Building Scalable Group Messaging with MLS (Message Layer Security)
⚠️ WARNING: This document is not finished. The details in this document are subject to change.
End-to-end encrypted messaging for two people is a solved problem—Signal Protocol has set the gold standard. But what happens when you want to scale that security to group chats with dozens or hundreds of participants? Traditional pairwise encryption becomes a nightmare: N participants require N(N-1)/2 encrypted channels, each with its own key management overhead.
Enter MLS (Message Layer Security), the IETF's RFC 9420 standard designed specifically for scalable group messaging. MLS provides the same strong security guarantees as Signal Protocol—forward secrecy, post-compromise security, authentication—but does so efficiently for groups of any size.
In this article, we'll explore how MLS works, why it's a game-changer for group messaging, and walk through a complete browser-based implementation using the ts-mls library. We'll cover everything from the TreeKEM algorithm to practical P2P integration with WebRTC.