Project Overview

Group Protocol Stack is a layered, end-to-end encrypted group-messaging protocol family built on top of MLS (RFC 9420).

What problem does it solve?

Existing group-messaging solutions either bolt encryption on top of transport-level primitives or rely on a trusted server for key distribution. Group Protocol Stack inverts this: the cryptographic group-membership layer (MLS) is the foundation, and all transport framing, replay protection, and stream multiplexing sit above it — so even the delivery server is treated as untrusted.

Protocol layers

┌── application ────────────────────────────────────────────────┐
│   GTP · GAP · GSP    (text / audio / signalling semantics)    │
├───────────────────────────────────────────────────────────────┤
│   GBP                (base: frames, AEAD, replay, FSM)        │
├───────────────────────────────────────────────────────────────┤
│   MLS (RFC 9420)     (group key agreement and exporter)       │
└───────────────────────────────────────────────────────────────┘

The design is intentionally analogous to the Internet protocol suite — GBP plays the role of IP, while GTP/GAP/GSP play the role of TCP, UDP, and SCTP.

Protocol	Full name	Responsibility
GBP	Group Base Protocol	Framing, AEAD encryption, replay window, FSM, control plane
GTP	Group Text Protocol	Text messages, delivery history, read watermarks
GAP	Group Audio Protocol	Opus audio streams, jitter buffer, SFrame E2EE
GSP	Group Signaling Protocol	Signalling, roles, capability negotiation

Key properties

End-to-end encrypted — payloads are AEAD-encrypted with keys derived by MLS; the Delivery Service sees only ciphertext.
Replay-protected — every stream maintains a per-member replay window; duplicate or replayed frames are rejected.
Epoch-safe — membership changes drive MLS epoch transitions; GBP enforces commit ordering via a coordinator-based control plane.
Stream-multiplexed — four stream classes (control / audio / text / signal) share one QUIC connection with independent congestion domains.

Implementations and packages

The reference implementation is a Rust workspace. The same protocol is published from one source tree to every major package manager, with pre-built native binaries for win-x64, win-arm64, linux-x64, linux-arm64, osx-x64, osx-arm64.