quicnprotochat

End-to-end encrypted group messaging over Noise_XX + MLS (RFC 9420), written in Rust.

Every byte on the wire is double-protected: the outer Noise_XX channel authenticates both sides and provides forward secrecy for the transport, while the inner MLS layer provides post-compromise security and ratcheted group key agreement across any number of participants. Messages are framed with Cap'n Proto, keeping serialisation zero-copy and schema-versioned.

---

Protocol stack

┌─────────────────────────────────────────────┐
│          Application / MLS ciphertext        │  <- group key ratchet (RFC 9420)
├─────────────────────────────────────────────┤
│              Cap'n Proto RPC                │  <- typed, schema-versioned framing
├─────────────────────────────────────────────┤
│   Noise_XX  (X25519 · ChaChaPoly · SHA-256) │  <- mutual auth + transport secrecy
├─────────────────────────────────────────────┤
│                  TCP                        │
└─────────────────────────────────────────────┘

Property	Mechanism
Transport confidentiality	Noise_XX (ChaCha20-Poly1305)
Transport authentication	Noise_XX static X25519 keys
Group key agreement	MLS MLS_128_DHKEMX25519_AES128GCM_SHA256_Ed25519
Post-compromise security	MLS epoch ratchet
Identity	Ed25519 (MLS credential + leaf node signature)
Message framing	Cap'n Proto (unpacked wire format)

---

Repository layout

quicnprotochat/
├── crates/
│   ├── quicnprotochat-core/        # Crypto primitives, Noise transport, MLS group state machine
│   │   ├── src/codec.rs     # LengthPrefixedCodec — Tokio Encoder + Decoder
│   │   ├── src/keypair.rs   # NoiseKeypair — X25519 static key, zeroize-on-drop
│   │   ├── src/identity.rs  # IdentityKeypair — Ed25519 identity + MLS Signer
│   │   ├── src/keypackage.rs# generate_key_package — standalone KeyPackage helper
│   │   ├── src/noise.rs     # handshake_initiator / handshake_responder / NoiseTransport
│   │   └── src/group.rs     # GroupMember — full MLS group lifecycle
│   │
│   ├── quicnprotochat-proto/       # Cap'n Proto schemas + generated types + serde helpers
│   │   └── schemas/ →       # (symlinked to workspace root schemas/)
│   │
│   ├── quicnprotochat-server/      # Authentication Service (AS) + Delivery Service (DS) binary
│   └── quicnprotochat-client/      # CLI client (ping, register, fetch-key, …)
│
└── schemas/
    ├── envelope.capnp       # Top-level wire envelope (MsgType discriminant + payload)
    ├── auth.capnp           # AuthenticationService RPC (KeyPackage upload / fetch)
    └── delivery.capnp       # DeliveryService RPC (enqueue / fetch MLS messages)

---

Services

Authentication Service (AS) — port 7000

Stores single-use MLS KeyPackages so peers can add each other to groups.

uploadKeyPackage(identityKey: Data, package: Data) -> (fingerprint: Data)
fetchKeyPackage(identityKey: Data)                 -> (package: Data)

Packages are indexed by the raw Ed25519 public key (32 bytes) and consumed exactly once on fetch, matching the MLS single-use KeyPackage requirement.

Delivery Service (DS) — port 7001

A simple store-and-forward relay for MLS messages. The DS never inspects payloads — it routes opaque blobs by recipient public key.

enqueue(recipientKey: Data, payload: Data) -> ()
fetch(recipientKey: Data)                  -> (payloads: List(Data))

fetch atomically drains the entire queue in FIFO order.

---

MLS group lifecycle

GroupMember::new(identity)
  │
  ├─ generate_key_package()    → upload bytes to AS
  │
  ├─ create_group(group_id)    → epoch 0, sole member
  │     └─ add_member(kp_bytes)→ (commit_bytes, welcome_bytes)
  │              ↑                  │            │
  │         fetched from AS    discard      send to joiner via DS
  │
  └─ join_group(welcome_bytes) → joined; ready to encrypt
        ├─ send_message(plain) → TLS-encoded PrivateMessage → DS
        └─ receive_message(ct) → Some(plaintext) | None (Commit)

The OpenMlsRustCrypto backend is persistent across calls on the same GroupMember instance — it holds the HPKE init private key in its in-memory key store between generate_key_package and join_group.

---

Building

Prerequisites:

• Rust (stable, 1.77+)
• capnp CLI — the Cap'n Proto schema compiler

# Debian / Ubuntu
apt-get install capnproto

# macOS
brew install capnp

Build everything:

cargo build --workspace

Run tests:

cargo test --workspace

---

Running

Start the server (AS on :7000, DS on :7001):

cargo run -p quicnprotochat-server
# or with custom ports:
cargo run -p quicnprotochat-server -- --listen 0.0.0.0:7000 --ds-listen 0.0.0.0:7001

Client commands:

# Check connectivity
cargo run -p quicnprotochat-client -- ping

# Generate a fresh identity + KeyPackage, upload to AS
# Prints your identity_key (hex) — share this with peers
cargo run -p quicnprotochat-client -- register

# Fetch a peer's KeyPackage (they must have registered first)
cargo run -p quicnprotochat-client -- fetch-key <64-hex-char identity key>

# Run an end-to-end Alice↔Bob demo against live AS + DS
cargo run -p quicnprotochat-client -- demo-group \
  --server 127.0.0.1:7000 \
  --ds-server 127.0.0.1:7001

# Persistent group CLI (stateful)
cargo run -p quicnprotochat-client -- register-state --state state.bin --server 127.0.0.1:7000
cargo run -p quicnprotochat-client -- create-group --state state.bin --group-id my-group
cargo run -p quicnprotochat-client -- invite --state state.bin --peer-key <peer hex> --server 127.0.0.1:7000 --ds-server 127.0.0.1:7001
cargo run -p quicnprotochat-client -- join --state state.bin --ds-server 127.0.0.1:7001
cargo run -p quicnprotochat-client -- send --state state.bin --peer-key <peer hex> --msg "hello" --ds-server 127.0.0.1:7001
cargo run -p quicnprotochat-client -- recv --state state.bin --ds-server 127.0.0.1:7001

Server address defaults to 127.0.0.1:7000; override with --server or QUICNPROTOCHAT_SERVER. Delivery Service defaults to 127.0.0.1:7001; override with --ds-server or QUICNPROTOCHAT_DS_SERVER.

State file notes: the persisted state stores your identity and MLS group state after you have joined. If you generate a KeyPackage (register-state) and then restart before consuming the Welcome, the join may fail because the HPKE init key is not retained; run join in the same session you register.

---

Milestones

#	Name	Status	What it adds
M1	Noise transport	✅	Noise_XX handshake, length-prefixed framing, Ping/Pong
M2	Authentication Service	✅	Ed25519 identity, KeyPackage generation, AS upload/fetch
M3	Delivery Service + MLS groups	✅	DS relay, GroupMember create/join/add/send/recv
M4	Group CLI subcommands	🔜	Persistent CLI (create-group, invite, join, send, recv); demo-group already available
M5	Multi-party groups	🔜	N > 2 members, Commit fan-out, Proposal handling
M6	Persistence	🔜	SQLite key store, durable group state
M7	Post-quantum	🔜	ML-KEM-768 hybrid in Noise layer

---

Security notes

• This is a proof-of-concept. It has not been audited.
• The server Noise keypair is ephemeral — regenerated on every restart. Clients perform no server key pinning in the current milestone.
• MLS credentials use CredentialType::Basic (public key only). A real deployment would bind credentials to a certificate authority.
• The Delivery Service does no authentication of the recipientKey field — anyone can enqueue for any recipient. Access control is a future milestone.

---

License

MIT