MLS for Engineers

"MLS for Engineers: Secure Group Messaging That Actually Scales"
Secure group messaging is easy to demo and notoriously hard to ship: membership churn, multi-device users, unreliable delivery, and real adversaries quickly turn “end-to-end encrypted” into a fragile promise. This book is written for experienced engineers and security-minded architects who need a precise, implementable understanding of Messaging Layer Security (MLS)—not as an abstract protocol, but as a production system that must survive concurrency, compromise, and scale.
You’ll learn what MLS (RFC 9420) truly guarantees—confidentiality, integrity, authentication, forward secrecy, and post-compromise security—and what your surrounding architecture must supply. The book provides an engineer’s tour of epochs and state invariants, transcript binding and validation gates, and TreeKEM’s ratchet tree mechanics that make large groups efficient. It then connects the key schedule and secret tree to high-volume per-message encryption, rigorous key erasure, and exporter patterns for integrating attachments and application features without breaking domain separation.
Finally, it tackles the parts teams often get wrong: KeyPackages and Welcome bootstrapping, proposal/commit workflows under abuse pressure, message framing and metadata realities, and the delivery/authentication service designs described in RFC 9750. Expect concrete failure modes, recovery playbooks, and interoperability strategy anchored in stable specifications. Familiarity with modern cryptography primitives (e.g., HPKE) and distributed systems will h