// Post-chirp pairing handshake.
//
// After both devices have exchanged ephemeral X25519 keys via the audio
// chirp, the actual trust transfer happens over an authenticated, encrypted
// channel built on top of those ephemerals. The handshake:
//
//   1. Both sides instantiate Noise NK using the chirp-derived ephemerals.
//      The Android device drives the initiator side; the extension drives
//      the responder side. Treating the chirp ephemerals as the NK "static"
//      key works because the chirp transports the ephemeral with checksum
//      integrity; an attacker who could substitute keys would have to do
//      so in the audio band, in real time, in the same room.
//
//   2. Over the resulting Noise transport, each side sends ONE JSON frame
//      with its `device_id`, `device_name`, `device_type`,
//      `long_term_public_key_hex`, and device-manifest signing key. Both
//      sides receive the counterpart's frame and persist it in the paired
//      device registry.
//
//   3. The Noise NK session and the chirp ephemerals are discarded —
//      future WebSocket sessions use Noise XX with the long-term static
//      keys exchanged in step 2.
//
// The transport for the Noise NK round is provided by the caller. In the
// real pairing flow it is a WebSocket (Android-as-server, extension-as-
// client). Tests pass an in-memory transport.

import { noiseNKInitiator, noiseNKResponder } from "./noise-js";
import type { DeviceType, PairedDevice } from "./sync-types";

export interface PairingTransport {
  send(msg: Uint8Array): Promise<void>;
  receive(): Promise<Uint8Array>;
}

/** Result of a pairing handshake: the peer's identity plus the SAS the user
 *  must confirm matches the peer device before the pair is persisted. */
export interface PairingHandshakeResult {
  device: PairedDevice;
  sas: string;
}

export interface LocalIdentity {
  deviceId: string;
  deviceName: string;
  deviceType: DeviceType;
  longTermPublicKeyHex: string;
  signingPublicKeyHex?: string;
  /** Server-only field — Android/iOS/desktop sync hosts announce their port. */
  wsPort?: number;
}

interface PairingFrame {
  type: "PAIR_IDENTITY";
  device_id: string;
  device_name: string;
  device_type: DeviceType;
  long_term_public_key_hex: string;
  signing_public_key_hex?: string;
  ws_port?: number;
}

function encodeFrame(frame: PairingFrame): Uint8Array {
  return new TextEncoder().encode(JSON.stringify(frame));
}

function decodeFrame(bytes: Uint8Array): PairingFrame {
  const parsed = JSON.parse(new TextDecoder().decode(bytes));
  if (parsed?.type !== "PAIR_IDENTITY") throw new Error("expected PAIR_IDENTITY frame");
  if (typeof parsed.device_id !== "string" || !parsed.device_id) throw new Error("device_id missing");
  if (typeof parsed.device_name !== "string") throw new Error("device_name missing");
  if (typeof parsed.device_type !== "string") throw new Error("device_type missing");
  if (typeof parsed.long_term_public_key_hex !== "string") throw new Error("long_term_public_key_hex missing");
  // Long-term pubkey is a hex-encoded 32-byte X25519 key.
  if (!/^[0-9a-f]{64}$/.test(parsed.long_term_public_key_hex)) {
    throw new Error("long_term_public_key_hex is not 32-byte hex");
  }
  if (
    parsed.signing_public_key_hex !== undefined &&
    !/^[0-9a-f]{64}$/.test(parsed.signing_public_key_hex)
  ) {
    throw new Error("signing_public_key_hex is not 32-byte hex");
  }
  return parsed as PairingFrame;
}

/**
 * Run the post-chirp pairing handshake from the INITIATOR side (Android).
 *
 * `remoteEphemeralPublicKey` is the responder's ephemeral pubkey, which we
 * learned from the inbound chirp. The Noise NK transport message 1 will
 * encrypt to that key; the responder must hold the matching private key.
 */
export async function runPairingInitiator(
  remoteEphemeralPublicKey: Uint8Array,
  ourIdentity: LocalIdentity,
  transport: PairingTransport,
): Promise<PairingHandshakeResult> {
  const session = await noiseNKInitiator(
    remoteEphemeralPublicKey,
    transport.send,
    transport.receive,
  );

  // Send our identity, then receive theirs.
  const ourFrame = encodeFrame({
    type: "PAIR_IDENTITY",
    device_id: ourIdentity.deviceId,
    device_name: ourIdentity.deviceName,
    device_type: ourIdentity.deviceType,
    long_term_public_key_hex: ourIdentity.longTermPublicKeyHex,
    signing_public_key_hex: ourIdentity.signingPublicKeyHex,
    ws_port: ourIdentity.wsPort,
  });
  await transport.send(session.encrypt(ourFrame));

  const theirCipher = await transport.receive();
  const theirPlain = session.decrypt(theirCipher);
  const theirFrame = decodeFrame(theirPlain);
  // The SAS is derived from the shared Noise handshake hash; the caller MUST
  // have the user confirm it matches the peer's before persisting the device.
  return { device: frameToPairedDevice(theirFrame), sas: derivePairingSAS(session.handshakeHash) };
}

// ───────────────────────────────────────────────────────────────────────────
// SECURITY — TRUST MODEL (PCH-02-003): the pairing handshake does NOT cryptographically
// authenticate the initiator. Noise NK authenticates only the RESPONDER (it
// proves the initiator encrypted to the responder's "static" key — here the
// chirp ephemeral the responder BROADCAST in the clear over audio). On its own
// it gives the extension (always the responder) zero assurance about *who*
// completed the handshake, and there is no proof-of-possession over the
// long_term_public_key_hex a peer advertises in its PAIR_IDENTITY frame.
//
// COMPENSATING CONTROL (IMPLEMENTED, MANDATORY on the extension side): a
// user-verified Short Authentication String. Both peers derive the SAME 6-digit
// SAS via `derivePairingSAS(session.handshakeHash)` (below) — identical on both
// ends because the Noise handshake hash is shared — and DISPLAY it. The
// extension pairing flow gates persistence on the user confirming the codes
// match: `gateOnSAS` in `src/popup/chirp-pair.ts` runs BEFORE any device is
// written to the registry and ABORTS pairing on mismatch/rejection. This is the
// Bluetooth-numeric-comparison / Signal-safety-number model and detects a
// MITM/relay despite the lack of full mutual auth. Residual risk is the standard
// 6-digit SAS bound: a MITM survives with probability 1e-6 per attempt, and the
// guarantee depends on the user actually comparing the codes.
//
// OPEN ITEM (NOT verifiable from this repository): the Android/native client
// MUST surface and enforce the identical SAS gate before it persists the peer.
// If the app skips or auto-confirms the SAS, the channel is only as strong as
// the app's weakest end — confirm the app-side gate against the Android source
// at kepr.uk/peach-android before any release that touches pairing. A MITM who
// intercepts the acoustic chirp and completes the handshake against a non-
// enforcing peer can pair without user awareness.
// A future hardening step (coordinated wire change with the app) could add
// proof-of-possession via `performInitiatorAuth`/`performResponderAuth`
// (noise-js) so each side proves possession of the key it advertises, making
// authentication structural rather than dependent on the user's SAS comparison.
//
// This is documented residual risk in the cross-repo trust model, not a bug in
// this codebase: the extension-side gateOnSAS is enforced here and regression-
// tested (PCH-01-024); the peer-side gate lives in Android and must be re-
// verified there on every pairing-touching Android release.
// ───────────────────────────────────────────────────────────────────────────

/**
 * Derive a 6-digit Short Authentication String from a Noise handshake hash.
 * Both pairing peers compute the SAME value because the Noise `handshakeHash`
 * is shared after the handshake. Showing it on both devices and having the
 * user confirm they match authenticates the channel out-of-band, detecting a
 * man-in-the-middle the NK handshake alone cannot (see SECURITY note above).
 */
export function derivePairingSAS(handshakeHash: Uint8Array): string {
  // Fold the first 4 bytes into an unsigned int, mod 1e6, zero-padded. Uniform
  // enough for a 6-digit comparison code; the handshake hash is already a PRF
  // output so no further mixing is needed.
  if (handshakeHash.length < 4) return "000000";
  const n =
    ((handshakeHash[0] << 24) >>> 0) +
    (handshakeHash[1] << 16) +
    (handshakeHash[2] << 8) +
    handshakeHash[3];
  return String((n >>> 0) % 1_000_000).padStart(6, "0");
}

/**
 * Run the post-chirp pairing handshake from the RESPONDER side (extension).
 *
 * `ourEphemeralPrivateKey` / `ourEphemeralPublicKey` come from the chirp
 * the responder broadcast; the initiator's chirp told them which key to
 * authenticate against.
 */
export async function runPairingResponder(
  ourEphemeralPrivateKey: Uint8Array,
  ourEphemeralPublicKey: Uint8Array,
  ourIdentity: LocalIdentity,
  transport: PairingTransport,
): Promise<PairingHandshakeResult> {
  const session = await noiseNKResponder(
    ourEphemeralPrivateKey,
    ourEphemeralPublicKey,
    transport.send,
    transport.receive,
  );

  // Receive theirs first, then send ours — the initiator sends first.
  const theirCipher = await transport.receive();
  const theirPlain = session.decrypt(theirCipher);
  const theirFrame = decodeFrame(theirPlain);

  const ourFrame = encodeFrame({
    type: "PAIR_IDENTITY",
    device_id: ourIdentity.deviceId,
    device_name: ourIdentity.deviceName,
    device_type: ourIdentity.deviceType,
    long_term_public_key_hex: ourIdentity.longTermPublicKeyHex,
    signing_public_key_hex: ourIdentity.signingPublicKeyHex,
    ws_port: ourIdentity.wsPort,
  });
  await transport.send(session.encrypt(ourFrame));

  // SAS derived from the shared handshake hash — caller must have the user
  // confirm it matches the peer's code before persisting (defeats the
  // initiator-impersonation / MITM attack the NK handshake alone allows).
  return { device: frameToPairedDevice(theirFrame), sas: derivePairingSAS(session.handshakeHash) };
}

function frameToPairedDevice(frame: PairingFrame): PairedDevice {
  return {
    deviceId: frame.device_id,
    deviceName: frame.device_name,
    deviceType: frame.device_type,
    longTermPublicKeyHex: frame.long_term_public_key_hex,
    signingPublicKeyHex: frame.signing_public_key_hex,
    trustSource: "direct",
    wsPort: typeof frame.ws_port === "number" ? frame.ws_port : undefined,
    pairedAt: Date.now(),
  };
}

/**
 * Test-helper: an in-memory pair of transports that connect to each other.
 * Use one half for the initiator and the other for the responder.
 */
export function memoryTransportPair(): [PairingTransport, PairingTransport] {
  const a: Uint8Array[] = [];
  const b: Uint8Array[] = [];
  const aReaders: Array<(b: Uint8Array) => void> = [];
  const bReaders: Array<(b: Uint8Array) => void> = [];

  function makeTransport(
    inbox: Uint8Array[],
    outbox: Uint8Array[],
    inboxReaders: Array<(b: Uint8Array) => void>,
    outboxReaders: Array<(b: Uint8Array) => void>,
  ): PairingTransport {
    return {
      async send(msg: Uint8Array) {
        const r = outboxReaders.shift();
        if (r) r(msg);
        else outbox.push(msg);
      },
      receive() {
        const msg = inbox.shift();
        if (msg) return Promise.resolve(msg);
        return new Promise<Uint8Array>((resolve) => inboxReaders.push(resolve));
      },
    };
  }
  return [makeTransport(a, b, aReaders, bReaders), makeTransport(b, a, bReaders, aReaders)];
}