Sideband Relay Protocol (SBRP)

Version: 0.1.0
Status: Draft

Authority: Navigation (Non-normative)
Purpose: Overview, intent, roles, and navigation to normative documents.

Overview

SBRP enables secure communication between local daemons and browser-based UIs via a relay server. The protocol uses a hybrid trust model:

Relay is trusted for token validation, access enforcement, and routing
Relay cannot decrypt application payloads (end-to-end encrypted)
Relay cannot perform undetectable MITM on established TOFU pins

This provides persistent multi-device access while ensuring message confidentiality.

text

┌─────────┐         ┌─────────┐         ┌─────────┐
│ Browser │◄──TLS──►│  Relay  │◄──TLS──►│ Daemon  │
│         │  auth   │ Server  │  auth   │         │
└─────────┘         └─────────┘         └─────────┘
     │                   │                   │
     │      [auth, routing, presence]        │
     │                   │                   │
     └────────── E2EE encrypted ─────────────┘
           [signed handshake, encrypted payloads]

Design Goals

Persistent access from any device
Multi-device support (mobile, desktop, multiple browsers)
End-to-end encryption for application data
Cryptographic MITM protection (not just trust-based)
Standard OAuth/session authentication
Zero local TLS/certificate configuration

Non-Goals

Protecting against compromised relay (auth/routing layer)
Transport-level anonymity
Hiding metadata (timing, message size, which daemon you're connecting to)

Terminology

Client: Session initiator authenticated via control plane. Generates ephemeral X25519 keys per session. No persistent cryptographic identity. Verifies daemon via TOFU. May be a browser, CLI, native app, or any non-daemon participant.

Daemon: Long-lived agent with Ed25519 identity keypair. Registers with control plane via API key; connects to relay using presence tokens. Reachable only through relay.

Relay: Routing authority and token validator. Not an encryption endpoint. Does not authenticate directly; validates tokens issued by control plane.

Trust Model

SBRP uses a layered trust model where the relay handles authentication and routing while being cryptographically excluded from message content.

What Relay Can Do	What Relay Cannot Do
Validate tokens from control plane	Decrypt message content (E2EE)
Route connections between parties	Perform undetectable MITM after TOFU trust
See metadata (timing, size)	Forge daemon identity (Ed25519 signatures)
Drop or delay messages (DoS)	—

TOFU Identity Pinning: Clients persist daemon identity public keys locally. First connection trusts the control-plane-provided key; subsequent connections verify against the pinned key. Key changes abort the connection and require user approval.

See threat-model.md for detailed threat analysis and attack resistance.

Protocol Flow

Daemon Registration (One-Time)

Daemon generates Ed25519 identity keypair on first run and registers with control plane.

text

Daemon                           Control Plane                    Relay
   │                                  │                              │
   │── register(identityPublicKey) ──►│                              │
   │◄─── { daemonId, presenceToken }  │                              │
   │                                  │                              │
   │── WSS relay?token=<jwt> ─────────┼─────────────────────────────►│

Client Connection

Client lists available daemons, obtains session token, and connects.

text

Client                           Control Plane                    Relay
   │                                  │                              │
   │── list daemons ─────────────────►│                              │
   │◄─── [{ id, identityPublicKey, status }]                         │
   │                                  │                              │
   │── request session(daemonId) ────►│                              │
   │◄─── { relayUrl, token } ─────────│                              │
   │                                  │                              │
   │── WSS relayUrl?token=<jwt> ──────┼─────────────────────────────►│

E2EE Handshake

After WebSocket connection, client and daemon perform authenticated key exchange.

text

Client                     Relay                    Daemon
   │                         │                         │
   │── HandshakeInit (0x01) ►│── forward ─────────────►│
   │   [32B X25519 key]      │                         │
   │                         │                         │  Generate ephemeral
   │                         │                         │  Sign with Ed25519
   │◄───────────────────────────── HandshakeAccept (0x02)
   │                         │   [32B id + 32B key + 64B sig]
   │  Verify signature       │                         │
   │  Derive session keys    │                         │
   │◄══════════ Encrypted frames (0x03) ══════════════►│

See wire-crypto.md for signature construction, key derivation, and wire format details.

End-to-End Flow

Happy Path Walkthrough

Daemon registers with control plane, stores Ed25519 identity keypair
Client authenticates, obtains session token targeting daemon
Client connects to relay, sends HandshakeInit with ephemeral X25519 key
Daemon signs its ephemeral key with identity key, sends HandshakeAccept
Client verifies signature against pinned identity (TOFU)
Both derive session keys via HKDF with transcript hash
Encrypted messages flow through relay as opaque bytes

mermaid

sequenceDiagram
    participant C as Client
    participant R as Relay
    participant D as Daemon

    C->>R: Connect + Session Token
    R->>D: Forward HandshakeInit
    D->>R: HandshakeAccept (signed ephemeral)
    R->>C: Forward
    Note over C,D: E2EE Session Established
    C->>R: Data (encrypted)
    R->>D: Forward (opaque bytes)
    D->>R: Data (encrypted)
    R->>C: Forward (opaque bytes)

Failure Path: Session Interruption

Daemon disconnects (network issue, process restart)
Relay sends Control(session_paused) to client
Client suspends sends, shows UI indicator
If daemon reconnects within grace period:
- Relay sends Control(session_pending) to client
- Daemon sends Signal(ready) for sessions with retained state
- Relay sends Control(session_resumed) to client
If state is lost or grace expires:
- Relay sends Control(session_expired) to client
- Client initiates full reconnect with new handshake

See state-machine.md for complete state transition rules.

Message Categories

Category	Frame Type	Direction	Purpose
Endpoint	0x01–0x03	E2EE	Handshake and encrypted data
Signal	0x04	Daemon → Relay	Session lifecycle commands
Control	0x20	Relay → Endpoint	Notifications and errors
Keepalive	0x10–0x11	Either	Connection liveness detection

Endpoint frames participate in E2EE and are forwarded by relay without inspection. Signal frames are daemon-to-relay commands (ready, close). Control frames are relay-generated notifications (session_paused, session_pending, rate_limited, backpressure). Keepalive frames are connection-scoped and never forwarded.

See wire-crypto.md for wire format details.

Relay Server Responsibilities

The relay operates as a frame router with these core responsibilities:

Responsibility	Description
Token validation	Validate JWTs from control plane; enforce claims
Routing	Route session-bound frames between paired connections
Presence tracking	Track daemon online/offline; notify clients of changes
Throttling	Rate-limit connections/throughput (non-terminal); close slow consumers (terminal)
Keepalive handling	Respond to Ping with Pong; detect dead connections

The relay MUST NOT interpret encrypted payloads or derive information from their content.

See authentication.md for token validation details.

Reconnection

Daemon Reconnection

Daemons may reconnect within a grace period and resume sessions if state is retained:

State retained: Daemon sends Signal(ready) → Client receives session_resumed
State lost: Daemon sends Signal(close) → Client receives session_expired
Non-resumable daemons: Relay auto-expires all sessions on reconnect

Client Reconnection

Clients always perform a full reconnect:

Obtain new session token from control plane
Connect to relay with fresh token
Full E2EE handshake (new ephemeral keys)

See state-machine.md for state transition details.

Multi-Device Support

Each client has an independent E2EE session with the daemon
Daemon maintains separate key state per client (keyed by SessionID)
Messages are not broadcast; each client gets its own responses
Identity pinning is per client/device storage (not synced)

Security Properties

Property	Mechanism
Message confidentiality	ChaCha20-Poly1305 E2EE
Message integrity	Poly1305 authentication tag
Daemon authenticity	Ed25519 identity signatures
Forward secrecy	Ephemeral X25519 per session
Replay protection	Sequence numbers + sliding window
MITM detection	TOFU identity pinning

See threat-model.md for attack resistance analysis.

Delegation

This protocol delegates:

Wire format: Defines own binary framing (does not use SBP frames directly)
Error codes: Defines own Control code taxonomy
Application protocol: Encrypted payloads contain SBP frames

This protocol defines:

E2EE session: Handshake, key derivation, encrypted transport
Control codes: Relay-to-endpoint notifications
Signal codes: Daemon-to-relay lifecycle commands
Authentication: Token-based relay access

Document Authority

Concern	Primary	Supporting
Crypto + wire format	wire-crypto.md	—
State transitions	state-machine.md	—
Control code semantics	state-machine.md	control-codes.md
Control code values	control-codes.md	—
Authentication	authentication.md	—
Threat model	threat-model.md	—
Test specification	conformance.md	—
Security verification	wire-crypto.md, state-machine.md	security-checklist.md

Local Terminology

Term	Definition	See also
Session Key	Derived HKDF key for this connection	glossary
TOFU	Trust-on-first-use identity pinning	glossary
Control Frame	Relay-generated notification (0x20)	glossary
Signal Frame	Daemon-to-relay lifecycle command (0x04)	—
Endpoint Frame	Session-bound E2EE frame (0x01–0x03)	—

Sideband Relay Protocol (SBRP) ​

Overview ​

Design Goals ​

Non-Goals ​

Terminology ​

Trust Model ​

Protocol Flow ​

Daemon Registration (One-Time) ​

Client Connection ​

E2EE Handshake ​

End-to-End Flow ​

Happy Path Walkthrough ​

Failure Path: Session Interruption ​

Message Categories ​

Relay Server Responsibilities ​

Reconnection ​

Daemon Reconnection ​

Client Reconnection ​

Multi-Device Support ​

Security Properties ​

Delegation ​

Document Authority ​

Local Terminology ​

Recommended Reading Order ​

Related Documents ​

Sideband Relay Protocol (SBRP)

Overview

Design Goals

Non-Goals

Terminology

Trust Model

Protocol Flow

Daemon Registration (One-Time)

Client Connection

E2EE Handshake

End-to-End Flow

Happy Path Walkthrough

Failure Path: Session Interruption

Message Categories

Relay Server Responsibilities

Reconnection

Daemon Reconnection

Client Reconnection

Multi-Device Support

Security Properties

Delegation

Document Authority

Local Terminology

Recommended Reading Order

Related Documents