Partial MLS

The Messaging Layer Security (MLS) protocol provides efficient asynchronous group key establishment for large groups with up to thousands of clients. In MLS, any member can commit a change to the group, and consequently, all members must download, validate, and maintain the full group state, which can incur a significant communication and computational costs, especially when joining a group. This document defines an MLS extension to support "partial MLS clients" that don't undertake these costs. A partial client cannot commit changes to the group, and only has partial authentication information for the other members of the group, but is otherwise able to participate in the group. In exchange for these limitations, a partial MLS client can participate in an MLS group with significantly lower requirements in terms of download, memory, and processing.