Messaging Layer Security R. Mahy Internet-Draft 16 October 2025 Intended status: Informational Expires: 19 April 2026 A more efficient FramedContentTBS structure in Messsaging Layer Security (MLS) draft-mahy-mls-new-framed-content-tbs-00 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." Mahy Expires 19 April 2026 [Page 1] Internet-Draft New MLS FramedContentTBS October 2025 This Internet-Draft will expire on 19 April 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 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 . . . . . . . . . . . . . . . . . . . . . 3 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 4 6.1. Normative References . . . . . . . . . . . . . . . . . . 4 6.2. Informative References . . . . . . . . . . . . . . . . . 4 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 5 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. Mahy Expires 19 April 2026 [Page 2] Internet-Draft New MLS FramedContentTBS October 2025 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 a hash (from the group's current MLS cipher suite) of the GroupContext. 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; case external: case new_member_proposal: struct{}; }; } FramedContentTBS; 4. Security Considerations TODO Security 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. Mahy Expires 19 April 2026 [Page 3] Internet-Draft New MLS FramedContentTBS October 2025 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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [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, . 6.2. Informative References [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-04, 7 July 2025, . Mahy Expires 19 April 2026 [Page 4] Internet-Draft New MLS FramedContentTBS October 2025 [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-05, 21 April 2025, . Acknowledgments TODO acknowledge. Author's Address Rohan Mahy Email: rohan.ietf@gmail.com Mahy Expires 19 April 2026 [Page 5]