group is rekeyed first and a new Gid value is established.

12.2.1.3.  Relation between Identifiers and Keying Material

   Figure 4 overviews the different identifiers and keying material
   components, considering their relation and possible reuse across
   group rekeying.

 Components changed in lockstep
     upon a group rekeying
 +----------------------------+            * Changing a kid does not
 |                            |              need changing the Group ID
 | Master               Group |<--> kid1
 | Secret <---> o <--->  ID   |            * A kid is not reassigned
 |              ^             |<--> kid2     under the ongoing usage of
 |              |             |              the current Group ID
 |              |             |<--> kid3
 |              v             |            * Upon changing the Group ID,
 |         Master Salt        | ... ...      every current kid should
 |         (optional)         |              be preserved for efficient
 |                            |              key rollover
 | The Key Generation Number  |
 | is incremented by 1        |            * After changing Group ID, an
 |                            |              unused kid can be assigned,
 +----------------------------+              even if it was used before
                                             the Group ID change

         Figure 4: Relations among keying material components.

12.3.  Support for Signature Checkers

   The Group Manager may serve signature checkers, e.g., intermediary
   gateways, which verify countersignatures of messages protected in
   group mode (see Section 7.5).  These entities do not join a group as
   members, but can retrieve authentication credentials of group members
   and other selected group data from the Group Manager.

   In order to verify countersignatures of messages in a group, a
   signature checker needs to retrieve the following information about
   the group:

   *  The current ID Context (Gid) used in the group.





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   *  The authentication credentials of the group members and of the
      Group Manager.

      If the signature checker is provided with a CWT or a CCS for a
      given entity, then the authentication credential associated with
      that entity is the untagged CWT or CCS.

      If the signature checker is provided with a chain or a bag of
      X.509 / C509 certificates, or of CWTs, or of CCSs for a given
      entity, then the authentication credential associated with that
      entity is the end-entity certificate or end-entity untagged CWT /
      CCS.

   *  The current Signature Encryption Key (see Section 2.1.9).

   *  The identifiers of the algorithms used in the group (see
      Section 2), i.e.: i) Group Encryption Algorithm and Signature
      Algorithm; and ii) AEAD Algorithm and Pairwise Key Agreement
      Algorithm, if such parameters are set in the Common Context (see
      Section 2.1.1 and Section 2.1.10).

   A signature checker MUST be authorized before it can retrieve such
   information, for example with the use of
   [I-D.ietf-ace-key-groupcomm-oscore].

13.  Implementation Status

   This section is to be removed before publishing as an RFC.

   Note to RFC Editor: when deleting this section, please also delete
   RFC 7942 from the references of this document.

   (Boilerplate as per Section 2.1 of [RFC7942]:)

   This section records the status of known implementations of the
   protocol defined by this specification at the time of posting of this
   Internet-Draft, and is based on a proposal described in [RFC7942].
   The description of implementations in this section is intended to
   assist the IETF in its decision processes in progressing drafts to
   RFCs.  Please note that the listing of any individual implementation
   here does not imply endorsement by the IETF.  Furthermore, no effort
   has been spent to verify the information presented here that was
   supplied by IETF contributors.  This is not intended as, and must not
   be construed to be, a catalog of available implementations or their
   features.  Readers are advised to note that other implementations may
   exist.





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   According to [RFC7942], "this will allow reviewers and working groups
   to assign due consideration to documents that have the benefit of
   running code, which may serve as evidence of valuable experimentation
   and feedback that have made the implemented protocols more mature.
   It is up to the individual working groups to use this information as
   they see fit".

13.1.  Implementation #1

   *  Responsible organization: RISE Research Institutes of Sweden AB

   *  Implementation's name: Group OSCORE for Eclipse Californium

   *  Available at: https://github.com/rikard-sics/californium/tree/
      group_oscore

   *  Description: Implementation in Java, building on Eclipse
      Californium, see:

      -  https://github.com/eclipse-californium/californium

      -  http://eclipse.dev/californium/

   *  Implementation's level of maturity: prototype

   *  The implementation supports:

      -  The group mode and the pairwise mode.

      -  Mapping of public keys for the curve Ed25519 into Montgomery
         coordinates to use with X25519.

      -  The following COSE encryption algorithms: AES-CCM-16-64-128,
         AES-CCM-16-128-128, AES-CCM-16-64-256, AES-CCM-16-128-256,
         AES_CCM-64-64-128, AES-CCM-64-128-128, AES-CCM-64-64-256, AES-
         CCM-64-128-256, A128GCM, A192GCM, A256GCM, ChaCha20/Poly1305,
         A128CTR, A192CTR, A256CTR.

      -  The following HKDF algorithms: HKDF SHA-256 (identified as the
         COSE Algorithm "HMAC 256/256") and HKDF SHA-512 (identified as
         the COSE Algorithm "HMAC 512/512").

      -  The following COSE signature algorithms: ECDSA with curves
         P-256, P-384, and P-521, as well as EdDSA with curve Ed25519.







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      -  The following COSE key agreement algorithms: ECDH-SS + HKDF-256
         and ECDH-SS + HKDF-512, both of which using either keys of COSE
         Key Type "EC2" with the curve P-256, P-384, and P-521, or keys
         of COSE Key Type "OKP" key with X25519.

      -  The following authentication credential format: CWT Claims Sets
         (CCSs).

   *  Version compatibility: From version -23 onwards.

   *  Licensing: according to the same dual license of Eclipse
      Californium, i.e., according to the "Eclipse Distribution License
      1.0" and the "Eclipse Public License 2.0".  See:

      -  https://github.com/eclipse-californium/californium/blob/main/
         LICENSE

      -  https://www.eclipse.org/org/documents/edl-v10.php

      -  https://www.eclipse.org/legal/epl-2.0/

   *  Contact information: Rikard Höglund - rikard.hoglund@ri.se

   *  Information last updated on: 2025-02-06

13.2.  Implementation #2

   *  Implementation's name: aiocoap

   *  Available at: https://codeberg.org/aiocoap/aiocoap

   *  Description: A Python CoAP library with support for multiple
      transports and security mechanisms.  The library provides also
      utility programs.

   *  Implementation's level of maturity: Stable support for OSCORE;
      Group OSCORE is minimal.

   *  The implementation supports:

      -  The group mode and the pairwise mode.

      -  Mapping of public keys for the curve Ed25519 into Montgomery
         coordinates to use with X25519.

      -  The following COSE encryption algorithms: 1-3, 10-13, 24,
         30-33.




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      -  The following HKDF algorithms: HKDF SHA-256, -384, -512.

      -  The following COSE signature algorithms: EdDSA on Ed25519,
         ECDSA w/ SHA-256 on P-256

      -  The following COSE key agreement algorithms: ECDH on P-256 and
         curve25519.

      -  The following authentication credential format: currently n/a
         (user provides pairs of credentials and keys)

   *  Version compatibility: -23

   *  Licensing: MIT

   *  Implementation experience: Mostly smooth; the differentiation
      between the regular and the group AEAD algorithm, and more
      generally finding the right parameters to input into the
      (abstracted) KDF part, was tedious and error prone (because Group
      OSCORE largely relies on OSCORE extension points that were
      anticipated, but that was not).

   *  Contact information: Christian Amsüss - christian@amsuess.com

   *  Information last updated on: 2025-02-06

13.3.  Interoperability

   The two implementations mentioned in Section 13.1 and Section 13.2
   have successfully completed interoperability tests.

   That occurred multiple times when covering earlier versions of the
   protocol, as well as specifically for version -23 of the Internet
   Draft, during the IETF 121 meeting in Dublin (Ireland) in November
   2024 and later on in February 2025.

   The scenarios considered during the interoperability tests are as
   follows:

   *  (A) Authentication credential format: CWT Claims Sets (CCSs).

   *  (B) Message protection:

      -  (B1) Both requests and responses protected in group mode.

      -  (B2) Requests protected in group mode and responses protected
         in pairwise mode.




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      -  (B3) Requests protected in pairwise mode and responses
         protected in group mode.

      -  (B4) Both requests and responses protected in pairwise mode.

   *  (C) Signature algorithm: EdDSA with curve Ed25519.

   *  (D) HKDF algorithms: HKDF SHA-256.

   *  (E) Key agreement algorithms: ECDH-SS + HKDF-256, following a
      mapping of public keys for the curve Ed25519 into Montgomery
      coordinates to use with X25519.

   *  (F) The following pairs of (Group Encryption Algorithm, AEAD
      Algorithm), for all the cases B1, B2, B3, and B4 above:

      -  (AES-CCM-16-64-128, AES-CCM-16-64-128).

      -  (ChaCha20/Poly1305, ChaCha20/Poly1305).

      -  (AES-CCM-16-64-128, ChaCha20/Poly1305).

      -  (ChaCha20/Poly1305, AES-CCM-16-64-128).

14.  Security Considerations

   The same considerations from Appendix D of [RFC8613] on the security
   properties of OSCORE hold for Group OSCORE, with the differences
   discussed in the following.  Those considerations are about the
   reference threat model, the support for Proxy operations, the
   protection of message fields, and the uniqueness of (key, nonce)
   pairs (which is further discussed in Section 14.3 of this document).

   For Group OSCORE, the Sender Context and Recipient Context
   additionally contain asymmetric keys, which are used to provide
   source authentication: in group mode, by means of countersignatures
   (see Section 14.1); in pairwise mode, by using Diffie-Hellman (see
   Section 14.2).  The key pair can, for example, be generated by the
   endpoint or provisioned during manufacturing.

   Note that, even if an endpoint is authorized to be a group member and
   to take part in group communications, there is a risk that it behaves
   inappropriately.  For instance, it can forward the content of
   messages in the group to unauthorized entities.  However, in many use
   cases, the devices in the group belong to a common authority and are
   configured by a commissioner (see Appendix B), which limits this risk
   in practice and enables a prompt detection/reaction in case of
   misbehaving.



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   With respect to unprotected message fields, the following holds.
   First, the 'kid context' of request messages is part of the
   Additional Authenticated Data, making it possible to keep long
   exchanges active safely beyond a possible change of ID Context (Gid)
   following a group rekeying (see Section 3.4).  Second, the
   countersignature included in a Group OSCORE message protected in
   group mode is also computed over the value of the OSCORE Option,
   which is also part of the Additional Authenticated Data used in the
   signing process.  This is further discussed in Section 14.7 of this
   document.

   In accordance with [RFC8613], all elements used in Group OSCORE as
   opaque binary values (e.g., Sender ID, ID Context) are not to be
   interpreted as text, Unicode, or otherwise.  Implementations ought
   not to apply encoding transformations to the content of those
   elements, e.g., UTF-8 decoding [RFC3629] or normalization.
   Information elements that may contain text such as those found within
   authentication credentials (e.g., X.509 distinguished names, CWT
   claims, or JSON Web Key fields) are to be treated as opaque
   structured data and to be interpreted only according to the rules of
   the credential format as defined in their respective specifications.
   This avoids misinterpretation, Unicode normalization attacks, or
   mismatches in identity comparison.

   As discussed in Section 6.2.3 of [I-D.ietf-core-groupcomm-bis], Group
   OSCORE addresses security attacks against CoAP listed in Sections
   11.2–11.6 of [RFC7252], especially when run over IP multicast.

   Group OSCORE does not aim to meet the following properties:

   *  Verification of server aliveness, as discussed in Section 5.2.

   *  Protection of network addressing information, as discussed in
      Section 14.9.

   *  Management of group membership and group keying material, which is
      entrusted to the Group Manager (see Section 12).  Related security
      considerations are discussed in Section 14.4 and Section 14.5.

   *  Confidentiality protection of the OSCORE Option.  Related privacy
      considerations are discussed in Section 14.18.

   The rest of this section first discusses security aspects to be taken
   into account when using Group OSCORE.  Then it goes through aspects
   covered in the security considerations of OSCORE (see Section 12 of
   [RFC8613]), and discusses how they hold when Group OSCORE is used.





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14.1.  Security of the Group Mode

   The group mode defined in Section 7 relies on shared group keying
   material to protect communication within a group.  Using the group
   mode has the implications discussed below.  This section uses the
   term 'group members' to describe endpoints which possess the latest
   version of the group keying material.

   *  Source authentication of messages sent to a group is ensured
      through a countersignature, which is computed by the sender
      endpoint using its own private key and then appended to the
      message payload.  The countersignature is also encrypted using a
      keystream derived from the group keying material (see Section 4.1
      and Section 4.2).  This ensures group privacy, i.e., an attacker
      cannot track an endpoint over two groups by linking messages
      between the two groups unless the attacker is also a member of
      both groups.

   *  Messages are encrypted at a group level (group-level data
      confidentiality), i.e., they can be decrypted by any member of the
      group, but not by an external adversary or other external entities
      other than the Group Manager responsible for the group.

   *  If the used Group Encryption Algorithm provides integrity
      protection, then it also ensures group authentication and proof of
      group membership, but not source authentication.  That is, it
      ensures that a message sent to a group has been sent by a member
      of that group, but not necessarily by the alleged sender.  In
      fact, any group member is able to derive the Sender Key used by
      the actual sender endpoint, and thus can compute a valid
      authentication tag.  Therefore, the message content could
      originate from any of the current group members.

      Furthermore, if the used Group Encryption Algorithm does not
      provide integrity protection, then it does not ensure any level of
      message authentication or proof of group membership.

      On the other hand, proof of group membership is always ensured by
      construction through the strict management of the group keying
      material (see Section 12.2).  That is, the group is rekeyed when
      members leave and the current group members are informed of former
      group members.  Thus, a current group member storing the latest
      group keying material does not store the authentication credential
      of any former group member.

      This allows a recipient endpoint to rely on the stored
      authentication credentials and public keys included therein, in
      order to always confidently verify the group membership of a



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      sender endpoint when processing an incoming message, i.e., to
      verify that the sender endpoint was a group member when it signed
      the message.  In turn, this prevents a former group member from
      possibly re-signing and injecting in the group a stored message
      that was protected with old keying material.  A case in point is
      discussed in Section 14.1.1.

   The security properties of the group mode are summarized below.

   1.  Asymmetric source authentication, by means of a countersignature.

   2.  Symmetric group authentication, by means of an authentication tag
       (only for Group Encryption Algorithms providing integrity
       protection).

   3.  Symmetric group confidentiality, by means of symmetric
       encryption.

   4.  Proof of group membership, by strictly managing the group keying
       material, as well as by means of integrity tags when using a
       Group Encryption Algorithm that provides also integrity
       protection.

   5.  Group privacy, by encrypting the countersignature.

   The group mode fulfills the security properties above while also
   displaying the following benefits.  First, the use of a Group
   Encryption Algorithm that does not provide integrity protection
   results in a minimal communication overhead, by limiting the message
   payload to the ciphertext without integrity tag together with the
   encrypted countersignature.  Second, it is possible to deploy semi-
   trusted entities such as signature checkers (see Section 12.3), which
   can break property 5, but cannot break properties 1, 2, 3, and 4.

14.1.1.  Example of Need for Proof of Group Membership

   As a case in point, the importance of reliable proof of group
   membership is evident in a group where the Group Encryption Algorithm
   does not provide integrity protection.  Suppose a group member leaves
   the group and, after the group rekeying, associates with the group as
   external signature checker (see Section 7.5).  When doing so, it
   obtains from the Group Manager the new Signature Encryption Key, from
   which it can derive keystreams for encrypting and decrypting the
   countersignatures of messages protected in group mode.

   However, when participating in the group rekeying, the current group
   members deleted the Recipient Context and authentication credential
   of the former group member.  Consequently, the signature checker is



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   not able to successfully inject messages protected in group mode, as
   encrypted with the old group keying material, signed with its own
   private key, and with the countersignature encrypted by means of the
   latest Signature Encryption Key. That is, if the signature checker
   attempts to do that, then the group members will fail to verify the
   messages from the signature checker and thus will discard those
   messages.

14.2.  Security of the Pairwise Mode

   The pairwise mode defined in Section 8 protects messages by using
   pairwise symmetric keys, derived from the static-static Diffie-
   Hellman shared secret computed from the asymmetric keys of the sender
   and recipient endpoint (see Section 2.5).

   The used AEAD Algorithm MUST provide integrity protection.
   Therefore, the pairwise mode ensures both pairwise data-
   confidentiality and source authentication of messages, without using
   countersignatures.  Furthermore, the recipient endpoint achieves
   proof of group membership for the sender endpoint, since only current
   group members have the required keying material to derive a valid
   Pairwise Sender/Recipient Key.

   Finally, the pairwise mode ensures group privacy, i.e., an attacker
   cannot track an endpoint over two groups by linking messages between
   the two groups unless the attacker is also a member of both groups.
   This follows from two different groups using different and
   uncorrelated group keying material, which yields different and
   uncorrelated pairwise keys for the same endpoint in any two groups.
   Therefore, the authentication tags generated by an endpoint in a
   group have no correlation with those generated by the same endpoint
   in another group.

   The security properties of the pairwise mode are summarized below.

   1.  Symmetric source authentication, by means of an authentication
       tag.

   2.  Symmetric pairwise confidentiality, by means of symmetric
       encryption.

   3.  Proof of group membership, by strictly managing the group keying
       material, as well as by means of integrity tags.

   4.  Group privacy, by virtue of the uncorrelated pairwise keys used
       in any two different groups.





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   The long-term storing of the Diffie-Hellman shared secret is a
   potential security issue.  In fact, if the shared secret of two group
   members is leaked, a third group member can exploit it to derive
   their pairwise keys and use those to impersonate either of the two
   group members to the other, or to decrypt previously stored messages
   exchanged between those two members and protected with their pairwise
   keys.  The possibility of such leakage should be considered more
   likely than the leakage of a private key, which could be rather
   protected at a significantly higher level than generic memory, e.g.,
   by using a Trusted Platform Module.  Therefore, there is a trade-off
   between the maximum amount of time a same shared secret is stored and
   the frequency of its re-computing.

14.3.  Uniqueness of (key, nonce)

   The proof for uniqueness of (key, nonce) pairs in Appendix D.4 of
   [RFC8613] is also valid in group communication scenarios.  That is,
   given an OSCORE group:

   *  Uniqueness of Sender IDs within the group is enforced by the Group
      Manager.  In fact, from the moment when a Gid is assigned to a
      group until the moment when a new Gid is assigned to that same
      group, the Group Manager does not reassign a Sender ID within the
      group (see Section 12.2.1.2).

   *  The case A in Appendix D.4 of [RFC8613] concerns all requests as
      well as all responses including a Partial IV (e.g., Observe
      notifications [RFC7641] or any other subsequent responses after
      the first one).  In this case, the same considerations from
      [RFC8613] apply here as well.

   *  The case B in Appendix D.4 of [RFC8613] concerns responses not
      including a Partial IV (e.g., a single response to a request).  In
      this case, the same considerations from [RFC8613] apply here as
      well.

   As a consequence, each message encrypted/decrypted with the same
   Sender Key is processed by using a different (ID_PIV, PIV) pair.
   This means that nonces used by any fixed encrypting endpoint are
   unique.  Thus, each message is processed with a different (key,
   nonce) pair.










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14.4.  Management of Group Keying Material

   The protocol described in this document should take into account the
   risk of compromise of group members.  In particular, this document
   specifies that a key management scheme for secure revocation and
   renewal of Security Contexts and group keying material MUST be
   adopted.

   [I-D.ietf-ace-key-groupcomm-oscore] specifies a simple rekeying
   scheme for renewing the Security Context in a group.

   Alternative rekeying schemes that are more scalable with the group
   size may be needed in dynamic, large groups where endpoints can join
   and leave at any time, in order to limit the impact on performance
   due to the Security Context and keying material update.

14.4.1.  Denial of Service

   An adversary may leverage the loss of Sender Contexts and/or
   Recipient Contexts as described in Section 2.6.1.1, in order to
   perform a Denial of Service attack and prevent some group members
   from communicating altogether.

   That is, the adversary can first block the communication path between
   the Group Manager and some individual group members.  This can be
   achieved, for instance, by injecting fake responses to DNS queries
   for the Group Manager hostname, or by removing a network link used
   for routing traffic towards the Group Manager.

   Then, the adversary can induce an unprepared reboot for some
   endpoints in the group, e.g., by triggering a short power outage.
   After that, such endpoints that have lost their Sender Context and/or
   Recipient Contexts following the reboot would not be able to obtain
   new Security Context parameters from the Group Manager, as specified
   in Section 2.6.1.1.  Thus, they would not be able to further
   communicate in the group until connectivity with the Group Manager is
   restored.

14.5.  Update of Security Context and Key Rotation

   A group member can receive a message shortly after the group has been
   rekeyed, and new security parameters and keying material have been
   distributed by the Group Manager.








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   This may result in a client using an old Security Context to protect
   a request, and a server using a different new Security Context to
   protect a corresponding response.  As a consequence, clients may
   receive a response protected with a Security Context different from
   the one used to protect the corresponding request.

   In particular, a server may first get a request protected with the
   old Security Context, then install the new Security Context, and only
   after that produce a response to send back to the client.  In such a
   case, as specified in Section 7.3, the server MUST protect the
   potential response using the new Security Context.  Specifically, the
   server MUST include its Sender Sequence Number as Partial IV in the
   response and use it to build the nonce to protect the response.  This
   prevents the nonce from the request from being reused with the new
   Security Context.

   The client will process that response using the new Security Context,
   provided that it has installed the new security parameters and keying
   material before the message processing.

   In case block-wise transfer [RFC7959] is used, the same
   considerations from Section 10.3 of [RFC9594] hold.

   Furthermore, as described below, a group rekeying may temporarily
   result in misaligned Security Contexts between the sender and
   recipient of a given message.

14.5.1.  Late Update on the Sender

   In this case, the sender protects a message using the old Security
   Context, i.e., before having installed the new Security Context.
   However, the recipient receives the message after having installed
   the new Security Context, and is thus unable to correctly process it.

   A possible way to mitigate this issue is to preserve the old retained
   Security Context for a maximum amount of time defined by the
   application.  By doing so, the recipient can still try to process the
   received message using the old retained Security Context.

   This makes particular sense when the recipient is a client, that
   would hence be able to process incoming responses protected with the
   old retained Security Context used to protect the associated request.
   If, as typically expected, the old Gid is not included in the
   response, then the client will first fail to process the response
   using the latest Security Context, and then use the old retained
   Security Context as a second attempt.





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   Instead, a recipient server can immediately process an incoming
   request with the old retained Security Context, as signaled by the
   old Gid that is always included in requests.  However, the server
   could simply discard such an incoming request, which is preferable
   from a security point of view.

   This tolerance preserves the processing of secure messages throughout
   a long-lasting key rotation, as group rekeying processes may likely
   take a long time to complete, especially in large groups.  On the
   other hand, a former (compromised) group member can abusively take
   advantage of this, and send messages protected with the old retained
   Security Context.  Therefore, a conservative application policy
   should not permit the retention of old Security Contexts.

14.5.2.  Late Update on the Recipient

   In this case, the sender protects a message using the new Security
   Context, but the recipient receives that message before having
   installed the new Security Context.  Therefore, the recipient would
   not be able to correctly process the message and hence discards it.

   If the recipient installs the new Security Context shortly after that
   and the sender endpoint retransmits the message, the former will
   still be able to receive and correctly process the message.

   In any case, the recipient should actively ask the Group Manager for
   an updated Security Context according to an application-defined
   policy, for instance after a given number of unsuccessfully decrypted
   incoming messages.

14.6.  Collision of Group Identifiers

   In case endpoints are deployed in multiple groups managed by
   different non-synchronized Group Managers, it is possible for Group
   Identifiers of different groups to coincide.

   This does not impair the security of the AEAD Algorithm and of the
   Group Encryption Algorithm.  In fact, as long as the Master Secret is
   different for different groups and this condition holds over time,
   keys are different among different groups.

   In case multiple groups use the same IP multicast address, the entity
   assigning that address may help limiting the chances to experience
   such collisions of Group Identifiers.  In particular, it may allow
   the Group Managers of those groups using the same IP multicast
   address to share their respective list of assigned Group Identifiers
   currently in use.




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14.7.  Cross-group Message Injection

   A given endpoint is allowed to and would likely use the same pair
   (private key, authentication credential) in multiple OSCORE groups,
   possibly administered by different Group Managers.

   When a sender endpoint sends a message protected in pairwise mode to
   a recipient endpoint in an OSCORE group, a malicious group member may
   attempt to inject the message to a different OSCORE group also
   including the same endpoints (see Section 14.7.1).

   This relies on altering the content of the OSCORE Option in such a
   way that the MAC in the ciphertext will still be correctly validated,
   which has a success probability depending on the size of the MAC.

   As discussed in Section 14.7.2, the attack is practically infeasible
   if the message is protected in group mode, thanks to the
   countersignature also bound to the OSCORE Option through the
   Additional Authenticated Data used in the signing process (see
   Section 3.4).

14.7.1.  Attack Description

   Let us consider:

   *  Two OSCORE groups G1 and G2, with ID Context (Group ID) Gid1 and
      Gid2, respectively.  Both G1 and G2 use the AEAD cipher AES-CCM-
      16-64-128, i.e., the MAC of the ciphertext is 8 bytes in size.

   *  A sender endpoint X which is member of both G1 and G2, and uses
      the same pair (private key, authentication credential) in both
      groups.  The endpoint X has Sender ID Sid1 in G1 and Sender ID
      Sid2 in G2.  The pairs (Sid1, Gid1) and (Sid2, Gid2) identify the
      same authentication credential of X in G1 and G2, respectively.

   *  A recipient endpoint Y which is member of both G1 and G2, and uses
      the same pair (private key, authentication credential) in both
      groups.  The endpoint Y has Sender ID Sid3 in G1 and Sender ID
      Sid4 in G2.  The pairs (Sid3, Gid1) and (Sid4, Gid2) identify the
      same authentication credential of Y in G1 and G2, respectively.

   *  A malicious endpoint Z is also member of both G1 and G2.  Hence, Z
      is able to derive the Sender Keys used by X in G1 and G2.

   When X sends a message M1 addressed to Y in G1 and protected in
   pairwise mode, Z can intercept M1, and attempt to forge a valid
   message M2 to be injected in G2, making it appear as still sent by X
   to Y and valid to be accepted.



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   In more detail, Z intercepts and stops message M1, and forges a
   message M2 by changing the value of the OSCORE Option from M1 as
   follows: the 'kid context' is set to G2 (rather than G1); and the
   'kid' is set to Sid2 (rather than Sid1).  Then, Z injects message M2
   as addressed to Y in G2.

   Upon receiving M2, there is a probability equal to 2^-64 that Y
   successfully verifies the same unchanged MAC by using the Pairwise
   Recipient Key associated with X in G2.

   Note that Z does not know the pairwise keys of X and Y, since it does
   not know and is not able to compute their shared Diffie-Hellman
   secret.  Therefore, Z is not able to check offline if a performed
   forgery is actually valid, before sending the forged message to G2.

14.7.2.  Attack Prevention in Group Mode

   When a Group OSCORE message is protected in group mode, the
   countersignature is also computed over the value of the OSCORE
   Option, which is part of the Additional Authenticated Data used in
   the signing process (see Section 3.4).

   That is, other than over the ciphertext, the countersignature is
   computed over: the ID Context (Gid) and the Partial IV, which are
   always present in requests; as well as the Sender ID of the message
   originator, which is always present in requests as well as in
   responses to requests protected in group mode.

   Since the signing process also takes as input the ciphertext of the
   COSE_Encrypt0 object, the countersignature is bound not only to the
   intended OSCORE group, hence to the triplet (Master Secret, Master
   Salt, ID Context), but also to a specific Sender ID in that group and
   to its specific symmetric key used for AEAD encryption, hence to the
   quartet (Master Secret, Master Salt, ID Context, Sender ID).

   This makes it practically infeasible to perform the attack described
   in Section 14.7.1, since it would require the adversary to
   additionally forge a valid countersignature that replaces the
   original one in the forged message M2.

14.8.  Prevention of Group Cloning Attack

   Both when using the group mode and the pairwise mode, the message
   protection covers also the Group Manager's authentication credential.
   This is included in the Additional Authenticated Data used in the
   signing process and/or in the integrity-protected encryption process
   (see Section 3.4).




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   By doing so, an endpoint X member of a group G1 cannot perform the
   following attack.

   1.  X sets up a group G2 where it acts as Group Manager.

   2.  X makes G2 a "clone" of G1, i.e., G1 and G2 use the same
       algorithms and have the same Master Secret, Master Salt, and ID
       Context.

   3.  X collects a message M sent to G1 and injects it in G2.

   4.  Members of G2 accept M and believe it to be originated in G2.

   The attack above is effectively prevented, since message M is
   protected by including the authentication credential of G1's Group
   Manager in the Additional Authenticated Data.  Therefore, members of
   G2 do not successfully verify and decrypt M, since they correctly use
   the authentication credential of X as Group Manager of G2 when
   attempting to.

14.9.  Group OSCORE for Unicast Requests

   If a request is intended to be sent over unicast as addressed to a
   single group member, it is NOT RECOMMENDED for the client to protect
   the request by using the group mode as defined in Section 7.1.

   This does not include the case where the client sends a request over
   unicast to a proxy, to be forwarded to multiple intended recipients
   over multicast [I-D.ietf-core-groupcomm-bis].  In this case, the
   client typically protects the request with the group mode, even
   though it is sent to the proxy over unicast (see Section 7).

   If the client uses the group mode with its own Sender Key to protect
   a unicast request to a group member, an on-path adversary can, right
   then or later on, redirect that request to one/many different group
   member(s) over unicast, or to the whole OSCORE group over multicast.
   By doing so, the adversary can induce the target group member(s) to
   perform actions intended for one group member only.  Note that the
   adversary can be external, i.e., they do not need to also be a member
   of the OSCORE group.











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   This is due to the fact that the client is not able to indicate the
   single intended recipient in a way which is secure and possible to
   process for Group OSCORE on the server side.  In particular, Group
   OSCORE does not protect network addressing information such as the IP
   address of the intended recipient server.  It follows that the
   server(s) receiving the redirected request cannot verify whether that
   was the original intention of the client, and would thus simply
   assume so.

   The impact of such an attack depends especially on the REST method of
   the request, i.e., the Inner CoAP Code of the OSCORE request message.
   In particular, safe methods such as GET and FETCH would trigger
   (several) unintended responses from the targeted server(s), while not
   resulting in destructive behavior.  On the other hand, non safe
   methods such as PUT, POST, and PATCH/iPATCH would result in the
   target server(s) taking active actions on their resources and
   possible cyber-physical environment, with the risk of destructive
   consequences and possible implications for safety.

   A client can instead use the pairwise mode as defined in Section 8.3,
   in order to protect a request sent to a single group member by using
   pairwise keying material (see Section 2.5).  This prevents the attack
   discussed above by construction, as only the intended server is able
   to derive the pairwise keying material used by the client to protect
   the request.

   Before delivering to the application an incoming request protected in
   group mode that was sent over unicast, a server should carefully
   consider the impact that processing the request would have, without
   dismissing the possibility that the request reception was in fact the
   result of the attack discussed above.  This is especially important
   for endpoints implementing only a silent server, since they do not
   support the pairwise mode and thus are able to process only requests
   protected in group mode.  Application policies can also define
   specific exceptional cases where it is safe for a server to deliver
   such requests to the application, which can then take a final
   decision about whether acting on the request or not.  For instance,
   such decision can leverage an application-level unique identifier of
   the server specified in the payload of the request.

   In a group where the AEAD Algorithm and Pairwise Key Agreement
   Algorithm are set in the Security Context, an endpoint supporting the
   pairwise mode SHOULD use it to protect requests sent to a single
   group member over unicast.  Using the group mode to protect such
   requests would make the request possible to decrypt and verify for
   any other group member; this may sometimes be a desired property
   (e.g. see [I-D.ietf-core-observe-multicast-notifications]).




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   The use of block-wise transfers [RFC7959] with group communication
   for CoAP is as discussed in Section 3.8 of
   [I-D.ietf-core-groupcomm-bis].  Note that, after the first request
   targeting all servers in the group over multicast, a client can
   switch to sending unicast requests for retrieving further blocks from
   the servers.  In particular, after a first block-wise request that
   targets all servers in the group and includes the CoAP Block2 Option,
   the retrieval of further blocks can use unicast requests, which
   should therefore be protected using the pairwise mode.  Similarly, if
   the first request that targeted all servers in the group did not
   include the CoAP Block2 Option and the corresponding responses
   included the Block2 Option at the servers' own initiative, then the
   retrieval of further blocks can use unicast requests, which should
   therefore be protected using the pairwise mode.

   Additional considerations are discussed in Section 9, with respect to
   requests including a CoAP Echo Option [RFC9175] that have to be sent
   over unicast, as a challenge-response method for servers to achieve
   freshness or to initialize as valid a previously invalid Replay
   Window.

14.10.  End-to-end Protection

   The same considerations from Section 12.1 of [RFC8613] hold for Group
   OSCORE.

   Additionally, (D)TLS and Group OSCORE can be combined for protecting
   message exchanges occurring over unicast.  However, it is not
   possible to combine (D)TLS and Group OSCORE for protecting message
   exchanges where messages are sent over multicast.

14.11.  Master Secret

   Group OSCORE derives the Security Context using the same construction
   used by OSCORE, and by using the Group Identifier of a group as the
   related ID Context.  Hence, the same required properties of the
   Security Context parameters discussed in Section 3.3 of [RFC8613]
   hold for this document.

   With particular reference to the OSCORE Master Secret, it has to be
   kept secret among the members of the respective OSCORE group and the
   Group Manager responsible for that group.  Also, the Master Secret
   must have a good amount of randomness, and the Group Manager can
   generate it offline using a good random number generator.  This
   includes the case where the Group Manager rekeys the group by
   generating and distributing a new Master Secret.





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14.12.  Replay Protection

   As in OSCORE [RFC8613], Group OSCORE relies on Sender Sequence
   Numbers included in the COSE message field 'Partial IV' and used to
   build nonces.

   Note that the Partial IV of an endpoint does not necessarily grow
   monotonically.  For instance, upon exhaustion of the endpoint's
   Sender Sequence Number space, the endpoint's Partial IV space also
   gets exhausted.  As discussed in Section 2.6.3, this results either
   in the endpoint being individually rekeyed and getting a new Sender
   ID, or in the establishment of a new Security Context in the group.
   Therefore, uniqueness of (key, nonce) pairs (see Section 14.3) is
   preserved when a new Security Context is established.

   Since one-to-many communication such as multicast usually involves
   unreliable transports, the simplification of the Replay Window to a
   size of 1 suggested in Section 7.4 of [RFC8613] is not viable with
   Group OSCORE, unless exchanges in the group rely only on unicast
   messages.

   A server's Replay Window may be initialized as invalid (see
   Section 2.6.1).  The server can either retrieve a new Group OSCORE
   Security Context, or make a Replay Window valid (see Section 9)
   before accepting further incoming messages from other group members.

14.13.  Message Ordering

   Assuming that the other endpoint is honest, Group OSCORE provides
   relative ordering of received messages.  For a given Group OSCORE
   Security Context, the received Partial IV (when included) allows the
   recipient endpoint to determine the order in which requests or
   responses were sent by the other endpoint.

   If the Partial IV was omitted in a response, this indicates that it
   was the oldest response from the sender endpoint to the corresponding
   request (like notification responses in OSCORE, see Section 7.4.1 of
   [RFC8613]).  A received response is not older than the corresponding
   request.

14.14.  Message Freshness

   As in OSCORE, Group OSCORE provides only the guarantee that the
   request is not older than the Group OSCORE Security Context used to
   protect it.  Other aspects of freshness are discussed in Section 5.2.






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   The challenge-response approach described in Section 9 provides an
   assurance of freshness of the request without depending on the
   honesty of the client.  However, it can result in an impact on
   performance which is undesirable or unbearable, especially in large
   groups where many endpoints at the same time might join as new
   members.

   Endpoints configured as silent servers are not able to perform the
   challenge-response described above, as they do not store a Sender
   Context to secure the 4.01 (Unauthorized) response to the client.
   Thus, silent servers should adopt alternative approaches to make
   their Replay Windows valid.  For example, a silent server can
   retrieve or wait for new Security Context parameters from the Group
   Manager and derive new Recipient Contexts.  When doing so, the Replay
   Windows of all Recipient Contexts become valid if they are not
   already.  In particular, any invalid Replay Window is re-initialized
   as valid and with 0 as its current lower limit.

   Since requests including the Echo Option are sent over unicast, a
   server can be the victim of the attack discussed in Section 14.9 if
   such requests are protected in group mode.  Instead, protecting those
   requests with the pairwise mode prevents the attack above.  In fact,
   only the server involved in the challenge-response exchange is able
   to derive the pairwise key used by the client to protect the request
   including the Echo Option.

   In either case, an internal on-path adversary would not be able to
   transpose the Echo Option value of two different unicast requests,
   sent by a same client to any two different servers in the group.  In
   fact, even if the group mode was used, this would require the
   adversary to forge the countersignature of both requests.  As a
   consequence, each of the two servers remains able to selectively
   accept a request with the Echo Option only if it is waiting for that
   exact integrity-protected Echo Option value, and is thus the intended
   recipient.

14.15.  Client Aliveness

   Like in OSCORE (see Section 12.5 of [RFC8613]), a server may verify
   the aliveness of the client by using the CoAP Echo Option [RFC9175]
   as described in Section 9.

   In the interest of avoiding otherwise unnecessary uses of such an
   approach, the server can exploit the fact that the received request
   cannot be older than the Security Context used to protect it.  This
   effectively allows the server to verify the client aliveness relative
   to the installation of the latest group keying material.




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14.16.  Cryptographic Considerations

   The same considerations from Section 12.6 of [RFC8613] about the
   maximum Sender Sequence Number hold for Group OSCORE.

   As discussed in Section 2.6.2, an endpoint that experiences an
   exhaustion of its own Sender Sequence Number space MUST NOT protect
   further messages including a Partial IV, until it has derived a new
   Sender Context.  This prevents the endpoint from reusing the same
   nonce with the same Sender Key.

   In order to renew its own Sender Context, the endpoint SHOULD inform
   the Group Manager, which can either renew the whole Security Context
   by means of group rekeying, or provide only that endpoint with a new
   Sender ID value.  In either case, the endpoint derives a new Sender
   Context, and in particular a new Sender Key.

   Additionally, the same considerations from Section 12.6 of [RFC8613]
   hold for Group OSCORE, about building the nonce and the secrecy of
   the Security Context parameters.

   The group mode uses the "encrypt-then-sign" construction, i.e., the
   countersignature is computed over the COSE_Encrypt0 object (see
   Section 3.1).  This is motivated by enabling signature checkers (see
   Section 12.3), which do not join a group as members but are allowed
   to verify countersignatures of messages protected in group mode
   without being able to decrypt those messages (see Section 7.5).

   If the Group Encryption Algorithm used in group mode provides
   integrity protection, countersignatures of COSE_Encrypt0 with short
   authentication tags do not provide the security properties associated
   with the same algorithm used in COSE_Sign (see Section 6 of
   [RFC9338]).  To provide 128-bit security against collision attacks,
   the tag length MUST be at least 256-bits.  A countersignature of a
   COSE_Encrypt0 with AES-CCM-16-64-128 provides at most 32 bits of
   integrity protection.

   The derivation of pairwise keys defined in Section 2.5.1 is
   compatible with ECDSA and EdDSA asymmetric keys, but is not
   compatible with RSA asymmetric keys.

   For the public key translation from Ed25519 (Ed448) to X25519 (X448)
   specified in Section 2.5.1, variable time methods can be used since
   the translation operates on public information.  Any byte string of
   appropriate length is accepted as a public key for X25519 (X448) in
   [RFC7748].  It is therefore not necessary for security to validate
   the translated public key (assuming the translation was successful).




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   The security of using the same key pair for Diffie-Hellman and for
   signing (by considering the ECDH procedure in Section 2.5 as a Key
   Encapsulation Mechanism (KEM)) is demonstrated in [Degabriele] and
   [Thormarker].

   Applications using ECDH (except X25519 and X448) based KEM in
   Section 2.5 are assumed to verify that a peer endpoint's public key
   is on the expected curve and that the shared secret is not the point
   at infinity.  The KEM in [Degabriele] checks that the shared secret
   is different from the point at infinity, as does the procedure in
   Section 5.7.1.2 of [NIST-800-56A] which is referenced in Section 2.5.

   By extending Theorem 2 of [Degabriele], [Thormarker] shows that the
   same key pair can be used with X25519 and Ed25519 (X448 and Ed448)
   for the KEM specified in Section 2.5.  By symmetry in the KEM used in
   this document, both endpoints can consider themselves to have the
   recipient role in the KEM - as discussed in Section 7 of [Thormarker]
   - and rely on the mentioned proofs for the security of their key
   pairs.

   Theorem 3 in [Degabriele] shows that the same key pair can be used
   for an ECDH based KEM and ECDSA.  The KEM uses a different KDF than
   in Section 2.5, but the proof only depends on that the KDF has
   certain required properties, which are the typical assumptions about
   HKDF, e.g., that output keys are pseudorandom.  In order to comply
   with the assumptions of Theorem 3, received public keys MUST be
   successfully validated, see Section 5.6.2.3.4 of [NIST-800-56A].  The
   validation MAY be performed by a trusted Group Manager.  For
   [Degabriele] to apply as it is written, public keys need to be in the
   expected subgroup.  For this, we rely on cofactor Diffie-Hellman as
   per Section 5.7.1.2 of [NIST-800-56A], which is referenced in
   Section 2.5.1.

   HashEdDSA variants of Ed25519 and Ed448 are not used by COSE (see
   Section 2.2 of [RFC9053]), and are not covered by the analysis in
   [Thormarker].  Hence, they MUST NOT be used with the public keys used
   to derive pairwise keys as specified in this document.

14.17.  Message Segmentation

   The same considerations from Section 12.7 of [RFC8613] hold for Group
   OSCORE.









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14.18.  Privacy Considerations

   Group OSCORE ensures end-to-end integrity protection and encryption
   of the message payload and of all the options that are not used for
   proxy operations.  In particular, options are processed according to
   the same class U/I/E that they have for OSCORE.  Therefore, the same
   privacy considerations from Section 12.8 of [RFC8613] hold for Group
   OSCORE, with the following addition.

   *  When protecting a message in group mode, the countersignature is
      encrypted by using a keystream derived from the group keying
      material (see Section 4.1 and Section 4.2).  This ensures group
      privacy.  That is, an attacker cannot track an endpoint over two
      groups by linking messages between the two groups, unless being
      also a member of those groups.

   Furthermore, the following privacy considerations hold about the
   OSCORE Option, which may reveal information on the communicating
   endpoints.

   *  The 'kid' parameter, which is intended to help a recipient
      endpoint to find the right Recipient Context, may reveal
      information about the Sender Endpoint.  When both a request and
      the corresponding responses include the 'kid' parameter, this may
      reveal information about both a client sending a request and all
      the possibly replying servers sending their own individual
      response.

   *  The 'kid context' parameter, which is intended to help a recipient
      endpoint to find the right Security Context, reveals information
      about the sender endpoint.  In particular, it reveals that the
      sender endpoint is a member of a particular OSCORE group, whose
      current Group ID is indicated in the 'kid context' parameter.

   When receiving a group request, each of the recipient endpoints can
   reply with a response that includes its Sender ID as 'kid' parameter.
   All these responses will be matchable with the request through the
   CoAP Token.  Thus, even if these responses do not include a 'kid
   context' parameter, it becomes possible to understand that the
   responder endpoints are in the same group of the requester endpoint.

   Furthermore, using the approach described in Section 9 to make Replay
   Windows valid may reveal when a server device goes through a reboot.
   This can be mitigated by the server device storing the precise state
   of the Replay Window of each known client on a clean shutdown.






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   Finally, the approach described in Section 14.6 to prevent collisions
   of Group Identifiers from different Group Managers may reveal
   information about events in the respective OSCORE groups.  In
   particular, a Group Identifier changes when the corresponding group
   is rekeyed.  Thus, Group Managers might use the shared list of Group
   Identifiers to infer the rate and patterns of group membership
   changes triggering a group rekeying, e.g., due to newly joined
   members or evicted (compromised) members.  In order to alleviate this
   privacy concern, it should be hidden from the Group Managers which
   exact Group Manager has currently assigned which Group Identifiers in
   its OSCORE groups.

15.  IANA Considerations

   Note to RFC Editor: Please replace "[RFC-XXXX]" with the RFC number
   of this document and delete this paragraph.

   This document has the following actions for IANA.

15.1.  OSCORE Flag Bits Registry

   IANA is asked to add the following entry to the "OSCORE Flag Bits"
   registry within the "Constrained RESTful Environments (CoRE)
   Parameters" registry group.

   +==========+=======+===================================+============+
   | Bit      | Name  | Description                       | Reference  |
   | Position |       |                                   |            |
   +==========+=======+===================================+============+
   | 2        | Group | For using a Group OSCORE          | [RFC-XXXX] |
   |          | Flag  | Security Context, set to 1        |            |
   |          |       | if the message is protected       |            |
   |          |       | with the group mode               |            |
   +----------+-------+-----------------------------------+------------+

          Table 1: Registrations in the OSCORE Flag Bits Registry

15.2.  Target Attributes Registry

   IANA is asked to add the following entry to the "Target Attributes"
   registry within the "Constrained RESTful Environments (CoRE)
   Parameters" registry group.

   Attribute Name: gosc
   Brief Description: Hint: resource only accessible
                      using Group OSCORE or OSCORE
   Change Controller: IETF
   Reference: [RFC-XXXX, Section 11]



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16.  References

16.1.  Normative References

   [COSE.Algorithms]
              IANA, "COSE Algorithms",
              <https://www.iana.org/assignments/cose/
              cose.xhtml#algorithms>.

   [I-D.ietf-core-groupcomm-bis]
              Dijk, E. and M. Tiloca, "Group Communication for the
              Constrained Application Protocol (CoAP)", Work in
              Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
              15, 25 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              groupcomm-bis-15>.

   [NIST-800-56A]
              Barker, E., Chen, L., Roginsky, A., Vassilev, A., and R.
              Davis, "Recommendation for Pair-Wise Key-Establishment
              Schemes Using Discrete Logarithm Cryptography - NIST
              Special Publication 800-56A, Revision 3", April 2018,
              <https://nvlpubs.nist.gov/nistpubs/SpecialPublications/
              NIST.SP.800-56Ar3.pdf>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/rfc/rfc2119>.

   [RFC5869]  Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
              Key Derivation Function (HKDF)", RFC 5869,
              DOI 10.17487/RFC5869, May 2010,
              <https://www.rfc-editor.org/rfc/rfc5869>.

   [RFC6979]  Pornin, T., "Deterministic Usage of the Digital Signature
              Algorithm (DSA) and Elliptic Curve Digital Signature
              Algorithm (ECDSA)", RFC 6979, DOI 10.17487/RFC6979, August
              2013, <https://www.rfc-editor.org/rfc/rfc6979>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/rfc/rfc7252>.







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   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/rfc/rfc7641>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/rfc/rfc7748>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,
              <https://www.rfc-editor.org/rfc/rfc8032>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.

   [RFC8288]  Nottingham, M., "Web Linking", RFC 8288,
              DOI 10.17487/RFC8288, October 2017,
              <https://www.rfc-editor.org/rfc/rfc8288>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/rfc/rfc8613>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/rfc/rfc8949>.

   [RFC9052]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Structures and Process", STD 96, RFC 9052,
              DOI 10.17487/RFC9052, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9052>.

   [RFC9053]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Initial Algorithms", RFC 9053, DOI 10.17487/RFC9053,
              August 2022, <https://www.rfc-editor.org/rfc/rfc9053>.





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   [RFC9175]  Amsüss, C., Preuß Mattsson, J., and G. Selander,
              "Constrained Application Protocol (CoAP): Echo, Request-
              Tag, and Token Processing", RFC 9175,
              DOI 10.17487/RFC9175, February 2022,
              <https://www.rfc-editor.org/rfc/rfc9175>.

   [RFC9338]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Countersignatures", STD 96, RFC 9338,
              DOI 10.17487/RFC9338, December 2022,
              <https://www.rfc-editor.org/rfc/rfc9338>.

   [RFC9459]  Housley, R. and H. Tschofenig, "CBOR Object Signing and
              Encryption (COSE): AES-CTR and AES-CBC", RFC 9459,
              DOI 10.17487/RFC9459, September 2023,
              <https://www.rfc-editor.org/rfc/rfc9459>.

16.2.  Informative References

   [Degabriele]
              Degabriele, J. P., Lehmann, A., Paterson, K. G., Smart, N.
              P., and M. Strefler, "On the Joint Security of Encryption
              and Signature in EMV", December 2011,
              <https://eprint.iacr.org/2011/615>.

   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M. and F. Palombini, "Key Management for Group
              Object Security for Constrained RESTful Environments
              (Group OSCORE) Using Authentication and Authorization for
              Constrained Environments (ACE)", Work in Progress,
              Internet-Draft, draft-ietf-ace-key-groupcomm-oscore-18, 28
              August 2025, <https://datatracker.ietf.org/doc/html/draft-
              ietf-ace-key-groupcomm-oscore-18>.

   [I-D.ietf-core-cacheable-oscore]
              Amsüss, C. and M. Tiloca, "Cacheable OSCORE", Work in
              Progress, Internet-Draft, draft-ietf-core-cacheable-
              oscore-00, 22 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              cacheable-oscore-00>.

   [I-D.ietf-core-groupcomm-proxy]
              Tiloca, M. and E. Dijk, "Proxy Operations for CoAP Group
              Communication", Work in Progress, Internet-Draft, draft-
              ietf-core-groupcomm-proxy-05, 3 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              groupcomm-proxy-05>.





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   [I-D.ietf-core-observe-multicast-notifications]
              Tiloca, M., Höglund, R., Amsüss, C., and F. Palombini,
              "Observe Notifications as CoAP Multicast Responses", Work
              in Progress, Internet-Draft, draft-ietf-core-observe-
              multicast-notifications-13, 20 October 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              observe-multicast-notifications-13>.

   [I-D.ietf-core-oscore-capable-proxies]
              Tiloca, M. and R. Höglund, "OSCORE-capable Proxies", Work
              in Progress, Internet-Draft, draft-ietf-core-oscore-
              capable-proxies-05, 3 September 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-core-
              oscore-capable-proxies-05>.

   [I-D.ietf-cose-cbor-encoded-cert]
              Mattsson, J. P., Selander, G., Raza, S., Höglund, J., and
              M. Furuhed, "CBOR Encoded X.509 Certificates (C509
              Certificates)", Work in Progress, Internet-Draft, draft-
              ietf-cose-cbor-encoded-cert-15, 18 August 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-
              cbor-encoded-cert-15>.

   [I-D.ietf-iotops-security-protocol-comparison]
              Mattsson, J. P., Palombini, F., and M. Vučinić,
              "Comparison of CoAP Security Protocols", Work in Progress,
              Internet-Draft, draft-ietf-iotops-security-protocol-
              comparison-09, 4 June 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-iotops-
              security-protocol-comparison-09>.

   [I-D.ietf-lwig-curve-representations]
              Struik, R., "Alternative Elliptic Curve Representations",
              Work in Progress, Internet-Draft, draft-ietf-lwig-curve-
              representations-23, 21 January 2022,
              <https://datatracker.ietf.org/doc/html/draft-ietf-lwig-
              curve-representations-23>.

   [I-D.irtf-cfrg-det-sigs-with-noise]
              Mattsson, J. P., Thormarker, E., and S. Ruohomaa, "Hedged
              ECDSA and EdDSA Signatures", Work in Progress, Internet-
              Draft, draft-irtf-cfrg-det-sigs-with-noise-05, 3 March
              2025, <https://datatracker.ietf.org/doc/html/draft-irtf-
              cfrg-det-sigs-with-noise-05>.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
              2003, <https://www.rfc-editor.org/rfc/rfc3629>.



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   [RFC4340]  Kohler, E., Handley, M., and S. Floyd, "Datagram
              Congestion Control Protocol (DCCP)", RFC 4340,
              DOI 10.17487/RFC4340, March 2006,
              <https://www.rfc-editor.org/rfc/rfc4340>.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
              <https://www.rfc-editor.org/rfc/rfc4944>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2",
              FYI 36, RFC 4949, DOI 10.17487/RFC4949, August 2007,
              <https://www.rfc-editor.org/rfc/rfc4949>.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/rfc/rfc5280>.

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,
              <https://www.rfc-editor.org/rfc/rfc6282>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/rfc/rfc6690>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/rfc/rfc7228>.

   [RFC7942]  Sheffer, Y. and A. Farrel, "Improving Awareness of Running
              Code: The Implementation Status Section", BCP 205,
              RFC 7942, DOI 10.17487/RFC7942, July 2016,
              <https://www.rfc-editor.org/rfc/rfc7942>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/rfc/rfc7959>.








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   [RFC8075]  Castellani, A., Loreto, S., Rahman, A., Fossati, T., and
              E. Dijk, "Guidelines for Mapping Implementations: HTTP to
              the Constrained Application Protocol (CoAP)", RFC 8075,
              DOI 10.17487/RFC8075, February 2017,
              <https://www.rfc-editor.org/rfc/rfc8075>.

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/rfc/rfc8392>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.

   [RFC9002]  Iyengar, J., Ed. and I. Swett, Ed., "QUIC Loss Detection
              and Congestion Control", RFC 9002, DOI 10.17487/RFC9002,
              May 2021, <https://www.rfc-editor.org/rfc/rfc9002>.

   [RFC9147]  Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
              <https://www.rfc-editor.org/rfc/rfc9147>.

   [RFC9200]  Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S., and
              H. Tschofenig, "Authentication and Authorization for
              Constrained Environments Using the OAuth 2.0 Framework
              (ACE-OAuth)", RFC 9200, DOI 10.17487/RFC9200, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9200>.

   [RFC9594]  Palombini, F. and M. Tiloca, "Key Provisioning for Group
              Communication Using Authentication and Authorization for
              Constrained Environments (ACE)", RFC 9594,
              DOI 10.17487/RFC9594, September 2024,
              <https://www.rfc-editor.org/rfc/rfc9594>.

   [Thormarker]
              Thormarker, E., "On using the same key pair for Ed25519
              and an X25519 based KEM", April 2021,
              <https://eprint.iacr.org/2021/509>.

Appendix A.  Assumptions and Security Objectives

   This section presents a set of assumptions and security objectives
   for the protocol described in this document.  The rest of this
   section refers to three types of groups:





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   *  Application group, i.e., a set of CoAP endpoints that share a
      common pool of resources.

   *  Security group, as defined in Section 1.1 of this document.
      Between security groups and application groups, there can be a
      many-to-many, one-to-many, many-to-one, or one-to-one
      relationship.

   *  CoAP group, i.e., a set of CoAP endpoints where each endpoint is
      configured to receive one-to-many CoAP requests, e.g., sent to the
      group's associated IP multicast address and UDP port as defined in
      [I-D.ietf-core-groupcomm-bis].  An endpoint may be a member of
      multiple CoAP groups.  Between application groups and CoAP groups,
      there can be a many-to-many, one-to-many, many-to-one, or one-to-
      one relationship.  Note that a device sending a CoAP request to a
      CoAP group is not necessarily itself a member of that group: it is
      a member only if it also has a CoAP server endpoint listening to
      requests for this CoAP group, sent to the associated IP multicast
      address and port.  In order to provide secure group communication,
      all members of a CoAP group as well as all further endpoints
      configured only as clients sending CoAP (multicast) requests to
      the CoAP group have to be member of a security group.  Between
      security groups and CoAP groups, there can be a many-to-many, one-
      to-many, many-to-one, or one-to-one relationship.

A.1.  Assumptions

   The following points are assumed to be already addressed and are out
   of the scope of this document.

   *  Multicast communication topology: this document considers both
      1-to-N (one sender and multiple recipients) and M-to-N (multiple
      senders and multiple recipients) communication topologies.  The
      1-to-N communication topology is the simplest group communication
      scenario that would serve the needs of a typical Low-power and
      Lossy Network (LLN).  Examples of use cases that benefit from
      secure group communication are provided in Appendix B.

      In a 1-to-N communication model, only a single client transmits
      data to the CoAP group, in the form of request messages; in an
      M-to-N communication model (where M and N do not necessarily have
      the same value), M clients transmit data to the CoAP group.
      According to [I-D.ietf-core-groupcomm-bis], any possible proxy
      entity is supposed to know about the clients.  Also, every client
      expects and is able to handle multiple response messages
      associated with a same request sent to the CoAP group.





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   *  Group size: security solutions for group communication should be
      able to adequately support different and possibly large security
      groups.  The group size is the current number of members in a
      security group.  In the use cases mentioned in this document, the
      number of clients (normally the controlling devices) is expected
      to be much smaller than the number of servers (i.e., the
      controlled devices).  A security solution for group communication
      that supports 1 to 50 clients would be able to properly cover the
      group sizes required for most use cases that are relevant for this
      document.  The maximum group size is expected to be in the range
      of hundreds to thousands of devices, with large groups easier to
      manage if including several silent servers.  Security groups
      larger than that should be divided into smaller independent
      groups.  One should not assume that the set of members of a
      security group remains fixed.  That is, the group membership is
      subject to changes, possibly on a frequent basis.

   *  Communication with the Group Manager: an endpoint must use a
      secure dedicated channel when communicating with the Group
      Manager, also when not registered as a member of the security
      group.

   *  Provisioning and management of Security Contexts: a Security
      Context must be established among the members of the security
      group.  A secure mechanism must be used to generate, revoke and
      (re-)distribute keying material, communication policies and
      security parameters in the security group.  The actual
      provisioning and management of the Security Context is out of the
      scope of this document.

   *  Multicast data security cipher suite: all members of a security
      group must use the same cipher suite to provide authenticity,
      integrity and confidentiality of messages in the group.  The
      cipher suite is specified as part of the Security Context.

   *  Ensuring backward security: a new device joining the security
      group should not have access to any old Security Contexts used
      before its joining.  This ensures that a new member of the
      security group is not able to decrypt confidential data sent
      before it has joined the security group.  The adopted key
      management scheme should ensure that the Security Context is
      updated to ensure backward confidentiality.  The actual mechanism
      to update the Security Context and renew the group keying material
      in the security group upon a new member's joining has to be
      defined as part of the group key management scheme.






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   *  Ensuring forward security: entities that leave the security group
      should not have access to any future Security Contexts or message
      exchanged within the security group after their leaving.  This
      ensures that a former member of the security group is not able to
      decrypt confidential data sent within the security group anymore.
      Also, it ensures that a former member is not able to send
      protected messages to the security group anymore.  The actual
      mechanism to update the Security Context and renew the group
      keying material in the security group upon a member's leaving has
      to be defined as part of the group key management scheme.

A.2.  Security Objectives

   The protocol described in this document aims at fulfilling the
   following security objectives:

   *  Data replay protection: request messages or response messages
      replayed within the security group must be detected.

   *  Data confidentiality: messages sent within the security group
      shall be encrypted.

   *  Group-level data confidentiality: the group mode provides group-
      level data confidentiality since messages are encrypted at a group
      level, i.e., in such a way that they can be decrypted by any
      member of the security group, but not by an external adversary or
      other external entities.

   *  Pairwise data confidentiality: the pairwise mode especially
      provides pairwise data confidentiality, since messages are
      encrypted using pairwise keying material shared between any two
      group members, hence they can be decrypted only by the intended
      single recipient.

   *  Source message authentication: messages sent within the security
      group shall be authenticated.  That is, it is essential to ensure
      that a message is originated by a member of the security group in
      the first place, and in particular by a specific, identifiable
      member of the security group.

   *  Message integrity: messages sent within the security group shall
      be integrity protected.  That is, it is essential to ensure that a
      message has not been tampered with, either by a group member, or
      by an external adversary or other external entities which are not
      members of the security group.






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   *  Message ordering: it must be possible to determine the ordering of
      messages coming from a single sender.  Like in OSCORE [RFC8613], a
      recipient endpoint can determine the relative order of requests or
      responses from another sender endpoint by means of their Partial
      IV.  It is not required to determine ordering of messages from
      different senders.

Appendix B.  List of Use Cases

   Group Communication for CoAP [I-D.ietf-core-groupcomm-bis] provides
   the necessary background for multicast-based CoAP communication, with
   particular reference to low-power and lossy networks (LLNs) and
   resource constrained environments.  The interested reader is
   encouraged to first read [I-D.ietf-core-groupcomm-bis] to understand
   the non-security related details.  This section discusses a number of
   use cases that benefit from secure group communication, and refers to
   the three types of groups from Appendix A.  Specific security
   requirements for these use cases are discussed in Appendix A.

   *  Lighting control: consider a building equipped with IP-connected
      lighting devices, switches, and border routers.  The lighting
      devices acting as servers are organized into application groups
      and CoAP groups, according to their physical location in the
      building.  For instance, lighting devices in a room or corridor
      can be configured as members of a single application group and
      corresponding CoAP group.  Those lighting devices together with
      the switches acting as clients in the same room or corridor can be
      configured as members of the corresponding security group.
      Switches are then used to control the lighting devices by sending
      on/off/dimming commands to all lighting devices in the CoAP group,
      while border routers connected to an IP network backbone (which is
      also multicast-enabled) can be used to interconnect routers in the
      building.  Consequently, this would also enable logical groups to
      be formed even if devices with a role in the lighting application
      may be physically in different subnets (e.g., on wired and
      wireless networks).  Connectivity between lighting devices may be
      realized, for instance, by means of IPv6 and (border) routers
      supporting 6LoWPAN [RFC4944][RFC6282].  Group communication
      enables synchronous operation of a set of connected lights,
      ensuring that the light preset (e.g., dimming level or color) of a
      large set of luminaires are changed at the same perceived time.
      This is especially useful for providing a visual synchronicity of
      light effects to the user.  As a practical guideline, events
      within a 200 ms interval are perceived as simultaneous by humans,
      which is necessary to ensure in many setups.  Devices may reply
      back to the switches that issue on/off/dimming commands, in order
      to report about the execution of the requested operation (e.g.,
      OK, failure, error) and their current operational status.  In a



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      typical lighting control scenario, a single switch is the only
      entity responsible for sending commands to a set of lighting
      devices.  In more advanced lighting control use cases, an M-to-N
      communication topology would be required, for instance in case
      multiple sensors (presence or day-light) are responsible to
      trigger events to a set of lighting devices.  Especially in
      professional lighting scenarios, the roles of client and server
      are configured by the lighting commissioner, and devices strictly
      follow those roles.

   *  Integrated building control: enabling Building Automation and
      Control Systems (BACSs) to control multiple heating, ventilation,
      and air-conditioning units to predefined presets.  Controlled
      units can be organized into application groups and CoAP groups in
      order to reflect their physical position in the building, e.g.,
      devices in the same room can be configured as members of a single
      application group and corresponding CoAP group.  As a practical
      guideline, events within intervals of seconds are typically
      acceptable.  Controlled units are expected to possibly reply back
      to the BACS issuing control commands, in order to report about the
      execution of the requested operation (e.g., OK, failure, error)
      and their current operational status.

   *  Software and firmware updates: software and firmware updates often
      comprise quite a large amount of data.  This can overload a Low-
      power and Lossy Network (LLN) that is otherwise typically used to
      deal with only small amounts of data, on an infrequent base.
      Rather than sending software and firmware updates as unicast
      messages to each individual device, multicasting such updated data
      to a larger set of devices at once displays a number of benefits.
      For instance, it can significantly reduce the network load and
      decrease the overall time latency for propagating this data to all
      devices.  Even if the complete whole update process itself is
      secured, securing the individual messages is important, in case
      updates consist of relatively large amounts of data.  In fact,
      checking individual received data piecemeal for tampering avoids
      that devices store large amounts of partially corrupted data and
      that they detect tampering hereof only after all data has been
      received.  Devices receiving software and firmware updates are
      expected to possibly reply back, in order to provide a feedback
      about the execution of the update operation (e.g., OK, failure,
      error) and their current operational status.

   *  Parameter and configuration update: by means of multicast
      communication, it is possible to update the settings of a set of
      similar devices, both simultaneously and efficiently.  Possible
      parameters are related, for instance, to network load management
      or network access control.  Devices receiving parameter and



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      configuration updates are expected to possibly reply back, to
      provide a feedback about the execution of the update operation
      (e.g., OK, failure, error) and their current operational status.

   *  Commissioning of Low-power and Lossy Network (LLN) systems: a
      commissioning device is responsible for querying all devices in
      the local network or a selected subset of them, in order to
      discover their presence, and be aware of their capabilities,
      default configuration, and operating conditions.  Queried devices
      displaying similarities in their capabilities and features, or
      sharing a common physical location can be configured as members of
      a single application group and corresponding CoAP group.  Queried
      devices are expected to reply back to the commissioning device, in
      order to notify their presence, and provide the requested
      information and their current operational status.

   *  Emergency multicast: a particular emergency-related information
      (e.g., natural disaster) is generated and multicast by an
      emergency notifier, and relayed to multiple devices.  The latter
      may reply back to the emergency notifier, in order to provide
      their feedback and local information related to the ongoing
      emergency.  This kind of setups should additionally rely on a
      fault-tolerant multicast algorithm, such as Multicast Protocol for
      Low-Power and Lossy Networks (MPL).

Appendix C.  Example of Group Identifier Format

   This section provides an example of how the Group Identifier (Gid)
   can be formatted.  That is, the Gid can be composed of two parts,
   namely a Group Prefix and a Group Epoch.

   For each group, the Group Prefix is constant over time and is
   uniquely defined in the set of all the groups associated with the
   same Group Manager.  The choice of the Group Prefix for a given
   group's Security Context is application specific.  The size of the
   Group Prefix directly impact on the maximum number of distinct groups
   under the same Group Manager.

   The Group Epoch is set to 0 upon the group's initialization, and is
   incremented by 1 each time new keying material, together with a new
   Gid, is distributed to the group in order to establish a new Security
   Context (see Section 12.2).

   As an example, a 3-byte Gid can be composed of: i) a 1-byte Group
   Prefix '0xb1' interpreted as a raw byte string; and ii) a 2-byte
   Group Epoch interpreted as an unsigned integer ranging from 0 to
   65535.  Then, after having established the Common Context 61532 times
   in the group, its Gid will assume value '0xb1f05c'.



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   Using an immutable Group Prefix for a group with a Group Manager that
   does not reassign Gid values (see Section 12.2.1.1) limits the total
   number of rekeying instances.  With a Group Manager that does
   reassign Gid values, it limits the maximum active number of rekeying
   instances that a CoAP observation [RFC7641] can persist through.  In
   either case, the group epoch size needs to be chosen depending on the
   expected rate of rekeying instances.

   As discussed in Section 14.6, if endpoints are deployed in multiple
   groups managed by different non-synchronized Group Managers, it is
   possible that Group Identifiers of different groups coincide at some
   point in time.  In this case, a recipient has to handle coinciding
   Group Identifiers, and has to try using different Security Contexts
   to process an incoming message, until the right one is found and the
   message is correctly verified.  Therefore, it is favorable that Group
   Identifiers from different Group Managers have a size that result in
   a small probability of collision.  How small this probability should
   be is up to system designers.

Appendix D.  Responsibilities of the Group Manager

   The Group Manager is responsible for performing the following tasks:

   1.   Creating and managing OSCORE groups.  This includes the
        assignment of a Gid to every newly created group, ensuring
        uniqueness of Gids within the set of its OSCORE groups and,
        optionally, the secure recycling of Gids.

   2.   Defining policies for authorizing the joining of its OSCORE
        groups.

   3.   Handling the join process to add new endpoints as group members.

   4.   Establishing the Common Context part of the Security Context,
        and providing it to authorized group members during the join
        process, together with the corresponding Sender Context.

   5.   Updating the Key Generation Number and the Gid of its OSCORE
        groups, upon renewing the respective Security Context.

   6.   Generating and managing Sender IDs within its OSCORE groups, as
        well as assigning and providing them to new endpoints during the
        join process, or to current group members upon request of
        renewal or re-joining.  This includes ensuring that:

        *  Each Sender ID is unique within each of the OSCORE groups;





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        *  Each Sender ID is not reassigned within the same group since
           the latest time when the current Gid value was assigned to
           the group.  That is, the Sender ID is not reassigned even to
           a current group member re-joining the same group, without a
           rekeying happening first.

   7.   Defining communication policies for each of its OSCORE groups,
        and signaling them to new endpoints during the join process.

   8.   Renewing the Security Context of an OSCORE group upon membership
        change, by revoking and renewing common security parameters and
        keying material (rekeying).

   9.   Providing the management keying material that a new endpoint
        requires to participate in the rekeying process, consistently
        with the key management scheme used in the group joined by the
        new endpoint.

   10.  Assisting a group member that has missed a group rekeying
        instance to understand which authentication credentials and
        Recipient Contexts to delete, as associated with former group
        members.

   11.  Acting as key repository, in order to handle the authentication
        credentials of the members of its OSCORE groups, and providing
        such authentication credentials to other members of the same
        group upon request.  The actual storage of authentication
        credentials may be entrusted to a separate secure storage device
        or service.

   12.  Validating that the format and parameters of authentication
        credentials of group members are consistent with the public key
        algorithm and related parameters used in the respective OSCORE
        group.

   The Group Manager specified in [I-D.ietf-ace-key-groupcomm-oscore]
   provides this functionality.

Appendix E.  Document Updates

   This section is to be removed before publishing as an RFC.

E.1.  Version -27 to -28

   *  Group Encryption Algorithm: admitted AES-CTR and ruled out AES-
      CBC.





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   *  Considerations on unreliable/reliable transports for group key
      management traffic.

   *  Explicit mentioning that group rekeying can also be regular/
      periodic.

   *  Clarifications:

      -  Clearer definition of Signature Encryption Key.

      -  Integrity protection for the Group Encryption Algorithm.

      -  Use and lifetime of static-static Diffie-Hellman keys.

      -  Fixed confusing text on keys used for signing and/or ECDH.

      -  Long exchanges do not actually last "indefinitely".

      -  In CWTs/CSSs, (kty, crv) indicates types of public key
         algorithms (not "fully described" public key algorithms).

      -  Applications can define the maximum number of Recipient
         Contexts.

      -  Possible means for the application to learn about deliberately
         deleted Recipient Contexts.

      -  Clearer requirement and examples on congestion control for
         group key management traffic.

   *  Avoided restatements with normative language.

   *  Avoided unnecessary normative language for parameters of the
      Security Context.

   *  Updated references.

   *  Editorial fixes and improvements.

E.2.  Version -26 to -27

   *  Clarified "SHOULD" for the Group Manager's authentication
      credential having the same format of that of the group members.

   *  Avoid unnecessary details on Sequence Number wrap-around.

   *  Clarified that using unreliable transports must not forego
      congestion control.



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   *  Clarified means for the Group Manager to determine compromised
      group members.

   *  Normative "SHOULD" for preserving current Sender IDs upon group
      rekeying.

   *  Updated security considerations:

      -  Explicit list of security properties of the pairwise mode.

      -  Clarified group-level data confidentiality in the group mode.

      -  Explicit list of security properties not aimed to be met.

      -  Security considerations on treating opaque binary data as such.

   *  Minor clarifications and editorial improvements.

E.3.  Version -25 to -26

   *  Terminology for Security Context: avoid "immutable"; use "long-
      term" and "varying".

   *  Reference on achieving proof of possession for group members and
      Group Manager.

   *  Not only CWTs but also CCSs can be tagged.

   *  Exceptional handling after deleting a Recipient Context.

   *  Clearer handling of incoming messages if the Replay Window is
      invalid.

   *  The exhaustion of Sender Sequence Numbers should be handled with
      margin.

   *  Highlighted overhead for accepting out-of-order responses within a
      long exchange.

   *  Generalization of exceptions to behaviors that are defined as
      SHOULD.

   *  Clearer phrasing for the requirement on early signature
      verification.

   *  Clearer generalization of delivery of messages protected in
      pairwise mode.




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   *  Generalized use of the Block2 Option in protected (group)
      requests.

   *  Clearer statements on implementation compliance.

   *  Suggested means for silent servers to make Replay Windows valid
      again.

   *  Optional procedure for reassigning Gids moved to the document
      body.

   *  Specific definition of reassignment of Sender IDs in a group.

   *  Discussed server-side mitigations against unicast requests
      protected in group mode.

   *  Removed hypothetical discussions on alternative protocol designs.

   *  Discussion on Denial of Service moved to security considerations.

   *  Expanded considerations on leakage of Diffie-Hellman shared
      secret.

   *  Editorial clarifications and fixes.

E.4.  Version -24 to -25

   *  Made RFC 5869 a normative reference.

   *  Removed request to add a note in the "CoAP Option Numbers"
      registry.

   *  More precise phrasing on using the encryption nonce.

E.5.  Version -23 to -24

   *  Added section "Implementation Status", according to RFC 7942.

   *  Fixed "P-521" (instead of "P-512").

E.6.  Version -22 to -23

   *  Examples of non-AEAD algorithms as Group Encryption Algorithm.

   *  Implementation compliance on non-AEAD algorithms.

   *  Clarified that the HKDF Algorithm must be an HMAC-based HKDF.




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   *  Easier description of using Common IV to compute the nonce.

   *  Consistent use of "nonce", "key", and "AEAD key".

   *  Repositioned content about the Group Manager.

   *  Editorial improvements.

E.7.  Version -21 to -22

   *  Removed mentioning of the CBOR encoding of the HKDF Algorithm.

   *  Rephrased consequences on loss of Recipient Contexts.

   *  Removed requirement on 0 as initial value of the Key Generation
      Number.

   *  Improved handling of responses from a server that changes Sender
      ID.

   *  Relax constrictions of Block-wise with group communication.

   *  Removed the concept of synchronization with the Client's Sender
      Sequence Number.

   *  Improved content on Challenge-Response based freshness and Replay
      Window recovery.

   *  Use the acronym CCSs for CWT Claims Sets.

   *  Mentioned wrap-around of the Key Generation Number.

   *  Added IANA consideration on the "CoAP Option Numbers" registry.

   *  Updated references.

   *  Editorial improvements.

E.8.  Version -20 to -21

   *  Updated author list.

   *  Terminology: improved definition of "group request".

   *  Editorial: removed quotation marks when using the CBOR simple
      values true, false, and null.

   *  Editorial: expanded name of the "CoRE Parameters" registry group.



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E.9.  Version -19 to -20

   *  Change Controller for the target attribute "gosc" set to "IETF".

E.10.  Version -18 to -19

   *  Unified presentation of handling of multiple responses.

   *  Added Rikard Höglund as Contributor.

E.11.  Version -17 to -18

   *  Changed document title.

   *  Possible use with CoAP-mappable HTTP.

   *  Added Common Context parameter "Authentication Credential Format".

   *  Renamed "Group Encryption Key" to "Signature Encryption Key".
      Consistent fixes in its derivation.

   *  Renamed "Signature Encryption Algorithm" to "Group Encryption
      Algorithm".

   *  Ensured a single Common IV, also when the two encryption
      algorithms have different nonce sizes.

   *  Guidelines on the Pairwise Key Agreement Algorithm and derivation
      of the Diffie-Hellman secret.

   *  The possible use of a mode follows from the set parameters.

   *  The Group Manager is always present; 'gm_cred' in the external_aad
      cannot be null anymore.

   *  The authentication credential of the Group Manager can have a
      different format than that of the group members'.

   *  Set-up of new endpoints moved to document body.

   *  The encrypted countersignature is a result of the header
      compression, not of COSE.

   *  Revised examples of compressed and non-compressed COSE object.

   *  Removed excessive requirements on group rekeying scheduling.

   *  More considerations on the strictness of group key management.



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   *  Clearer alternatives on retaining an old Security Context.

   *  Revised used of terminology on freshness.

   *  Clarifications, fixes and editorial improvements.

E.12.  Version -16 to -17

   *  Definition and registration of the target attribute "gosc".

   *  Reference update and editorial fixes.

E.13.  Version -15 to -16

   *  Clients "SHOULD" use the group mode for one-to-many requests.

   *  Handling of multiple non-notification responses.

   *  Revised presentation of security properties.

   *  Improved listing of operations defined for the group mode that are
      inherited by the pairwise mode.

   *  Editorial improvements.

E.14.  Version -14 to -15

   *  Updated references and editorial fixes.

E.15.  Version -13 to -14

   *  Replaced "node" with "endpoint" where appropriate.

   *  Replaced "owning" with "storing" (of keying material).

   *  Distinction between "authentication credential" and "public key".

   *  Considerations on storing whole authentication credentials.

   *  Considerations on Denial of Service.

   *  Recycling of Group IDs by tracking the "Birth Gid" of each group
      member is now optional to support and use for the Group Manager.

   *  Fine-grained suppression of error responses.

   *  Changed section title "Mandatory-to-Implement Compliance
      Requirements" to "Implementation Compliance".



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   *  "Challenge-Response Synchronization" moved to the document body.

   *  RFC 7641 and draft-ietf-core-echo-request-tag as normative
      references.

   *  Clarifications and editorial improvements.

E.16.  Version -12 to -13

   *  Fixes in the derivation of the Group Encryption Key.

   *  Added Mandatory-to-Implement compliance requirements.

   *  Changed UCCS to CCS.

E.17.  Version -11 to -12

   *  No mode of operation is mandatory to support.

   *  Revised parameters of the Security Context, COSE object and
      external_aad.

   *  Revised management of keying material for the Group Manager.

   *  Informing of former members when rekeying the group.

   *  Admit encryption-only algorithms in group mode.

   *  Encrypted countersignature through a keystream.

   *  Added public key of the Group Manager as key material and
      protected data.

   *  Clarifications about message processing, especially notifications.

   *  Guidance for message processing of external signature checkers.

   *  Updated derivation of pairwise keys, with more security
      considerations.

   *  Termination of ongoing observations as client, upon leaving or
      before re-joining the group.

   *  Recycling Group IDs by tracking the "Birth Gid" of each group
      member.

   *  Expanded security and privacy considerations about the group mode.




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   *  Removed appendices on skipping signature verification and on COSE
      capabilities.

   *  Fixes and editorial improvements.

E.18.  Version -10 to -11

   *  Loss of Recipient Contexts due to their overflow.

   *  Added diagram on keying material components and their relation.

   *  Distinction between anti-replay and freshness.

   *  Preservation of Sender IDs over rekeying.

   *  Clearer cause-effect about reset of SSN.

   *  The GM provides public keys of group members with associated
      Sender IDs.

   *  Removed 'par_countersign_key' from the external_aad.

   *  One single format for the external_aad, both for encryption and
      signing.

   *  Presence of 'kid' in responses to requests protected in pairwise
      mode.

   *  Inclusion of 'kid_context' in notifications following a group
      rekeying.

   *  Pairwise mode presented with OSCORE as baseline.

   *  Revised examples with signature values.

   *  Decoupled growth of clients' Sender Sequence Numbers and loss of
      synchronization for server.

   *  Sender IDs not recycled in the group under the same Gid.

   *  Processing and description of the Group Flag bit in the OSCORE
      Option.

   *  Usage of the pairwise mode for multicast requests.

   *  Clarifications on synchronization using the Echo Option.





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   *  General format of context parameters and external_aad elements,
      supporting future registered COSE algorithms (new Appendix).

   *  Fixes and editorial improvements.

E.19.  Version -09 to -10

   *  Removed 'Counter Signature Key Parameters' from the Common
      Context.

   *  New parameters in the Common Context covering the DH secret
      derivation.

   *  New countersignature header parameter from draft-ietf-cose-
      countersign.

   *  Stronger policies non non-recycling of Sender IDs and Gid.

   *  The Sender Sequence Number is reset when establishing a new
      Security Context.

   *  Added 'request_kid_context' in the aad_array.

   *  The server can respond with 5.03 if the client's public key is not
      available.

   *  The observer client stores an invariant identifier of the group.

   *  Relaxed storing of original 'kid' for observer clients.

   *  Both client and server store the 'kid_context' of the original
      observation request.

   *  The server uses a fresh PIV if protecting the response with a
      Security Context different from the one used to protect the
      request.

   *  Clarifications on MTI algorithms and curves.

   *  Removed optimized requests.

   *  Overall clarifications and editorial revision.

E.20.  Version -08 to -09

   *  Pairwise keys are discarded after group rekeying.

   *  Signature mode renamed to group mode.



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   *  The parameters for countersignatures use the updated COSE
      registries.  Newly defined IANA registries have been removed.

   *  Pairwise Flag bit renamed as Group Flag bit, set to 1 in group
      mode and set to 0 in pairwise mode.

   *  Dedicated section on updating the Security Context.

   *  By default, sender sequence numbers and replay windows are not
      reset upon group rekeying.

   *  An endpoint implementing only a silent server does not support the
      pairwise mode.

   *  Separate section on general message reception.

   *  Pairwise mode moved to the document body.

   *  Considerations on using the pairwise mode in non-multicast
      settings.

   *  Optimized requests are moved as an appendix.

   *  Normative support for the signature and pairwise mode.

   *  Revised methods for synchronization with clients' sender sequence
      number.

   *  Appendix with example values of parameters for countersignatures.

   *  Clarifications and editorial improvements.

E.21.  Version -07 to -08

   *  Clarified relation between pairwise mode and group communication
      (Section 1).

   *  Improved definition of "silent server" (Section 1.1).

   *  Clarified when a Recipient Context is needed (Section 2).

   *  Signature checkers as entities supported by the Group Manager
      (Section 2.3).

   *  Clarified that the Group Manager is under exclusive control of Gid
      and Sender ID values in a group, with Sender ID values under each
      Gid value (Section 2.3).




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   *  Mitigation policies in case of recycled 'kid' values
      (Section 2.4).

   *  More generic exhaustion (not necessarily wrap-around) of sender
      sequence numbers (Sections 2.5 and 10.11).

   *  Pairwise key considerations, as to group rekeying and Sender
      Sequence Numbers (Section 3).

   *  Added reference to static-static Diffie-Hellman shared secret
      (Section 3).

   *  Note for implementation about the external_aad for signing
      (Sectino 4.3.2).

   *  Retransmission by the application for group requests over
      multicast as Non-confirmable (Section 7).

   *  A server MUST use its own Partial IV in a response, if protecting
      it with a different context than the one used for the request
      (Section 7.3).

   *  Security considerations: encryption of pairwise mode as
      alternative to group-level security (Section 10.1).

   *  Security considerations: added approach to reduce the chance of
      global collisions of Gid values from different Group Managers
      (Section 10.5).

   *  Security considerations: added implications for block-wise
      transfers when using the signature mode for requests over unicast
      (Section 10.7).

   *  Security considerations: (multiple) supported signature algorithms
      (Section 10.13).

   *  Security considerations: added privacy considerations on the
      approach for reducing global collisions of Gid values
      (Section 10.15).

   *  Updates to the methods for synchronizing with clients' sequence
      number (Appendix E).

   *  Simplified text on discovery services supporting the pairwise mode
      (Appendix G.1).

   *  Editorial improvements.




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E.22.  Version -06 to -07

   *  Updated abstract and introduction.

   *  Clarifications of what pertains a group rekeying.

   *  Derivation of pairwise keying material.

   *  Content re-organization for COSE Object and OSCORE header
      compression.

   *  Defined the Pairwise Flag bit for the OSCORE Option.

   *  Supporting CoAP Observe for group requests and responses.

   *  Considerations on message protection across switching to new
      keying material.

   *  New optimized mode based on pairwise keying material.

   *  More considerations on replay protection and Security Contexts
      upon key renewal.

   *  Security considerations on Group OSCORE for unicast requests, also
      as affecting the usage of the Echo Option.

   *  Clarification on different types of groups considered
      (application/security/CoAP).

   *  New pairwise mode, using pairwise keying material for both
      requests and responses.

E.23.  Version -05 to -06

   *  Group IDs mandated to be unique under the same Group Manager.

   *  Clarifications on parameter update upon group rekeying.

   *  Updated external_aad structures.

   *  Dynamic derivation of Recipient Contexts made optional and
      application specific.

   *  Optional 4.00 response for failed signature verification on the
      server.

   *  Removed client handling of duplicated responses to multicast
      requests.



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   *  Additional considerations on public key retrieval and group
      rekeying.

   *  Added Group Manager responsibility on validating public keys.

   *  Updates IANA registries.

   *  Reference to RFC 8613.

   *  Editorial improvements.

E.24.  Version -04 to -05

   *  Added references to draft-dijk-core-groupcomm-bis.

   *  New parameter Counter Signature Key Parameters (Section 2).

   *  Clarification about Recipient Contexts (Section 2).

   *  Two different external_aad for encrypting and signing
      (Section 3.1).

   *  Updated response verification to handle Observe notifications
      (Section 6.4).

   *  Extended Security Considerations (Section 8).

   *  New "Counter Signature Key Parameters" IANA Registry
      (Section 9.2).

E.25.  Version -03 to -04

   *  Added the new "Counter Signature Parameters" in the Common Context
      (see Section 2).

   *  Added recommendation on using "deterministic ECDSA" if ECDSA is
      used as countersignature algorithm (see Section 2).

   *  Clarified possible asynchronous retrieval of keying material from
      the Group Manager, in order to process incoming messages (see
      Section 2).

   *  Structured Section 3 into subsections.

   *  Added the new 'par_countersign' to the aad_array of the
      external_aad (see Section 3.1).





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   *  Clarified non reliability of 'kid' as identity identifier for a
      group member (see Section 2.1).

   *  Described possible provisioning of new Sender ID in case of
      Partial IV wrap-around (see Section 2.2).

   *  The former signature bit in the Flag Byte of the OSCORE Option
      value is reverted to reserved (see Section 4.1).

   *  Updated examples of compressed COSE object, now with the sixth
      less significant bit in the Flag Byte of the OSCORE Option value
      set to 0 (see Section 4.3).

   *  Relaxed statements on sending error messages (see Section 6).

   *  Added explicit step on computing the countersignature for outgoing
      messages (see Sections 6.1 and 6.3).

   *  Handling of just created Recipient Contexts in case of
      unsuccessful message verification (see Sections 6.2 and 6.4).

   *  Handling of replied/repeated responses on the client (see
      Section 6.4).

   *  New IANA Registry "Counter Signature Parameters" (see
      Section 9.1).

E.26.  Version -02 to -03

   *  Revised structure and phrasing for improved readability and better
      alignment with draft-ietf-core-object-security.

   *  Added discussion on wrap-Around of Partial IVs (see Section 2.2).

   *  Separate sections for the COSE Object (Section 3) and the OSCORE
      Header Compression (Section 4).

   *  The countersignature is now appended to the encrypted payload of
      the OSCORE message, rather than included in the OSCORE Option (see
      Section 4).

   *  Extended scope of Section 5, now titled " Message Binding,
      Sequence Numbers, Freshness and Replay Protection".

   *  Clarifications about Non-confirmable messages in Section 5.1
      "Synchronization of Sender Sequence Numbers".





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   *  Clarifications about error handling in Section 6 "Message
      Processing".

   *  Compacted list of responsibilities of the Group Manager in
      Section 7.

   *  Revised and extended security considerations in Section 8.

   *  Added IANA considerations for the OSCORE Flag Bits Registry in
      Section 9.

   *  Revised Appendix D, now giving a short high-level description of a
      new endpoint set-up.

E.27.  Version -01 to -02

   *  Terminology has been made more aligned with RFC7252 and draft-
      ietf-core-object-security: i) "client" and "server" replace the
      old "multicaster" and "listener", respectively; ii) "silent
      server" replaces the old "pure listener".

   *  Section 2 has been updated to have the Group Identifier stored in
      the 'ID Context' parameter defined in draft-ietf-core-object-
      security.

   *  Section 3 has been updated with the new format of the Additional
      Authenticated Data.

   *  Major rewriting of Section 4 to better highlight the differences
      with the message processing in draft-ietf-core-object-security.

   *  Added Sections 7.2 and 7.3 discussing security considerations
      about uniqueness of (key, nonce) and collision of group
      identifiers, respectively.

   *  Minor updates to Appendix A.1 about assumptions on multicast
      communication topology and group size.

   *  Updated Appendix C on format of group identifiers, with practical
      implications of possible collisions of group identifiers.

   *  Updated Appendix D.2, adding a pointer to draft-palombini-ace-key-
      groupcomm about retrieval of nodes' public keys through the Group
      Manager.

   *  Minor updates to Appendix E.3 about Challenge-Response
      synchronization of sequence numbers based on the Echo Option from
      draft-ietf-core-echo-request-tag.



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E.28.  Version -00 to -01

   *  Section 1.1 has been updated with the definition of group as
      "security group".

   *  Section 2 has been updated with:

      -  Clarifications on establishment/derivation of Security
         Contexts.

      -  A table summarizing the the additional context elements
         compared to OSCORE.

   *  Section 3 has been updated with:

      -  Examples of request and response messages.

      -  Use of CounterSignature0 rather than CounterSignature.

      -  Additional Authenticated Data including also the signature
         algorithm, while not including the Group Identifier any longer.

   *  Added Section 6, listing the responsibilities of the Group
      Manager.

   *  Added Appendix A (former section), including assumptions and
      security objectives.

   *  Appendix B has been updated with more details on the use cases.

   *  Added Appendix C, providing an example of Group Identifier format.

   *  Appendix D has been updated to be aligned with draft-palombini-
      ace-key-groupcomm.

Acknowledgments

   Jiye Park contributed as a co-author of initial versions of this
   document.

   The authors sincerely thank Christian Amsüss, Stefan Beck, Mike
   Bishop, Rolf Blom, Carsten Bormann, Mohamed Boucadair, Deb Cooley,
   Esko Dijk, Gorry Fairhurst, Patrik Fältström, Martin Gunnarsson,
   Klaus Hartke, Richard Kelsey, Paul Kyzivat, Joerg Ott, Dave Robin,
   Jim Schaad, Ludwig Seitz, Orie Steele, Peter van der Stok, Ketan
   Talaulikar, Erik Thormarker, Mališa Vučinić, and Paul Wouters for
   their feedback and comments.




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   The work on this document has been partly supported by the Sweden's
   Innovation Agency VINNOVA and the Celtic-Next projects CRITISEC and
   CYPRESS; the H2020 projects SIFIS-Home (Grant agreement 952652) and
   ARCADIAN-IoT (Grant agreement 101020259); the SSF project SEC4Factory
   under the grant RIT17-0032; and the EIT-Digital High Impact
   Initiative ACTIVE.

Authors' Addresses

   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   SE-16440 Stockholm Kista
   Sweden
   Email: marco.tiloca@ri.se


   Göran Selander
   Ericsson AB
   Torshamnsgatan 23
   SE-16440 Stockholm Kista
   Sweden
   Email: goran.selander@ericsson.com


   Francesca Palombini
   Ericsson AB
   Torshamnsgatan 23
   SE-16440 Stockholm Kista
   Sweden
   Email: francesca.palombini@ericsson.com


   John Preuß Mattsson
   Ericsson AB
   Torshamnsgatan 23
   SE-16440 Stockholm Kista
   Sweden
   Email: john.mattsson@ericsson.com


   Rikard Höglund
   RISE AB
   Isafjordsgatan 22
   SE-16440 Stockholm Kista
   Sweden
   Email: rikard.hoglund@ri.se




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