MLS Extension for Out-Of-Band Additional Authenticated Data

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Table of Contents

1. Introduction

The Messaging Layer Security (MLS) protocol includes message types with Additional Authenticated Data.¶

[I-D.pham-mls-additional-wire-formats] defined new MLS Wire Formats for a similar purpose, however the Wire Format should be orthogonal to out-of-band AAD functionality.¶

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.¶

Out-Of-Band AAD and In-Band AAD.¶

3. Mechanism

struct {
    opaque group_id<V>;
    uint64 epoch;
    ContentType content_type;
    SafeAADItem oob_aad<V>;        /*  <== this is new     */
    opaque authenticated_data<V>;  /* usually just SafeAAD */
} PrivateContentAAD;

struct {
    ProtocolVersion version = mls10;
    WireFormat wire_format;
    FramedContent content;         /* can contain SafeAAD  */
    SafeAADItem oob_aad<V>;        /*  <== this is new     */
    select (FramedContentTBS.content.sender.sender_type) {
        case member:
        case new_member_commit:
            GroupContext context;
        case external:
        case new_member_proposal:
            struct{};
    };
} FramedContentTBS;

Consists of:¶

• a oob-aad extension type. This specification cannot be a "safe" extension according to [I-D.ietf-mls-extensions] since it requires the MLS stack to modify its behavior outside of the concept of the specific components it also defines. The presence of the oob-aad extension type in the GroupContext means that all members of the group support the extensions and that if an out-of-band AAD component is signaled, the members of the room will include its actual value in Out-Of-Band AAD, without communicating it in the authenticated_data.¶

• an default-oob-aad-component component type. When present in the app_data_dictionary in the GroupContext, the out-of-band value of each signaled as-needed component is also included in the Out-Of-Band AAD.¶

• an as-needed-oob-aad-component component type.¶

  • When present in the app_data_dictionary in the GroupContext it enumerates OOB AAD that can be signaled on a per-message basis.¶

  • When present as a SafeAadItem in the aad_items list in the authenticated_content, the out-of-band value of each signaled as-needed component is also included in the Out-Of-Band AAD.¶

4. Security Considerations

TODO Security¶

5. IANA Considerations

This document has no IANA actions.¶

6. References

Acknowledgments

TODO acknowledge.¶

Author's Address