MLS

The Messaging Layer Security (MLS) protocol (RFC 9420) provides a group key agreement protocol for messaging applications. MLS is designed to protect against eavesdropping, tampering, and message forgery, and to provide forward secrecy (FS) and post-compromise security (PCS).

This document describes the architecture for using MLS in a general secure group messaging infrastructure and defines the security goals for MLS. It provides guidance on building a group messaging system and discusses security and privacy trade-offs offered by multiple security mechanisms that are part of the MLS protocol (e.g., frequency of public encryption key rotation). The document also provides guidance for parts of the infrastructure that are not standardized by MLS and are instead left to the application.

While the recommendations of this document are not mandatory to follow in order to interoperate at the protocol level, they affect the overall security guarantees that are achieved by a messaging application. This is especially true in the case of active adversaries that are able to compromise clients, the Delivery Service (DS), or the Authentication Service (AS).

Messaging applications are increasingly making use of end-to-end security mechanisms to ensure that messages are only accessible to the communicating endpoints, and not to any servers involved in delivering messages. Establishing keys to provide such protections is challenging for group chat settings, in which more than two clients need to agree on a key but may not be online at the same time. In this document, we specify a key establishment protocol that provides efficient asynchronous group key establishment with forward secrecy (FS) and post-compromise security (PCS) for groups in size ranging from two to thousands.

This document describes an optional method for encoding explicit packet Sensitivity Labels on IPv6 packets. It is intended for use only within Multi-Level Secure (MLS) networking environments that are both trusted and trustworthy. This memo provides information for the Internet community.