Messaging Layer Security R. Mahy Internet-Draft 1 November 2026 Intended status: Informational Expires: 5 May 2027 Credentials By-Reference in the Messaging Layer Security (MLS) Protocol draft-mahy-mls-credential-by-ref-00 Abstract Credentials in MLS can grow to become quite large. Many credentials appear repeatedly in several MLS groups. This extension allows MLS credentials to be included by using a hash reference of the actual credential. 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-credential-by-ref/draft-mahy-mls- credential-by-ref.html. Status information for this document may be found at https://datatracker.ietf.org/doc/draft-mahy-mls-credential- by-ref/. 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-credential-by-ref. 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 5 May 2027 [Page 1] Internet-Draft MLS By-Ref Credentials November 2026 This Internet-Draft will expire on 5 May 2027. 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 . . . . . . . . . . . . . . . . . 2 3. Credential Format . . . . . . . . . . . . . . . . . . . . . . 2 4. Security Considerations . . . . . . . . . . . . . . . . . . . 3 5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3 6. Normative References . . . . . . . . . . . . . . . . . . . . 3 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction This new credential type allows an MLS LeafNode to contain a hash reference to another MLS credential. The value of the referenced credential could be be communicated out-of-band or in another MLS group. 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. Credential Format Mahy Expires 5 May 2027 [Page 2] Internet-Draft MLS By-Ref Credentials November 2026 struct { CredentialType referenced_credential_type; HashReference referenced_credential_hash; } ByRefCredential; by_ref = ByRefCredential; The referenced_credential_hash is the hash of the entire Credential struct of the referenced_credential_type, using the hash function from the cipher suite of the KeyPackage or the MLS group. The referenced_credential_type MUST NOT be a by_ref Credential type. 4. Security Considerations The security of this mechanism depends on the strength of the hash function's resistance to birthday attacks. Clients implementing this extension need to consider that the lookup of credential hashes could potentially leak timing information. Implementations must be careful that the scope of applicability (ex: the audience in a web token, or the domain in an X.509 certificate) is relevant to the context of the LeafNode (in an MLS group or in a KeyPackage). 5. IANA Considerations This document requests IANA to add the following entries to the MLS Credential Types registry. Please replace RFCXXXX with the RFC of this document * Value: 0x0007 (suggested) * Name: by_ref * Recommended: Y * Reference: RFCXXXX 6. 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, . Mahy Expires 5 May 2027 [Page 3] Internet-Draft MLS By-Ref Credentials November 2026 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . Author's Address Rohan Mahy Email: rohan.ietf@gmail.com Mahy Expires 5 May 2027 [Page 4]