[MLS] MLS Virtual Clients (subgroups/user-groups) and Paired MLS

[Konrad Kohbrok <[e-mail]>]

Hi everyone,

Today two drafts have been published that relate to two individual topics that we have discussed over the past interims: Subgroups (also called user-groups) and MLS pairing (also called guardianship).

The virtual clients (or subgroups) draft (https://datatracker.ietf.org/doc/draft-kohbrok-mls-virtual-clients/) describes how multiple clients can collaborate in emulating a virtual client by forming a subgroup underneath a leaf in one or more MLS groups. This can be used, for example, to allow a user’s devices to share membership in one or more groups, or generally to model any hierarchical structure using MLS groups. While this introduces some complexity, it also brings performance benefits, as trees generally shrink and updates in a subgroup can be re-used across multiple higher-level groups.

The Paired MLS draft (https://datatracker.ietf.org/doc/html/draft-fondevik-mls-pairedmls) allows two MLS clients to pair up, following the protocol described here: https://eprint.iacr.org/2023/1761. Paired-up devices share a randomness source, thus allowing one client to issue PCS updates for the other. This allows a client with more resources to take load off of another, weaker client. This is achieved by the two clients sharing an “anchor”, i.e. a single leaf in one or more MLS groups.

As you may have noticed, the “anchor” concept in Paired MLS is conceptually the same as that of subgroups, which is why we wanted to jointly announce them to the WG.

We are confident that both documents are of interest to the WG and are looking forward to feedback and discussions on the list and in the next interims.

Both documents are still works in progress. In particular, we’re still in the process of lining up terminology, but we’re confident that they are mature enough to open up the discussion.

Cheers,
Konrad