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rohan.ietf@gmail.com

Security Messaging Layer Security credential type smaller LeafNode smaller Credential MLS scaling 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 The latest revision of this draft can be found at . Status information for this document may be found at . Discussion of this document takes place on the Messaging Layer Security Working Group mailing list (), which is archived at . Subscribe at . Source for this draft and an issue tracker can be found at .

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.

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 when, and only when, they appear in all capitals, as shown here.

Credential Format ; } 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.

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).

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

Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings.