| Internet-Draft | MLS By-Ref Credentials | November 2026 |
| Mahy | Expires 5 May 2027 | [Page] |
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.¶
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.¶
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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.¶
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.¶
struct {
CredentialType referenced_credential_type;
HashReference referenced_credential_hash<V>;
} 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.¶
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).¶
This document requests IANA to add the following entries to the MLS Credential Types registry. Please replace RFCXXXX with the RFC of this document¶