chirp-handshake.ts

1// Post-chirp pairing handshake.2//3// After both devices have exchanged ephemeral X25519 keys via the audio4// chirp, the actual trust transfer happens over an authenticated, encrypted5// channel built on top of those ephemerals. The handshake:6//7//   1. Both sides instantiate Noise NK using the chirp-derived ephemerals.8//      The Android device drives the initiator side; the extension drives9//      the responder side. Treating the chirp ephemerals as the NK "static"10//      key works because the chirp transports the ephemeral with checksum11//      integrity; an attacker who could substitute keys would have to do12//      so in the audio band, in real time, in the same room.13//14//   2. Over the resulting Noise transport, each side sends ONE JSON frame15//      with its `device_id`, `device_name`, `device_type`,16//      `long_term_public_key_hex`, and device-manifest signing key. Both17//      sides receive the counterpart's frame and persist it in the paired18//      device registry.19//20//   3. The Noise NK session and the chirp ephemerals are discarded —21//      future WebSocket sessions use Noise XX with the long-term static22//      keys exchanged in step 2.23//24// The transport for the Noise NK round is provided by the caller. In the25// real pairing flow it is a WebSocket (Android-as-server, extension-as-26// client). Tests pass an in-memory transport.2728import { noiseNKInitiator, noiseNKResponder } from "./noise-js";29import type { DeviceType, PairedDevice } from "./sync-types";3031export interface PairingTransport {32  send(msg: Uint8Array): Promise<void>;33  receive(): Promise<Uint8Array>;34}3536/** Result of a pairing handshake: the peer's identity plus the SAS the user37 *  must confirm matches the peer device before the pair is persisted. */38export interface PairingHandshakeResult {39  device: PairedDevice;40  sas: string;41}4243export interface LocalIdentity {44  deviceId: string;45  deviceName: string;46  deviceType: DeviceType;47  longTermPublicKeyHex: string;48  signingPublicKeyHex?: string;49  /** Server-only field — Android/iOS/desktop sync hosts announce their port. */50  wsPort?: number;51}5253interface PairingFrame {54  type: "PAIR_IDENTITY";55  device_id: string;56  device_name: string;57  device_type: DeviceType;58  long_term_public_key_hex: string;59  signing_public_key_hex?: string;60  ws_port?: number;61}6263function encodeFrame(frame: PairingFrame): Uint8Array {64  return new TextEncoder().encode(JSON.stringify(frame));65}6667function decodeFrame(bytes: Uint8Array): PairingFrame {68  const parsed = JSON.parse(new TextDecoder().decode(bytes));69  if (parsed?.type !== "PAIR_IDENTITY") throw new Error("expected PAIR_IDENTITY frame");70  if (typeof parsed.device_id !== "string" || !parsed.device_id) throw new Error("device_id missing");71  if (typeof parsed.device_name !== "string") throw new Error("device_name missing");72  if (typeof parsed.device_type !== "string") throw new Error("device_type missing");73  if (typeof parsed.long_term_public_key_hex !== "string") throw new Error("long_term_public_key_hex missing");74  // Long-term pubkey is a hex-encoded 32-byte X25519 key.75  if (!/^[0-9a-f]{64}$/.test(parsed.long_term_public_key_hex)) {76    throw new Error("long_term_public_key_hex is not 32-byte hex");77  }78  if (79    parsed.signing_public_key_hex !== undefined &&80    !/^[0-9a-f]{64}$/.test(parsed.signing_public_key_hex)81  ) {82    throw new Error("signing_public_key_hex is not 32-byte hex");83  }84  return parsed as PairingFrame;85}8687/**88 * Run the post-chirp pairing handshake from the INITIATOR side (Android).89 *90 * `remoteEphemeralPublicKey` is the responder's ephemeral pubkey, which we91 * learned from the inbound chirp. The Noise NK transport message 1 will92 * encrypt to that key; the responder must hold the matching private key.93 */94export async function runPairingInitiator(95  remoteEphemeralPublicKey: Uint8Array,96  ourIdentity: LocalIdentity,97  transport: PairingTransport,98): Promise<PairingHandshakeResult> {99  const session = await noiseNKInitiator(100    remoteEphemeralPublicKey,101    transport.send,102    transport.receive,103  );104105  // Send our identity, then receive theirs.106  const ourFrame = encodeFrame({107    type: "PAIR_IDENTITY",108    device_id: ourIdentity.deviceId,109    device_name: ourIdentity.deviceName,110    device_type: ourIdentity.deviceType,111    long_term_public_key_hex: ourIdentity.longTermPublicKeyHex,112    signing_public_key_hex: ourIdentity.signingPublicKeyHex,113    ws_port: ourIdentity.wsPort,114  });115  await transport.send(session.encrypt(ourFrame));116117  const theirCipher = await transport.receive();118  const theirPlain = session.decrypt(theirCipher);119  const theirFrame = decodeFrame(theirPlain);120  // The SAS is derived from the shared Noise handshake hash; the caller MUST121  // have the user confirm it matches the peer's before persisting the device.122  return { device: frameToPairedDevice(theirFrame), sas: derivePairingSAS(session.handshakeHash) };123}124125// ───────────────────────────────────────────────────────────────────────────126// SECURITY — TRUST MODEL: the pairing handshake does NOT cryptographically127// authenticate the initiator. Noise NK authenticates only the RESPONDER (it128// proves the initiator encrypted to the responder's "static" key — here the129// chirp ephemeral the responder BROADCAST in the clear over audio). On its own130// it gives the extension (always the responder) zero assurance about *who*131// completed the handshake, and there is no proof-of-possession over the132// long_term_public_key_hex a peer advertises in its PAIR_IDENTITY frame.133//134// COMPENSATING CONTROL (IMPLEMENTED, MANDATORY on the extension side): a135// user-verified Short Authentication String. Both peers derive the SAME 6-digit136// SAS via `derivePairingSAS(session.handshakeHash)` (below) — identical on both137// ends because the Noise handshake hash is shared — and DISPLAY it. The138// extension pairing flow gates persistence on the user confirming the codes139// match: `gateOnSAS` in `src/popup/chirp-pair.ts` runs BEFORE any device is140// written to the registry and ABORTS pairing on mismatch/rejection. This is the141// Bluetooth-numeric-comparison / Signal-safety-number model and detects a142// MITM/relay despite the lack of full mutual auth. Residual risk is the standard143// 6-digit SAS bound: a MITM survives with probability 1e-6 per attempt, and the144// guarantee depends on the user actually comparing the codes.145//146// OPEN ITEM (NOT verifiable from this repository): the Android/native client147// MUST surface and enforce the identical SAS gate before it persists the peer.148// If the app skips or auto-confirms the SAS, the channel is only as strong as149// the app's weakest end — confirm the app-side gate against the Android source.150// A future hardening step (coordinated wire change with the app) could add151// proof-of-possession via `performInitiatorAuth`/`performResponderAuth`152// (noise-js) so each side proves possession of the key it advertises, making153// authentication structural rather than dependent on the user's SAS comparison.154// ───────────────────────────────────────────────────────────────────────────155156/**157 * Derive a 6-digit Short Authentication String from a Noise handshake hash.158 * Both pairing peers compute the SAME value because the Noise `handshakeHash`159 * is shared after the handshake. Showing it on both devices and having the160 * user confirm they match authenticates the channel out-of-band, detecting a161 * man-in-the-middle the NK handshake alone cannot (see SECURITY note above).162 */163export function derivePairingSAS(handshakeHash: Uint8Array): string {164  // Fold the first 4 bytes into an unsigned int, mod 1e6, zero-padded. Uniform165  // enough for a 6-digit comparison code; the handshake hash is already a PRF166  // output so no further mixing is needed.167  if (handshakeHash.length < 4) return "000000";168  const n =169    ((handshakeHash[0] << 24) >>> 0) +170    (handshakeHash[1] << 16) +171    (handshakeHash[2] << 8) +172    handshakeHash[3];173  return String((n >>> 0) % 1_000_000).padStart(6, "0");174}175176/**177 * Run the post-chirp pairing handshake from the RESPONDER side (extension).178 *179 * `ourEphemeralPrivateKey` / `ourEphemeralPublicKey` come from the chirp180 * the responder broadcast; the initiator's chirp told them which key to181 * authenticate against.182 */183export async function runPairingResponder(184  ourEphemeralPrivateKey: Uint8Array,185  ourEphemeralPublicKey: Uint8Array,186  ourIdentity: LocalIdentity,187  transport: PairingTransport,188): Promise<PairingHandshakeResult> {189  const session = await noiseNKResponder(190    ourEphemeralPrivateKey,191    ourEphemeralPublicKey,192    transport.send,193    transport.receive,194  );195196  // Receive theirs first, then send ours — the initiator sends first.197  const theirCipher = await transport.receive();198  const theirPlain = session.decrypt(theirCipher);199  const theirFrame = decodeFrame(theirPlain);200201  const ourFrame = encodeFrame({202    type: "PAIR_IDENTITY",203    device_id: ourIdentity.deviceId,204    device_name: ourIdentity.deviceName,205    device_type: ourIdentity.deviceType,206    long_term_public_key_hex: ourIdentity.longTermPublicKeyHex,207    signing_public_key_hex: ourIdentity.signingPublicKeyHex,208    ws_port: ourIdentity.wsPort,209  });210  await transport.send(session.encrypt(ourFrame));211212  // SAS derived from the shared handshake hash — caller must have the user213  // confirm it matches the peer's code before persisting (defeats the214  // initiator-impersonation / MITM attack the NK handshake alone allows).215  return { device: frameToPairedDevice(theirFrame), sas: derivePairingSAS(session.handshakeHash) };216}217218function frameToPairedDevice(frame: PairingFrame): PairedDevice {219  return {220    deviceId: frame.device_id,221    deviceName: frame.device_name,222    deviceType: frame.device_type,223    longTermPublicKeyHex: frame.long_term_public_key_hex,224    signingPublicKeyHex: frame.signing_public_key_hex,225    trustSource: "direct",226    wsPort: typeof frame.ws_port === "number" ? frame.ws_port : undefined,227    pairedAt: Date.now(),228  };229}230231/**232 * Test-helper: an in-memory pair of transports that connect to each other.233 * Use one half for the initiator and the other for the responder.234 */235export function memoryTransportPair(): [PairingTransport, PairingTransport] {236  const a: Uint8Array[] = [];237  const b: Uint8Array[] = [];238  const aReaders: Array<(b: Uint8Array) => void> = [];239  const bReaders: Array<(b: Uint8Array) => void> = [];240241  function makeTransport(242    inbox: Uint8Array[],243    outbox: Uint8Array[],244    inboxReaders: Array<(b: Uint8Array) => void>,245    outboxReaders: Array<(b: Uint8Array) => void>,246  ): PairingTransport {247    return {248      async send(msg: Uint8Array) {249        const r = outboxReaders.shift();250        if (r) r(msg);251        else outbox.push(msg);252      },253      receive() {254        const msg = inbox.shift();255        if (msg) return Promise.resolve(msg);256        return new Promise<Uint8Array>((resolve) => inboxReaders.push(resolve));257      },258    };259  }260  return [makeTransport(a, b, aReaders, bReaders), makeTransport(b, a, bReaders, aReaders)];261}