Messaging Layer Security (MLS) is an IETF standard for end-to-end encrypted (E2EE) messaging. It lets people on laptops and phones communicate with each other in a secure way that no one in between can see.

MLS is designed to use pluggable lower-level protocols. This specification defines an envelope format for distributing MLS messages through the network, and an Activity Streams 2.0 profile for the packets of application data stored inside the messages.

This specification is ready for review from both ActivityPub developers and security analysts. It’s time to start making proof-of-concept implementations and testing interoperability.

Source: https://socialwebfoundation.org/2025/06/13/mls-over-activitypub-draft/