A more efficient FramedContentTBS structure in Messsaging Layer Security (MLS)

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Table of Contents

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.¶

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<V>;
        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.¶

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

Acknowledgments

TODO acknowledge.¶

Author's Address