



ACE Working Group                                              M. Tiloca
Internet-Draft                                                   RISE AB
Updates: 9202 (if approved)                            J. Preuß Mattsson
Intended status: Standards Track                             Ericsson AB
Expires: 11 July 2026                                     7 January 2026


   Additional Formats of Authentication Credentials for the Datagram
     Transport Layer Security (DTLS) Profile for Authentication and
            Authorization for Constrained Environments (ACE)
                draft-ietf-ace-authcred-dtls-profile-03

Abstract

   This document updates the Datagram Transport Layer Security (DTLS)
   profile for Authentication and Authorization for Constrained
   Environments (ACE).  In particular, it specifies the use of
   additional formats of authentication credentials for establishing a
   DTLS session, when peer authentication is based on asymmetric
   cryptography.  Therefore, this document updates RFC 9202.  What is
   defined in this document is seamlessly applicable also if the profile
   uses Transport Layer Security (TLS) instead, as defined in RFC 9430.

Discussion Venues

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

   Discussion of this document takes place on the Authentication and
   Authorization for Constrained Environments Working Group mailing list
   (ace@ietf.org), which is archived at
   https://mailarchive.ietf.org/arch/browse/ace/.

   Source for this draft and an issue tracker can be found at
   https://github.com/ace-wg/ace-authcred-dtls-profile.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.







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   Internet-Drafts are draft documents valid for a maximum of six months
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   time.  It is inappropriate to use Internet-Drafts as reference
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   This Internet-Draft will expire on 11 July 2026.

Copyright Notice

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   document authors.  All rights reserved.

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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  Updates to the RPK Mode . . . . . . . . . . . . . . . . . . .   6
     2.1.  Raw Public Keys as CCSs . . . . . . . . . . . . . . . . .   6
       2.1.1.  Examples  . . . . . . . . . . . . . . . . . . . . . .   8
     2.2.  Raw Public Keys as COSE_Keys Identified by Reference  . .   9
       2.2.1.  Examples  . . . . . . . . . . . . . . . . . . . . . .  10
   3.  Certificate Mode  . . . . . . . . . . . . . . . . . . . . . .  11
     3.1.  Examples  . . . . . . . . . . . . . . . . . . . . . . . .  14
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  22
   Appendix A.  Examples with Hybrid Settings  . . . . . . . . . . .  23
     A.1.  RPK Mode (Raw Public Keys of Different Formats) . . . . .  23
     A.2.  Certificate Mode (Certificates of Different Formats)  . .  24
     A.3.  Combination of RPK Mode and Certificate Mode  . . . . . .  27
   Appendix B.  CDDL Model . . . . . . . . . . . . . . . . . . . . .  31
   Appendix C.  Document Updates . . . . . . . . . . . . . . . . . .  31
     C.1.  Version -02 to -03  . . . . . . . . . . . . . . . . . . .  31
     C.2.  Version -01 to -02  . . . . . . . . . . . . . . . . . . .  31
     C.3.  Version -00 to -01  . . . . . . . . . . . . . . . . . . .  32
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  32
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  32



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1.  Introduction

   The Authentication and Authorization for Constrained Environments
   (ACE) framework [RFC9200] defines an architecture to enforce access
   control for constrained devices.  A client (C) requests an evidence
   of granted permissions from an authorization server (AS) in the form
   of an access token, then uploads the access token to the target
   resource server (RS), and finally accesses protected resources at the
   RS according to what is specified in the access token.

   The framework has as main building blocks the OAuth 2.0 framework
   [RFC6749], the Constrained Application Protocol (CoAP) [RFC7252] for
   message transfer, Concise Binary Object Representation (CBOR)
   [RFC8949] for compact encoding, and CBOR Object Signing and
   Encryption (COSE) [RFC9052][RFC9053] for self-contained protection of
   access tokens.

   Separate profile documents define in detail how the participants in
   the ACE architecture communicate, especially as to the security
   protocols that they use.  In particular, the ACE profile defined in
   [RFC9202] specifies how Datagram Transport Layer Security (DTLS)
   [RFC6347][RFC9147] is used to protect communications with transport-
   layer security in the ACE architecture.  The profile has been
   extended in [RFC9430], in order to allow the alternative use of
   Transport Layer Security (TLS) [RFC8446] when CoAP is transported
   over TCP or WebSockets [RFC8323].

   The DTLS profile defined in [RFC9202] allows C and the RS to
   establish a DTLS session with peer authentication based on symmetric
   or asymmetric cryptography.  For the latter case, the profile defines
   a Raw Public Key (RPK) mode (see Section 3.2 of [RFC9202]), where
   authentication relies on the public keys of the two peers as raw
   public keys [RFC7250].

   That is, C specifies its public key to the AS when requesting an
   access token and the AS provides such public key to the target RS as
   included in the issued access token.  When issuing the access token,
   the AS also provides C with the public key of the RS.  Then, C and
   the RS use their asymmetric keys when performing the DTLS handshake,
   as defined in [RFC7250].

   Per [RFC9202], the DTLS profile admits only a COSE_Key object
   [RFC9052] as the format of authentication credentials to use for
   specifying the public keys of C and the RS as raw public keys.
   However, it is desirable that additional formats of authentication
   credentials can be used, such as enhanced raw public keys or public
   key certificates.




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   This document enables the use of such additional formats in the DTLS
   profile, by defining how the public keys of C and the RS can be
   specified by means of CBOR Web Token (CWT) Claims Sets (CCSs)
   [RFC8392], X.509 certificates [RFC5280], or C509 certificates
   [I-D.ietf-cose-cbor-encoded-cert].

   This document also enables the DTLS profile to use the CWT
   Confirmation Method 'ckt' defined in [RFC9679] when using a COSE_Key
   object for specifying a raw public key, thus allowing to identifying
   the COSE_Key object by reference alternatively to transporting it by
   value.

   In particular, this document updates [RFC9202] as follows.

   *  Section 2 of this document extends the RPK mode defined in
      Section 3.2 of [RFC9202], by enabling:

      -  The use of CCSs to wrap the raw public keys of C and the RS,
         i.e., as a new format of authentication credentials that can be
         used for specifying the public keys of C and the RS as raw
         public keys (see Section 2.1).

      -  The use of the CWT Confirmation Method 'ckt' to identify a
         COSE_Key object by reference, when that is the format of
         authentication credentials used for specifying the public keys
         of C and the RS as raw public keys (see Section 2.2).

   *  Section 3 of this document defines a new certificate mode, which
      enables the use of X.509 or C509 certificates to specify the
      public keys of C and the RS.  In either case, certificates can be
      transported by value or instead identified by reference.

   When using the updated RPK mode, the raw public keys of C and the RS
   do not have to be of the same format.  That is, it is possible to
   have both public keys as a COSE_Key object or as a CCS, or instead
   one as a COSE_Key object while the other one as a CCS.  When both raw
   public keys are COSE_Keys, it is possible to have both COSE_Keys
   transported by value, or both identified by reference, or one
   transported by value while the other one identified by reference.

   When using the certificate mode, the certificates of C and the RS do
   not have to be of the same format.  That is, it is possible to have
   both as X.509 certificates, or both as C509 certificates, or one as
   an X.509 certificate while the other one as a C509 certificate.
   Furthermore, it is possible to have both certificates transported by
   value, or both identified by reference, or one transported by value
   while the other one identified by reference.




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   Also, the RPK mode and the certificate mode can be combined.  That
   is, it is possible that one of the two authentication credentials is
   a certificate, while the other one is a raw public key.

   The effective provisioning of an authentication credential identified
   by reference builds on the assumption that the recipient is storing
   the authentication credential by value, or is able to retrieve it
   from a trusted source by means of the reference obtained.  If that
   assumption does not hold, the authentication credential will have to
   be provided by value.

   The decision about whether providing authentication credentials by
   value or by reference depending on the specific situation is left to
   application policies at C and the AS.  Furthermore, C and AS could
   explicitly coordinate with each other about exchanging the
   authentication credentials of C and the RS as transported by value or
   instead identified by reference, e.g., by relying on the coordination
   method defined in [I-D.ietf-ace-workflow-and-params].

   When using the formats introduced in this document, authentication
   credentials are specified by means of the CWT Confirmation Methods
   "kccs", "x5bag", "x5chain", "x5t", "x5u", "c5b", "c5c", "c5t", and
   "c5u" that are defined in [I-D.ietf-ace-edhoc-oscore-profile].

   What is defined in this document is seamlessly applicable if TLS is
   used instead, as defined in [RFC9430].

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 the ACE framework for Authentication and Authorization
   [RFC9200][RFC9201] and in its DTLS profile [RFC9202], as well as with
   terms and concepts related to CBOR Web Tokens (CWTs) [RFC8392] and
   CWT Confirmation Methods [RFC8747].

   The terminology for entities in the considered architecture is
   defined in OAuth 2.0 [RFC6749].  In particular, this includes client
   (C), resource server (RS), and authorization server (AS).







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   Readers are also expected to be familiar with the terms and concepts
   related to CoAP [RFC7252], CBOR [RFC8949], Concise Data Definition
   Language (CDDL) [RFC8610], COSE [RFC9052][RFC9053], DTLS
   [RFC6347][RFC9147], and the use of raw public keys in DTLS [RFC7250].

   Note that the term "endpoint" is used here following its OAuth
   definition [RFC6749], aimed at denoting resources such as /token and
   /introspect at the AS, and /authz-info at the RS.  The CoAP
   definition, which is "[a]n entity participating in the CoAP protocol"
   [RFC7252], is not used in this document.

   This document also refers to the term "authentication credential",
   which denotes the information associated with an entity, including
   that entity's public key and parameters associated with the public
   key.  Examples of authentication credentials are CWT Claims Sets
   (CCSs) [RFC8392], X.509 certificates [RFC5280], and C509 certificates
   [I-D.ietf-cose-cbor-encoded-cert].

   Examples throughout this document are expressed in CBOR diagnostic
   notation as defined in Section 8 of [RFC8949] and Appendix G of
   [RFC8610].  Diagnostic notation comments are often used to provide a
   textual representation of the parameters' keys and values.

   In the CBOR diagnostic notation used in this document, constructs of
   the form e'SOME_NAME' are replaced by the value assigned to SOME_NAME
   in the CDDL model shown in Figure 19 of Appendix B.  For example,
   {e'x5chain' : h'3081...cb02'} stands for {24 : h'3081...cb02'}.

   Note to RFC Editor: Please delete the paragraph immediately preceding
   this note.  Also, in the CBOR diagnostic notation used in this
   document, please replace the constructs of the form e'SOME_NAME' with
   the value assigned to SOME_NAME in the CDDL model shown in Figure 19
   of Appendix B.  Finally, please delete this note.

2.  Updates to the RPK Mode

   This section updates the RPK mode defined in Section 3.2 of
   [RFC9202], as detailed in the following Section 2.1 and Section 2.2.

2.1.  Raw Public Keys as CCSs

   This section defines how the raw public key of C and the RS can be
   provided as wrapped by a CCS [RFC8392], instead of as a COSE_Key
   object [RFC9052].  Note that only the differences from [RFC9202] are
   compiled below.

   If the raw public key of C is wrapped by a CCS, then the following
   applies.



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   *  The payload of the Access Token Request (see Section 5.8.1 of
      [RFC9200]) is as defined in Section 3.2.1 of [RFC9202], with the
      difference that the "req_cnf" parameter [RFC9201] MUST contain a
      "kccs" structure, with value a CCS specifying the public key of C
      that has to be bound to the access token.

      In particular, the CCS MUST include the "cnf" claim specifying the
      public key of C as a COSE_Key object, SHOULD include the "sub"
      claim specifying the subject name of C associated with the public
      key of C, and MAY include additional claims.

   *  The content of the access token that the AS provides to C in the
      Access Token Response (see Section 5.8.2 of [RFC9200]) is as
      defined in Section 3.2.1 of [RFC9202], with the difference that
      the "cnf" claim of the access token MUST contain a "kccs"
      structure, with value a CCS specifying the public key of C that is
      bound to the access token.

      In particular, the CCS MUST include the "cnf" claim specifying the
      public key of C as a COSE_Key object, SHOULD include the "sub"
      claim specifying the subject name of C associated with the public
      key of C, and MAY include additional claims.

   If the raw public key of the RS is wrapped by a CCS, then the
   following applies.

   *  The payload of the Access Token Response is as defined in
      Section 3.2.1 of [RFC9202], with the difference that the "rs_cnf"
      parameter [RFC9201] MUST contain a "kccs" structure, with value a
      CCS specifying the public key of the RS.

      In particular, the CCS MUST include the "cnf" claim specifying the
      public key of the RS as a COSE_Key object, SHOULD include the
      "sub" claim specifying the subject name of the RS associated with
      the public key of the RS, and MAY include additional claims.

   For the "req_cnf" parameter of the Access Token Request, the "rs_cnf"
   parameter of the Access Token Response, and the "cnf" claim of the
   access token, the Confirmation Method "kccs" structure and its
   identifier are defined in [I-D.ietf-ace-edhoc-oscore-profile].

   It is not required that both public keys are wrapped by a CCS.  That
   is, one of the two authentication credentials can be a CCS, while the
   other one can be a COSE_Key object transported by value as per
   Section 3.2 of [RFC9202] or identified by reference as per
   Section 2.2 of this document.





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2.1.1.  Examples

   Figure 1 shows an example of Access Token Request from C to the AS.

      POST coaps://as.example.com/token
      Content-Format: 19 (application/ace+cbor)
      Payload:
      {
        / grant_type / 33 : 2 / client_credentials /,
        / audience /    5 : "tempSensor1",
        / req_cnf /     4 : {
          e'kccs' : {
            / sub / 2 : "42-50-31-FF-EF-37-32-39",
            / cnf / 8 : {
              / COSE_Key / 1 : {
                / kty /    1 : 2 / EC2 /,
                / crv /   -1 : 1 / P-256 /,
                / x /     -2 : h'd7cc072de2205bdc1537a543d53c60a6
                                 acb62eccd890c7fa27c9e354089bbe13',
                / y /     -3 : h'f95e1d4b851a2cc80fff87d8e23f22af
                                 b725d535e515d020731e79a3b4e47120'
              }
            }
          }
        }
      }

       Figure 1: Access Token Request Example for RPK Mode, with the
         Public Key of C Wrapped by a CCS Conveyed within "req_cnf"

   Figure 2 shows an example of Access Token Response from the AS to C.




















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      2.01 Created
      Content-Format: 19 (application/ace+cbor)
      Max-Age: 3560
      Payload:
      {
        / access_token / 1 : h'd83dd083...643b',
          / (remainder of CWT omitted for brevity;
          CWT contains the client's RPK in the cnf claim) /
        / expires_in /   2 : 3600,
        / rs_cnf /      41 : {
          e'kccs' : {
            / sub / 2 : "AA-BB-CC-00-01-02-03-04",
            / cnf / 8 : {
              / COSE_Key / 1 : {
                / kty /  1 : 2 / EC2 /,
                / crv / -1 : 1 / P-256 /,
                / x /   -2 : h'bbc34960526ea4d32e940cad2a234148
                               ddc21791a12afbcbac93622046dd44f0',
                / y /   -3 : h'4519e257236b2a0ce2023f0931f1f386
                               ca7afda64fcde0108c224c51eabf6072'
              }
            }
          }
        }
      }

       Figure 2: Access Token Response Example for RPK Mode, with the
      Public Key of the RS Wrapped by a CCS, Conveyed within "rs_cnf"

2.2.  Raw Public Keys as COSE_Keys Identified by Reference

   As per Section 3.2 of [RFC9202], COSE_Key objects [RFC9052] used for
   specifying raw public keys are transported by value in the Access
   Token Request and Response messages, as well as within access tokens.

   This section extends the DTLS profile by allowing to identifying
   those COSE_Key objects by reference, alternatively to transporting
   those by value.  Note that only the differences from [RFC9202] are
   compiled below.

   The following relies on the CWT Confirmation Method 'ckt' defined in
   [RFC9679].  When using a 'ckt' structure, this conveys the thumbprint
   of a COSE_Key object computed as per Section 3 of [RFC9679].  In
   particular, the hash function used MUST be SHA-256 [SHA-256], which
   is mandatory to support when supporting COSE Key thumbprints.






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   If the raw public key of C is specified as a COSE_Key object
   COSE_KEY_C and the intent is to identify it by reference, then the
   following applies.

   *  The payload of the Access Token Request (see Section 5.8.1 of
      [RFC9200]) is as defined in Section 3.2.1 of [RFC9202], with the
      difference that the "req_cnf" parameter [RFC9201] MUST contain a
      "ckt" structure, with value the thumbprint of COSE_KEY_C.

   *  The content of the access token that the AS provides to C in the
      Access Token Response (see Section 5.8.2 of [RFC9200]) is as
      defined in Section 3.2.1 of [RFC9202], with the difference that
      the "cnf" claim of the access token MUST contain a "ckt"
      structure, with value the thumbprint of COSE_KEY_C.

   If the raw public key of the RS is specified as a COSE_Key object
   COSE_KEY_RS and the intent is to identify it by reference, then the
   following applies.

   *  The payload of the Access Token Response is as defined in
      Section 3.2.1 of [RFC9202], with the difference that the "rs_cnf"
      parameter [RFC9201] MUST contain a "ckt" structure, with value the
      thumbprint of COSE_KEY_RS.

   When both public keys are specified as COSE_Key objects, it is
   possible to have both transported by value, or both identified by
   reference, or one transported by value while the other one identified
   by reference.

   Note that the use of COSE Key thumbprints per [RFC9679] is applicable
   only to authentication credentials that are COSE_Key objects.  That
   is, the 'ckt' structure MUST NOT be used to identify authentication
   credentials of other formats and that include a COSE_Key object as
   part of their content, such as CCSs as defined in Section 2.1 of this
   document.

2.2.1.  Examples

   Figure 3 shows an example of Access Token Request from C to the AS.












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      POST coaps://as.example.com/token
      Content-Format: 19 (application/ace+cbor)
      Payload:
      {
        / grant_type / 33 : 2 / client_credentials /,
        / audience /    5 : "tempSensor2",
        / req_cnf /     4 : {
          / ckt / 5 : h'd3550f1b5b763ee09d058fc7aef69900
                        1279903a4a15bdc3953d32b10f7cb8b1'
        }
      }

       Figure 3: Access Token Request Example for RPK Mode, with the
        Public Key of C Specified as a COSE_Key Object Identified by
                         Reference within "req_cnf"

   Figure 4 shows an example of Access Token Response from the AS to C.

      2.01 Created
      Content-Format: 19 (application/ace+cbor)
      Max-Age: 3560
      Payload:
      {
        / access_token / 1 : h'd83dd083...5532',
          / (remainder of CWT omitted for brevity;
          CWT contains the client's RPK in the cnf claim) /
        / expires_in /   2 : 3600,
        / rs_cnf /      41 : {
          / ckt / 5 : h'db60f4d371fffac3e1040566154a5c36
                        1e0bf835a4ad4c58069cf6edc9ac58a3'
        }
      }

       Figure 4: Access Token Response Example for RPK Mode, with the
     Public Key of the RS Specified as a COSE_Key Object Identified by
                         Reference within "rs_cnf"

3.  Certificate Mode

   This section defines a new certificate mode of the DTLS profile,
   which enables the use of public key certificates to specify the
   public keys of C and the RS.  Compared to the RPK mode defined in
   Section 3.2 of [RFC9202] and extended in Section 2 of this document,
   the certificate mode displays the differences compiled below.

   The authentication credential of C and/or of the RS is a public key
   certificate, i.e., an X.509 certificate [RFC5280] or a C509
   certificate [I-D.ietf-cose-cbor-encoded-cert].



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   *  The CWT Confirmation Methods "x5chain", "x5bag", "c5c", and "c5b"
      defined in [I-D.ietf-ace-edhoc-oscore-profile] are used to
      transport such authentication credentials by value.

   *  The CWT Confirmation Methods "x5t", "x5u", "c5t", and "c5u"
      defined in [I-D.ietf-ace-edhoc-oscore-profile] are used to
      identify such authentication credentials by reference.

   If the authentication credential AUTH_CRED_C of C is a public key
   certificate, then the following applies.

   *  The "req_cnf" parameter [RFC9201] of the Access Token Request (see
      Section 5.8.1 of [RFC9200]) specifies AUTH_CRED_C as follows.

      If AUTH_CRED_C is an X.509 certificate, the "req_cnf" parameter
      MUST contain:

      -  An "x5chain" or "x5bag" structure, in case AUTH_CRED_C is
         transported by value within a certificate chain or a
         certificate bag, respectively; or

      -  An "x5t" or "x5u" structure, in case AUTH_CRED_C is identified
         by reference through a hash value (a thumbprint) or a URI
         [RFC3986], respectively.

      If AUTH_CRED_C is a C509 certificate, the "req_cnf" parameter MUST
      contain:

      -  A "c5c" or "c5b" structure, in case AUTH_CRED_C is transported
         by value within a certificate chain or a certificate bag,
         respectively; or

      -  A "c5t" or "c5u" structure, in case AUTH_CRED_C is identified
         by reference through a hash value (a thumbprint) or a URI
         [RFC3986], respectively.

   *  The "cnf" claim of the access token that the AS provides to C in
      the Access Token Response (see Section 5.8.2 of [RFC9200])
      specifies AUTH_CRED_C as follows.

      If AUTH_CRED_C is an X.509 certificate, the "cnf" claim MUST
      contain:

      -  An "x5chain" or "x5bag" structure, in case AUTH_CRED_C is
         transported by value within a certificate chain or a
         certificate bag, respectively; or





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      -  An "x5t" or "x5u" structure, in case AUTH_CRED_C is identified
         by reference through a hash value (a thumbprint) or a URI
         [RFC3986], respectively.

      If AUTH_CRED_C is a C509 certificate, the "cnf" claim MUST
      contain:

      -  A "c5c" or "c5b" structure, in case AUTH_CRED_C is transported
         by value within a certificate chain or a certificate bag,
         respectively; or

      -  A "c5t" or "c5u" structure, in case AUTH_CRED_C is identified
         by reference through a hash value (a thumbprint) or a URI
         [RFC3986], respectively.

   If the authentication credential AUTH_CRED_RS of the RS is a public
   key certificate, then the following applies.

   *  The "rs_cnf" parameter [RFC9201] of the Access Token Response
      specifies AUTH_CRED_RS as follows.

      If AUTH_CRED_RS is an X.509 certificate, the "rs_cnf" parameter
      MUST contain:

      -  An "x5chain" or "x5bag" structure, in case AUTH_CRED_RS is
         transported by value within a certificate chain or a
         certificate bag, respectively; or

      -  An "x5t" or "x5u" structure, in case AUTH_CRED_RS is identified
         by reference through a hash value (a thumbprint) or a URI
         [RFC3986], respectively.

      If AUTH_CRED_RS is a C509 certificate, the "rs_cnf" parameter MUST
      contain:

      -  A "c5c" or "c5b" structure, in case AUTH_CRED_RS is transported
         by value within a certificate chain or a certificate bag,
         respectively; or

      -  A "c5t" or "c5u" structure, in case AUTH_CRED_RS is identified
         by reference through a hash value (a thumbprint) or a URI
         [RFC3986], respectively.

   For the "req_cnf" parameter of the Access Token Request, the "rs_cnf"
   parameter of the Access Token Response, and the "cnf" claim of the
   access token, the structures "x5bag", "x5chain", "x5t", "x5u", "c5b",
   "c5c", "c5t", and "c5u" are defined in
   [I-D.ietf-ace-edhoc-oscore-profile], together with their identifiers.



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   When using either of the structures, the specified authentication
   credential is just the end-entity certificate.

   As per [RFC6347] and [RFC9147], a public key certificate is specified
   in the Certificate message of the DTLS handshake.  For X.509
   certificates, the TLS Certificate Type is "X509", as defined in
   [RFC6091].  For C509 certificates, the TLS certificate type is "C509
   Certificate", as defined in [I-D.ietf-cose-cbor-encoded-cert].

   It is not required that AUTH_CRED_C and AUTH_CRED_RS are both X.509
   certificates or both C509 certificates.  Also, it is not required
   that AUTH_CRED_C and AUTH_CRED_RS are both transported by value or
   both identified by reference.

   Finally, one of the two authentication credentials can be a public
   key certificate, while the other one can be a raw public key.  This
   is consistent with the admitted, combined use of raw public keys and
   certificates, as discussed in Section 5.3 of [RFC7250].

3.1.  Examples

   Figure 5 shows an example of Access Token Request from C to the AS.
   In the example, C specifies its authentication credential by means of
   an "x5chain" structure, transporting by value only its X.509
   certificate.


























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      POST coaps://as.example.com/token
      Content-Format: 19 (application/ace+cbor)
      Payload:
      {
        / grant_type / 33 : 2 / client_credentials /,
        / audience /    5 : "tempSensor3",
        / req_cnf /     4 : {
          e'x5chain' : h'3081ee3081a1a003020102020462319ec430
                         0506032b6570301d311b301906035504030c
                         124544484f4320526f6f7420456432353531
                         39301e170d3232303331363038323433365a
                         170d3239313233313233303030305a302231
                         20301e06035504030c174544484f43205265
                         73706f6e6465722045643235353139302a30
                         0506032b6570032100a1db47b95184854ad1
                         2a0c1a354e418aace33aa0f2c662c00b3ac5
                         5de92f9359300506032b6570034100b723bc
                         01eab0928e8b2b6c98de19cc3823d46e7d69
                         87b032478fecfaf14537a1af14cc8be829c6
                         b73044101837eb4abc949565d86dce51cfae
                         52ab82c152cb02'
        }
      }

      Figure 5: Access Token Request Example for Certificate Mode with
          an X.509 Certificate as Authentication Credential of C,
                   Transported by Value within "req_cnf"

   Figure 6 shows an example of Access Token Response from the AS to C.
   In the example, the AS specifies the authentication credential of the
   RS by means of an "x5chain" structure, transporting by value only the
   X.509 certificate of the RS.



















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     2.01 Created
     Content-Format: 19 (application/ace+cbor)
     Max-Age: 3560
     Payload:
     {
       / access_token / 1 : h'd83dd083...2fa6',
         / (remainder of CWT omitted for brevity;
         CWT contains the client's X.509 certificate in the cnf claim) /
       / expires_in /   2 : 3600,
       / rs_cnf /      41 : {
         e'x5chain' : h'3081ee3081a1a003020102020462319ea030
                        0506032b6570301d311b301906035504030c
                        124544484f4320526f6f7420456432353531
                        39301e170d3232303331363038323430305a
                        170d3239313233313233303030305a302231
                        20301e06035504030c174544484f4320496e
                        69746961746f722045643235353139302a30
                        0506032b6570032100ed06a8ae61a829ba5f
                        a54525c9d07f48dd44a302f43e0f23d8cc20
                        b73085141e300506032b6570034100521241
                        d8b3a770996bcfc9b9ead4e7e0a1c0db353a
                        3bdf2910b39275ae48b756015981850d27db
                        6734e37f67212267dd05eeff27b9e7a813fa
                        574b72a00b430b'
       }
     }

    Figure 6: Access Token Response Example for Certificate Mode with
       an X.509 Certificate as Authentication Credential of the RS,
                   Transported by Value within "rs_cnf"

   The following shows a variation of the two previous examples, where
   X.509 certificates used as authentication credentials are instead
   identified by reference.

   Figure 7 shows an example of Access Token Request from C to the AS.
   In the example, C specifies its authentication credential by means of
   an "x5t" structure, identifying by reference its X.509 certificate.













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      POST coaps://as.example.com/token
      Content-Format: 19 (application/ace+cbor)
      Payload:
      {
        / grant_type / 33 : 2 / client_credentials /,
        / audience /    5 : "tempSensor4",
        / req_cnf /     4 : {
          e'x5t' : [-15, h'79f2a41b510c1f9b']
          / SHA-2 256-bit Hash truncated to 64-bits /
        }
      }

      Figure 7: Access Token Request Example for Certificate Mode with
          an X.509 Certificate as Authentication Credential of C,
                  Identified by Reference within "req_cnf"

   Figure 8 shows an example of Access Token Response from the AS to C.
   In the example, the AS specifies the authentication credential of the
   RS by means of an "x5t" structure, identifying by reference the X.509
   certificate of the RS.

     2.01 Created
     Content-Format: 19 (application/ace+cbor)
     Max-Age: 3560
     Payload:
     {
       / access_token / 1 : h'd83dd083...cda0',
         / (remainder of CWT omitted for brevity;
         CWT contains the client's X.509 certificate in the cnf claim) /
       / expires_in /   2 : 3600,
       / rs_cnf /      41 : {
         e'x5t' : [-15, h'c24ab2fd7643c79f']
         / SHA-2 256-bit Hash truncated to 64-bits /
       }
     }

    Figure 8: Access Token Response Example for Certificate Mode with
       an X.509 Certificate as Authentication Credential of the RS,
                 Identified by Reference within "rs_cnf"

4.  Security Considerations

   The security considerations from [RFC9200] and [RFC9202] apply to
   this document as well.  Furthermore:

   *  When using the CWT Confirmation Method 'ckt' for identifying by
      reference a COSE_Key object that is used for specifying a raw
      public key, the security considerations from [RFC9679] apply.



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   *  When using public key certificates as authentication credentials,
      the security considerations from Appendix C.2 of [RFC8446] apply.

   *  When using X.509 certificates as authentication credentials, the
      security considerations from [RFC5280], [RFC6818], [RFC9549],
      [RFC9598], [RFC9608], and [RFC9618] apply.

   *  When using C509 certificates as authentication credentials, the
      security considerations from [I-D.ietf-cose-cbor-encoded-cert]
      apply.

   Consistently with the ACE architecture, C and the RS securely obtain
   each others' authentication credential from the AS acting as trusted
   third party, i.e., through the Access Token Response sent to C and
   through the issued access token uploaded to the RS, respectively.

   Nevertheless, C and the RS are responsible for verifying the
   integrity and validity of obtained authentication credentials when
   those are CCSs or public key certificates as defined in this
   document.

   For public key certificates, verifying their validity may require
   using a Real-Time Clock (RTC).  Trusted certification authorities
   (CAs) should be selected very carefully and certificate revocation
   should be supported.  The revocation mechanism specifically used
   depends on the application.  For example Certificate Revocation Lists
   [RFC5280] or the Online Certificate Status Protocol (OCSP) [RFC6960]
   may be used when authentication credentials are X.509 certificates.

   Similarly for CCSs, verifying their validity and handling their
   revocation require C and the RS to very carefully select relevant
   trust anchors and to have a well-defined trust-establishment process.

   Note that self-signed certificates or CCSs provided to C and the RS
   cannot result in modifying the set of trust anchors.  A common way
   for a new trust anchor to be added to (or removed from) a device is
   by performing a firmware upgrade.  A longer discussion on trust and
   validation in constrained devices is provided by [RFC9360].

5.  IANA Considerations

   This document has no actions for IANA.

6.  References

6.1.  Normative References





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   [I-D.ietf-ace-edhoc-oscore-profile]
              Selander, G., Mattsson, J. P., Tiloca, M., and R. Höglund,
              "Ephemeral Diffie-Hellman Over COSE (EDHOC) and Object
              Security for Constrained Environments (OSCORE) Profile for
              Authentication and Authorization for Constrained
              Environments (ACE)", Work in Progress, Internet-Draft,
              draft-ietf-ace-edhoc-oscore-profile-09, 20 October 2025,
              <https://datatracker.ietf.org/doc/html/draft-ietf-ace-
              edhoc-oscore-profile-09>.

   [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>.

   [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>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/rfc/rfc3986>.

   [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>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/rfc/rfc6347>.

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, DOI 10.17487/RFC6749, October 2012,
              <https://www.rfc-editor.org/rfc/rfc6749>.

   [RFC6818]  Yee, P., "Updates to the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 6818, DOI 10.17487/RFC6818, January
              2013, <https://www.rfc-editor.org/rfc/rfc6818>.





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   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <https://www.rfc-editor.org/rfc/rfc7250>.

   [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>.

   [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>.

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/rfc/rfc8323>.

   [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>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/rfc/rfc8446>.

   [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>.

   [RFC8747]  Jones, M., Seitz, L., Selander, G., Erdtman, S., and H.
              Tschofenig, "Proof-of-Possession Key Semantics for CBOR
              Web Tokens (CWTs)", RFC 8747, DOI 10.17487/RFC8747, March
              2020, <https://www.rfc-editor.org/rfc/rfc8747>.

   [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>.







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   [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>.

   [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>.

   [RFC9201]  Seitz, L., "Additional OAuth Parameters for Authentication
              and Authorization for Constrained Environments (ACE)",
              RFC 9201, DOI 10.17487/RFC9201, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9201>.

   [RFC9202]  Gerdes, S., Bergmann, O., Bormann, C., Selander, G., and
              L. Seitz, "Datagram Transport Layer Security (DTLS)
              Profile for Authentication and Authorization for
              Constrained Environments (ACE)", RFC 9202,
              DOI 10.17487/RFC9202, August 2022,
              <https://www.rfc-editor.org/rfc/rfc9202>.

   [RFC9430]  Bergmann, O., Preuß Mattsson, J., and G. Selander,
              "Extension of the Datagram Transport Layer Security (DTLS)
              Profile for Authentication and Authorization for
              Constrained Environments (ACE) to Transport Layer Security
              (TLS)", RFC 9430, DOI 10.17487/RFC9430, July 2023,
              <https://www.rfc-editor.org/rfc/rfc9430>.

   [RFC9549]  Housley, R., "Internationalization Updates to RFC 5280",
              RFC 9549, DOI 10.17487/RFC9549, March 2024,
              <https://www.rfc-editor.org/rfc/rfc9549>.

   [RFC9598]  Melnikov, A., Chuang, W., and C. Bonnell,
              "Internationalized Email Addresses in X.509 Certificates",
              RFC 9598, DOI 10.17487/RFC9598, May 2024,
              <https://www.rfc-editor.org/rfc/rfc9598>.




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   [RFC9608]  Housley, R., Okubo, T., and J. Mandel, "No Revocation
              Available for X.509 Public Key Certificates", RFC 9608,
              DOI 10.17487/RFC9608, June 2024,
              <https://www.rfc-editor.org/rfc/rfc9608>.

   [RFC9618]  Benjamin, D., "Updates to X.509 Policy Validation",
              RFC 9618, DOI 10.17487/RFC9618, August 2024,
              <https://www.rfc-editor.org/rfc/rfc9618>.

   [RFC9679]  Isobe, K., Tschofenig, H., and O. Steele, "CBOR Object
              Signing and Encryption (COSE) Key Thumbprint", RFC 9679,
              DOI 10.17487/RFC9679, December 2024,
              <https://www.rfc-editor.org/rfc/rfc9679>.

   [SHA-256]  NIST, "Secure Hash Standard", NIST FIPS PUB 180-4, DOI
              10.6028/NIST.FIPS.180-4 , August 2015,
              <https://nvlpubs.nist.gov/nistpubs/FIPS/
              NIST.FIPS.180-4.pdf>.

6.2.  Informative References

   [I-D.ietf-ace-workflow-and-params]
              Tiloca, M. and G. Selander, "Short Distribution Chain
              (SDC) Workflow and New OAuth Parameters for the
              Authentication and Authorization for Constrained
              Environments (ACE) Framework", Work in Progress, Internet-
              Draft, draft-ietf-ace-workflow-and-params-06, 20 October
              2025, <https://datatracker.ietf.org/doc/html/draft-ietf-
              ace-workflow-and-params-06>.

   [RFC6091]  Mavrogiannopoulos, N. and D. Gillmor, "Using OpenPGP Keys
              for Transport Layer Security (TLS) Authentication",
              RFC 6091, DOI 10.17487/RFC6091, February 2011,
              <https://www.rfc-editor.org/rfc/rfc6091>.

   [RFC6960]  Santesson, S., Myers, M., Ankney, R., Malpani, A.,
              Galperin, S., and C. Adams, "X.509 Internet Public Key
              Infrastructure Online Certificate Status Protocol - OCSP",
              RFC 6960, DOI 10.17487/RFC6960, June 2013,
              <https://www.rfc-editor.org/rfc/rfc6960>.

   [RFC9360]  Schaad, J., "CBOR Object Signing and Encryption (COSE):
              Header Parameters for Carrying and Referencing X.509
              Certificates", RFC 9360, DOI 10.17487/RFC9360, February
              2023, <https://www.rfc-editor.org/rfc/rfc9360>.






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Appendix A.  Examples with Hybrid Settings

   This section provides additional examples where, within the same ACE
   execution workflow, C and the RS use different formats of raw public
   keys (see Appendix A.1), or different formats of certificates (see
   Appendix A.2), or a combination of the RPK mode and certificate mode
   (see Appendix A.3).

A.1.  RPK Mode (Raw Public Keys of Different Formats)

   Figure 9 shows an example of Access Token Request from C to the AS,
   where the public key of C is conveyed as a COSE Key.

      POST coaps://as.example.com/token
      Content-Format: 19 (application/ace+cbor)
      Payload:
      {
        / grant_type / 33 : 2 / client_credentials /,
        / audience /    5 : "tempSensor5",
        / req_cnf /     4 : {
          / COSE_Key / 1 : {
            / kty /    1 : 2 / EC2 /,
            / crv /   -1 : 1 / P-256 /,
            / x /     -2 : h'd7cc072de2205bdc1537a543d53c60a6
                             acb62eccd890c7fa27c9e354089bbe13',
            / y /     -3 : h'f95e1d4b851a2cc80fff87d8e23f22af
                             b725d535e515d020731e79a3b4e47120'
          }
        }
      }

       Figure 9: Access Token Request Example for RPK Mode, with the
          Public Key of C Conveyed as a COSE Key within "req_cnf"

   Figure 10 shows an example of Access Token Response from the AS to C,
   where the public key of the RS is wrapped by a CCS.















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      2.01 Created
      Content-Format: 19 (application/ace+cbor)
      Max-Age: 3560
      Payload:
      {
        / access_token / 1 : h'd83dd083...c41a',
          / (remainder of CWT omitted for brevity;
          CWT contains the client's RPK in the cnf claim) /
        / expires_in /   2 : 3600,
        / rs_cnf /      41 : {
          e'kccs' : {
            / sub / 2 : "DD-EE-FF-05-06-07-08-09",
            / cnf / 8 : {
              / COSE_Key / 1 : {
                / kty /  1 : 2 / EC2 /,
                / crv / -1 : 1 / P-256 /,
                / x /   -2 : h'ac75e9ece3e50bfc8ed6039988952240
                               5c47bf16df96660a41298cb4307f7eb6',
                / y /   -3 : h'6e5de611388a4b8a8211334ac7d37ecb
                               52a387d257e6db3c2a93df21ff3affc8'
              }
            }
          }
        }
      }

      Figure 10: Access Token Response Example for RPK Mode, with the
           Public Key of the RS Wrapped by a CCS within "rs_cnf"

A.2.  Certificate Mode (Certificates of Different Formats)

   Figure 11 shows an example of Access Token Request from C to the AS.
   In the example, C specifies its authentication credential by means of
   an "x5chain" structure, transporting by value only its X.509
   certificate.
















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      POST coaps://as.example.com/token
      Content-Format: 19 (application/ace+cbor)
      Payload:
      {
        / grant_type / 33 : 2 / client_credentials /,
        / audience /    5 : "tempSensor6",
        / req_cnf /     4 : {
          e'x5chain' : h'308201383081dea003020102020301f50d30
                         0a06082a8648ce3d04030230163114301206
                         035504030c0b524643207465737420434130
                         1e170d3233303130313030303030305a170d
                         3236303130313030303030305a3022312030
                         1e06035504030c1730312d32332d34352d46
                         462d46452d36372d38392d41423059301306
                         072a8648ce3d020106082a8648ce3d030107
                         03420004b1216ab96e5b3b3340f5bdf02e69
                         3f16213a04525ed44450b1019c2dfd3838ab
                         ac4e14d86c0983ed5e9eef2448c6861cc406
                         547177e6026030d051f7792ac206a30f300d
                         300b0603551d0f040403020780300a06082a
                         8648ce3d0403020349003046022100d4320b
                         1d6849e309219d30037e138166f2508247dd
                         dae76cceea55053c108e90022100d551f6d6
                         0106f1abb484cfbe6256c178e4ac3314ea19
                         191e8b607da5ae3bda16'
        }
      }

     Figure 11: Access Token Request Example for Certificate Mode with
          an X.509 Certificate as Authentication Credential of C,
                   Transported by Value within "req_cnf"

   Figure 12 shows an example of Access Token Response from the AS to C.
   In the example, the AS specifies the authentication credential of the
   RS by means of a "c5c" structure, transporting by value only the C509
   certificate of the RS.















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      2.01 Created
      Content-Format: 19 (application/ace+cbor)
      Max-Age: 3560
      Payload:
      {
        / access_token / 1 : h'd83dd083...001a',
          / (remainder of CWT omitted for brevity;
          CWT contains the client's C509 certificate in the cnf claim) /
        / expires_in /   2 : 3600,
        / rs_cnf /      41 : {
          e'c5c' : h'03487e7661d7b54e46328a23625553066243
                     41086b4578616d706c6520496e63096d6365
                     7274696669636174696f6e016a3830322e31
                     41522043411a5c52dc0cf68c236255530662
                     434105624c41086b6578616d706c6520496e
                     630963496f542266577431323334015821fd
                     c8b421f11c25e47e3ac57123bf2d9fdc494f
                     028bc351cc80c03f150bf50cff958a042101
                     5496600d8716bf7fd0e752d0ac760777ad66
                     5d02a0075468d16551f951bfc82a431d0d9f
                     08bc2d205b1160210503822082492b060104
                     01b01f0a014401020304005840c0d81996d2
                     507d693f3c48eaa5ee9491bda6db214099d9
                     8117c63b361374cd86a774989f4c321a5cf2
                     5d832a4d336a08ad67df20f1506421188a0a
                     de6d349236'
        }
      }

       Figure 12: Access Token Response Example for Certificate Mode
      with a C509 Certificate as Authentication Credential of the RS,
                    Transported by Value within "rs_cnf"

   The following shows a variation of the two previous examples, where
   certificates used as authentication credentials are instead
   identified by reference.

   Figure 13 shows an example of Access Token Request from C to the AS.
   In the example, C specifies its authentication credential by means of
   an "x5t" structure, identifying by reference its X.509 certificate.











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      POST coaps://as.example.com/token
      Content-Format: 19 (application/ace+cbor)
      Payload:
      {
        / grant_type / 33 : 2 / client_credentials /,
        / audience /    5 : "tempSensor7",
        / req_cnf /     4 : {
          e'x5t' : [-15, h'6ac62b8f41ba5d99']
          / SHA-2 256-bit Hash truncated to 64-bits /
        }
      }

     Figure 13: Access Token Request Example for Certificate Mode with
          an X.509 Certificate as Authentication Credential of C,
                  Identified by Reference within "req_cnf"

   Figure 14 shows an example of Access Token Response from the AS to C.
   In the example, the AS specifies the authentication credential of the
   RS by means of a "c5t" structure, identifying by reference the C509
   certificate of the RS.

     2.01 Created
     Content-Format: 19 (application/ace+cbor)
     Max-Age: 3560
     Payload:
     {
       / access_token / 1 : h'd83dd083...cc04',
         / (remainder of CWT omitted for brevity;
         CWT contains the client's X.509 certificate in the cnf claim) /
       / expires_in /   2 : 3600,
       / rs_cnf /      41 : {
         e'c5t' : [-15, h'cb247f29c82b933a']
         / SHA-2 256-bit Hash truncated to 64-bits /
       }
     }

      Figure 14: Access Token Response Example for Certificate Mode
     with a C509 Certificate as Authentication Credential of the RS,
                 Identified by Reference within "rs_cnf"

A.3.  Combination of RPK Mode and Certificate Mode

   Figure 15 shows an example of Access Token Request from C to the AS,
   where the public key of C is wrapped by a CCS.







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      POST coaps://as.example.com/token
      Content-Format: 19 (application/ace+cbor)
      Payload:
      {
        / grant_type / 33 : 2 / client_credentials /,
        / audience /    5 : "tempSensor8",
        / req_cnf /     4 : {
          e'kccs' : {
            / sub / 2 : "55-11-44-AB-CD-EF-00-00",
            / cnf / 8 : {
              / COSE_Key / 1 : {
                / kty /    1 : 2 / EC2 /,
                / crv /   -1 : 1 / P-256 /,
                / x /     -2 : h'cd4177ba62433375ede279b5e18e8b91
                                 bc3ed8f1e174474a26fc0edb44ea5373',
                / y /     -3 : h'a0391de29c5c5badda610d4e301eaaa1
                                 8422367722289cd18cbe6624e89b9cfd'
              }
            }
          }
        }
      }

       Figure 15: Access Token Request Example for RPK Mode, with the
             Public Key of C Wrapped by a CCS within "req_cnf"

   Figure 16 shows an example of Access Token Response from the AS to C.
   In the example, the AS specifies the authentication credential of the
   RS by means of an "x5chain" structure, transporting by value only the
   X.509 certificate of the RS.





















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     2.01 Created
     Content-Format: 19 (application/ace+cbor)
     Max-Age: 3560
     Payload:
     {
       / access_token / 1 : h'd83dd083...0f7b',
         / (remainder of CWT omitted for brevity;
         CWT contains the client's X.509 certificate in the cnf claim) /
       / expires_in /   2 : 3600,
       / rs_cnf /      41 : {
         e'x5chain' : h'3082023d308201e2a00302010202087e7661
                        d7b54e4632300a06082a8648ce3d04030230
                        5d310b3009060355040613025553310b3009
                        06035504080c02434131143012060355040a
                        0c0b4578616d706c6520496e633116301406
                        0355040b0c0d63657274696669636174696f
                        6e3113301106035504030c0a3830322e3141
                        522043413020170d31393031333131313239
                        31365a180f39393939313233313233353935
                        395a305c310b300906035504061302555331
                        0b300906035504080c024341310b30090603
                        5504070c024c4131143012060355040a0c0b
                        6578616d706c6520496e63310c300a060355
                        040b0c03496f54310f300d06035504051306
                        5774313233343059301306072a8648ce3d02
                        0106082a8648ce3d03010703420004c8b421
                        f11c25e47e3ac57123bf2d9fdc494f028bc3
                        51cc80c03f150bf50cff958d75419d81a6a2
                        45dffae790be95cf75f602f9152618f816a2
                        b23b5638e59fd9a3818a3081873009060355
                        1d1304023000301d0603551d0e0416041496
                        600d8716bf7fd0e752d0ac760777ad665d02
                        a0301f0603551d2304183016801468d16551
                        f951bfc82a431d0d9f08bc2d205b1160300e
                        0603551d0f0101ff0404030205a0302a0603
                        551d1104233021a01f06082b060105050708
                        04a013301106092b06010401b43b0a010404
                        01020304300a06082a8648ce3d0403020349
                        003046022100c0d81996d2507d693f3c48ea
                        a5ee9491bda6db214099d98117c63b361374
                        cd86022100a774989f4c321a5cf25d832a4d
                        336a08ad67df20f1506421188a0ade6d3492
                        36'
       }
     }






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      Figure 16: Access Token Response Example for Certificate Mode
    with an X.509 Certificate as Authentication Credential of the RS,
                   Transported by Value within "rs_cnf"

   The following shows a variation of the two previous examples, where
   one authentication credential is a raw public key specified by a
   COSE_Key Object and the other authentication credential is an X.509
   certificate, with both credentials identified by reference.

   Figure 17 shows an example of Access Token Request from C to the AS.
   In the example, C specifies its authentication credential by means of
   a "ckt" structure, identifying by reference the COSE_Key Object that
   specifies its public key.

      POST coaps://as.example.com/token
      Content-Format: 19 (application/ace+cbor)
      Payload:
      {
        / grant_type / 33 : 2 / client_credentials /,
        / audience /    5 : "tempSensor9",
        / req_cnf /     4 : {
          / ckt / 5 : h'29e8a588da26249fc88f3b3f059f2144
                        475c895619d64b2ad4aa2f8a051e8dc9'
        }
      }

       Figure 17: Access Token Request Example for RPK Mode, with the
        Public Key of C Specified as a COSE_Key Object Identified by
                         Reference within "req_cnf"

   Figure 18 shows an example of Access Token Response from the AS to C.
   In the example, the AS specifies the authentication credential of the
   RS by means of an "x5t" structure, identifying by reference the X.509
   certificate of the RS.

















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     2.01 Created
     Content-Format: 19 (application/ace+cbor)
     Max-Age: 3560
     Payload:
     {
       / access_token / 1 : h'd83dd083...f3c5',
         / (remainder of CWT omitted for brevity;
         CWT contains the client's X.509 certificate in the cnf claim) /
       / expires_in /   2 : 3600,
       / rs_cnf /      41 : {
         e'x5t' : [-15, h'e35464981de8d29c']
         / SHA-2 256-bit Hash truncated to 64-bits /
       }
     }

      Figure 18: Access Token Response Example for Certificate Mode
    with an X.509 Certificate as Authentication Credential of the RS,
                 Identified by Reference within "rs_cnf"

Appendix B.  CDDL Model

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

   ; CWT Confirmation Methods
   x5t = 6
   c5t = 8
   kccs = 11
   x5chain = 24
   c5c = 26

                           Figure 19: CDDL Model

Appendix C.  Document Updates

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

C.1.  Version -02 to -03

   *  Editorial fixes and improvements.

C.2.  Version -01 to -02

   *  Considerations on providing credentials by value or by reference.

   *  Minor fixes in examples.

   *  Added more examples with hybrid settings.




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   *  Extended security considerations.

   *  Updated CBOR abbreviations for a more efficient use of codepoints.

   *  Updated references.

   *  Editorial improvements.

C.3.  Version -00 to -01

   *  Enabled use of COSE Keys identified by reference with a
      thumbprint.

   *  Changed CBOR abbreviations to not collide with existing
      codepoints.

   *  Fixes in the examples in CBOR diagnostic notation.

   *  Updated references.

   *  Editorial improvements.

Acknowledgments

   The authors sincerely thank Rikard Höglund and Göran Selander for
   their comments and feedback.

   This work was supported by the Sweden's Innovation Agency VINNOVA
   within the EUREKA CELTIC-NEXT project CYPRESS; and by the H2020
   project SIFIS-Home (Grant agreement 952652).

Authors' Addresses

   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   SE-164 40 Kista
   Sweden
   Email: marco.tiloca@ri.se


   John Preuß Mattsson
   Ericsson AB
   SE-164 80 Stockholm
   Sweden
   Email: john.mattsson@ericsson.com





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