



Messaging Layer Security                                         R. Mahy
Internet-Draft                                              2 March 2026
Intended status: Informational                                          
Expires: 3 September 2026


A more efficient FramedContentTBS structure in Messsaging Layer Security
                                 (MLS)
                draft-mahy-mls-new-framed-content-tbs-01

Abstract

   Most MLS signatures are signed over the relatively large GroupContext
   structure.  This document defines a way to safely sign using a pre-
   hashed version of the GroupContext structure for better efficiency.

About This Document

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

   The latest revision of this draft can be found at
   https://rohanmahy.github.io/mls-new-framed-content-tbs/draft-mahy-
   mls-new-framed-content-tbs.html.  Status information for this
   document may be found at https://datatracker.ietf.org/doc/draft-mahy-
   mls-new-framed-content-tbs/.

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

   Source for this draft and an issue tracker can be found at
   https://github.com/rohanmahy/mls-new-framed-content-tbs.

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/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."



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   This Internet-Draft will expire on 3 September 2026.

Copyright Notice

   Copyright (c) 2026 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
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Conventions and Definitions . . . . . . . . . . . . . . . . .   3
   3.  Mechanism . . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   3
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   4
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .   4
     6.2.  Informative References  . . . . . . . . . . . . . . . . .   4
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   In MLS [RFC9420] all in-member application, commit, and proposal
   messages (whether sent via a PublicMessage, PrivateMessage or
   SemiPrivateMessage [I-D.mahy-mls-semiprivatemessage]); and all
   external commits contain a signature of the FramedContentTBS
   structure.  This structure contains the full GroupContext of the
   group, which can store a large amount of group state.  (For example,
   in the MIMI protocol [I-D.ietf-mimi-protocol] it contains the
   participant list for the group, which could contain thousands of
   URIs.)

   The GroupContext only changes once per epoch, therefore it is an
   excellent candidate to pre-hash and cache for efficiency reasons.

   This document defines an a replacement for the FramedContentTBS
   structure that uses a pre-hashed GroupContext, and an MLS extension
   for safely negotiating the use of the new struct.





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2.  Conventions and Definitions

   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.

3.  Mechanism

   This document defines the new_framed_content_tbs extension.  When
   present in a LeafNode.capabilities.extensions list, it indicates that
   the client supports this extension.  When present in the
   required_capabilities.extension_types list in GroupContext.extensions
   it indicates that every member of the group MUST use the new
   FramedContentTBS structure.

   The FramedContentTBS structure is replaced with the new structure
   below.  The only change is that the GroupContext is replaced with
   context_hash: a hash (using the current hash function from the
   group's MLS cipher suite) of the GroupContext.

   Since the context_hash can be cached for an entire epoch, this can
   result in a substantial efficiency improvement for additional
   messages sent during the same epoch.

   context_hash = RefHash(GroupContext);

   struct {
       ProtocolVersion version = mls10;
       WireFormat wire_format;
       FramedContent content;
       select (FramedContentTBS.content.sender.sender_type) {
           case member:
           case new_member_commit:
               opaque context_hash<V>;
           case external:
           case new_member_proposal:
               struct{};
       };
   } FramedContentTBS;

4.  Security Considerations

   This proposal replaces the GroupContext with the hash of the
   GroupContext.





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   The primary security consequence of this change is that if a non-
   member is aware of the context_hash, but not the entire GroupContext,
   it can still validate member signatures.  This may have minor privacy
   implications.

5.  IANA Considerations

   This document requests the addition of a new MLS Extension Type value
   under the heading of "Messaging Layer Security".

   RFC EDITOR: Please replace XXXX throughout with the RFC number
   assigned to this document.

   The new_framed_content_tbs MLS Extension Type is used inside LeafNode
   and GroupContext objects.  In a LeafNode it indicates support the the
   extension.  In the required_capabilities of the GroupContext it
   indicates the extension is being used.

   Value: 0x000b (suggested)

   Name: new_framed_content_tbs

   Message(s): GC,LN: This extension may appear in GroupContext and
   LeafNode objects

   Recommended: Y

   Reference: RFC XXXX

6.  References

6.1.  Normative References

   [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>.

   [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>.

   [RFC9420]  Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
              Omara, E., and K. Cohn-Gordon, "The Messaging Layer
              Security (MLS) Protocol", RFC 9420, DOI 10.17487/RFC9420,
              July 2023, <https://www.rfc-editor.org/rfc/rfc9420>.

6.2.  Informative References



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   [I-D.ietf-mimi-protocol]
              Barnes, R., Hodgson, M., Kohbrok, K., Mahy, R., Ralston,
              T., and R. Robert, "More Instant Messaging
              Interoperability (MIMI) using HTTPS and MLS", Work in
              Progress, Internet-Draft, draft-ietf-mimi-protocol-05, 20
              October 2025, <https://datatracker.ietf.org/doc/html/
              draft-ietf-mimi-protocol-05>.

   [I-D.mahy-mls-semiprivatemessage]
              Mahy, R., "Semi-Private Messages in the Messaging Layer
              Security (MLS) Protocol", Work in Progress, Internet-
              Draft, draft-mahy-mls-semiprivatemessage-06, 16 October
              2025, <https://datatracker.ietf.org/doc/html/draft-mahy-
              mls-semiprivatemessage-06>.

Author's Address

   Rohan Mahy
   Email: rohan.ietf@gmail.com
































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