# Change 3 — Desktop helper pairing handler

> **DRAFT.** Conforms to `_GROUND-TRUTH.md`. Proposed new module:
> `/Users/june/projects/peach-browser/src/background/desktop-pairing.ts`.
> Reuses Change 1 (`vault_id` in `PAIR_IDENTITY`) and Change 5 (plain
> port probe). Nothing here is applied — it is an apply-ready draft.

## What this change adds

A net-new browser-side **code pairing** client. The existing
`chirp-handshake.ts` only implements the *audio chirp* NK flow where the
**extension is the NK responder**. Desktop pairing is the OPPOSITE: the
**browser is the NK initiator** against the helper's ephemeral key learned
from `PEACH_HELLO_ACK` (`_GROUND-TRUTH.md` §3).

## Verified dependencies (real exported symbols)

| Need | Symbol | Location |
| --- | --- | --- |
| NK initiator primitive | `noiseNKInitiator(responderStaticPublicKey, send, receive)` | `src/shared/noise-js.ts:71` |
| SAS derivation (REUSE) | `derivePairingSAS(handshakeHash: Uint8Array): string` | `src/shared/chirp-handshake.ts:171` |
| Persist paired device | `upsertPairedDevice(device: PairedDevice)` | `src/background/paired-devices.ts:56` |
| Cache peer IP (optional) | `setLastKnownIp(pubkeyHex, ip)` | `src/background/paired-devices.ts:120` |
| Build local identity | `buildLocalIdentity(masterKey): Promise<LocalChirpIdentity>` | `src/background/chirp-identity.ts:79` |
| Vault id for `vault_id` | `computeVaultId(masterKey): Promise<string>` | `src/shared/sync-identity.ts:75` |
| Root key (for masterKey) | `getRootKeyHex()` / `isUnlocked()` | `src/background/vault.ts` (used at `messages.ts:1324`) |
| Plain port probe (Change 5) | `probeSubnetForPort(subnet, port, opts?)` | `src/background/desktop-pairing-probe.ts` (NEW — see `05-subnet-probe.md`) |
| PairedDevice / DeviceType | `PairedDevice`, `DeviceType` | `src/shared/sync-types.ts:86,11` (`"desktop"` already a member) |

> **Why not reuse `probeSubnetForPeer`?** It is already port-parameterized
> (`chirp-sync-transport.ts:238`), BUT it runs a full **Noise XX** handshake
> against an *expected long-term pubkey* per host. In code pairing we have no
> long-term pubkey yet (we only get the helper's *ephemeral* key, after the
> plaintext HELLO). So Change 5 supplies a lighter "is there a WebSocket
> server on this port" probe. See `05-subnet-probe.md`.

## The PEACH_HELLO / PEACH_HELLO_ACK frames (PINNED, see §6 of ground truth)

```ts
interface PeachHelloFrame { type: "PEACH_HELLO"; code: string; }
interface PeachHelloAck   { type: "PEACH_HELLO_ACK"; ephemeralPublicKeyHex: string; }
```

There is **no** error/OK frame. After sending `PEACH_HELLO`, the browser
expects EITHER a `PEACH_HELLO_ACK` binary frame OR the socket to close.
Socket close before an ACK ⇒ wrong/expired code ⇒ `onError("invalid_code")`.

## Transport notes (from ground truth §1)

- Port **49142** (pairing). The persistent sync port arrives inside the
  encrypted `PAIR_IDENTITY` as `ws_port` (helper always announces `8765`).
- WebSocket over TCP, **binary frames only**, one Noise (or one JSON) frame
  per WS binary message. The browser `WebSocket` client masks frames
  automatically — no manual masking needed on this side.
- Pre-handshake HELLO/ACK frames are PLAINTEXT JSON, sent as binary WS
  messages (`new TextEncoder().encode(JSON.stringify(...))`).

---

## Apply-ready module — `src/background/desktop-pairing.ts`

```ts
// Desktop helper code-pairing client (Change 3).
//
// Pairs the extension with the `peach-sync` desktop helper:
//   1. probe the /24 for a host accepting WebSocket on the pairing port
//   2. open ws://<host>:49142
//   3. send PEACH_HELLO { code } in PLAINTEXT
//   4. await PEACH_HELLO_ACK { ephemeralPublicKeyHex } OR socket close
//        (socket close before ACK == wrong/expired code — peach-sync sends
//         NO error frame; it silently closes and rotates the code)
//   5. run Noise NK as INITIATOR against the helper's ephemeral key
//   6. exchange PAIR_IDENTITY (send ours incl. vault_id, then read theirs)
//   7. derive the SAS, surface it for user confirmation
//   8. on confirm -> persist the PairedDevice (deviceType "desktop"); on
//        reject -> abort without persisting
//
// ROLE: the BROWSER is the NK INITIATOR here — the OPPOSITE of the audio
// chirp flow (chirp-handshake.ts), where the extension is the responder.
// The helper's `ephemeralPublicKeyHex` is the NK responder static `s`.

import { noiseNKInitiator } from "../shared/noise-js";
import { derivePairingSAS } from "../shared/chirp-handshake";
import type { LocalIdentity } from "../shared/chirp-handshake";
import type { DeviceType, PairedDevice } from "../shared/sync-types";

/** The pairing-only port peach-sync listens on while `peach-sync pair` runs. */
export const DESKTOP_PAIRING_PORT = 49142;
/** Default sync port the helper announces; used when ws_port is absent. */
const DEFAULT_SYNC_PORT = 8765;

export type DesktopPairingError =
  | "invalid_code"      // socket closed before PEACH_HELLO_ACK (wrong/expired)
  | "no_host_found"     // no WebSocket server answered on the pairing port
  | "handshake_failed"  // Noise NK / PAIR_IDENTITY decode failed
  | "sas_mismatch"      // user rejected the SAS
  | "vault_locked";     // no master key available

/**
 * Minimal binary WebSocket transport seam. Production wires the real
 * `WebSocket`; tests inject a fake. One binary message per frame.
 */
export interface DesktopPairingSocket {
  send(bytes: Uint8Array): void;
  /** Resolves with the next inbound binary frame, or rejects on close. */
  receive(): Promise<Uint8Array>;
  close(): void;
  /** Resolves true if the socket closed before the next frame arrived. */
  closedBeforeFrame(): boolean;
}

export interface DesktopPairingDeps {
  /** Change 5 — find a host answering WebSocket on the pairing port. */
  findHost(port: number): Promise<string | null>;
  /** Open a binary WebSocket to ws://<ip>:<port>/ . */
  openSocket(ip: string, port: number): Promise<DesktopPairingSocket>;
  /** Public identity for our outbound PAIR_IDENTITY (incl. vaultId). */
  getLocalIdentity(): Promise<LocalIdentity>;
  /** Persist on user confirm. */
  persistDevice(device: PairedDevice): Promise<void>;
  /** Optional fast-path cache for the next sync round. */
  cacheLastKnownIp?(pubkeyHex: string, ip: string): Promise<void>;
}

export interface PairWithDesktopCallbacks {
  /** Show the 6-digit SAS; the popup must compare against the helper's. */
  onSasReady(sas: string): void;
  /** Pairing succeeded and the device was persisted. */
  onComplete(device: PairedDevice): void;
  /** Pairing failed at some stage. */
  onError(error: DesktopPairingError): void;
}

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;
  vault_id?: string; // Change 1 — additive, optional
}

function encodeJson(obj: unknown): Uint8Array {
  return new TextEncoder().encode(JSON.stringify(obj));
}

function decodeJson<T>(bytes: Uint8Array): T {
  return JSON.parse(new TextDecoder().decode(bytes)) as T;
}

function hexToBytes(hex: string): Uint8Array {
  if (!/^[0-9a-f]{64}$/.test(hex)) throw new Error("expected 64-char lowercase hex");
  const out = new Uint8Array(32);
  for (let i = 0; i < 32; i++) out[i] = parseInt(hex.slice(i * 2, i * 2 + 2), 16);
  return out;
}

function decodePairIdentity(bytes: Uint8Array): PairingFrame {
  const p = decodeJson<Record<string, unknown>>(bytes);
  if (p?.type !== "PAIR_IDENTITY") throw new Error("expected PAIR_IDENTITY");
  if (typeof p.device_id !== "string" || !p.device_id) throw new Error("device_id missing");
  if (typeof p.device_name !== "string") throw new Error("device_name missing");
  if (typeof p.device_type !== "string") throw new Error("device_type missing");
  if (typeof p.long_term_public_key_hex !== "string" ||
      !/^[0-9a-f]{64}$/.test(p.long_term_public_key_hex)) {
    throw new Error("long_term_public_key_hex is not 32-byte hex");
  }
  return p as unknown as PairingFrame;
}

/**
 * Drive a complete desktop-helper code pairing.
 *
 * `confirm` is a function the caller resolves AFTER the user has compared
 * the SAS shown here against the SAS the helper displays. Resolving false
 * (or rejecting) aborts before any device is persisted — this is the
 * out-of-band MITM defence (see SECURITY note in chirp-handshake.ts).
 */
export async function pairWithDesktopHelper(
  code: string,
  cb: PairWithDesktopCallbacks,
  deps: DesktopPairingDeps,
  confirm: (sas: string) => Promise<boolean>,
): Promise<void> {
  // The helper compares the code byte-for-byte against a [6]u8. Normalise to
  // exactly 6 ASCII digits, zero-padded, no separators (ground truth §2).
  const normalizedCode = String(code).replace(/\D/g, "").padStart(6, "0").slice(-6);

  let identity: LocalIdentity;
  try {
    identity = await deps.getLocalIdentity();
  } catch {
    cb.onError("vault_locked");
    return;
  }

  // 1) Find a host on the pairing port.
  const host = await deps.findHost(DESKTOP_PAIRING_PORT);
  if (!host) {
    cb.onError("no_host_found");
    return;
  }

  // 2) Open the WebSocket.
  let sock: DesktopPairingSocket;
  try {
    sock = await deps.openSocket(host, DESKTOP_PAIRING_PORT);
  } catch {
    cb.onError("no_host_found");
    return;
  }

  try {
    // 3) Send PEACH_HELLO (PLAINTEXT binary frame).
    sock.send(encodeJson({ type: "PEACH_HELLO", code: normalizedCode }));

    // 4) Await PEACH_HELLO_ACK OR socket close.
    //    peach-sync sends NO error frame; a wrong/expired code closes the
    //    socket. So "receive() rejected / closed before a frame" == invalid_code.
    let ackBytes: Uint8Array;
    try {
      ackBytes = await sock.receive();
    } catch {
      cb.onError("invalid_code");
      return;
    }
    if (sock.closedBeforeFrame() || ackBytes.length === 0) {
      cb.onError("invalid_code");
      return;
    }

    let ephemeralPublicKeyHex: string;
    try {
      const ack = decodeJson<{ type?: string; ephemeralPublicKeyHex?: string }>(ackBytes);
      if (ack.type !== "PEACH_HELLO_ACK" || typeof ack.ephemeralPublicKeyHex !== "string") {
        // A non-ACK frame on a matching code should not happen; treat the
        // missing ACK exactly like a wrong code.
        cb.onError("invalid_code");
        return;
      }
      ephemeralPublicKeyHex = ack.ephemeralPublicKeyHex;
    } catch {
      cb.onError("invalid_code");
      return;
    }

    // 5) Noise NK as INITIATOR against the helper's ephemeral static key.
    const responderStatic = hexToBytes(ephemeralPublicKeyHex);
    const session = await noiseNKInitiator(
      responderStatic,
      async (msg) => sock.send(msg),
      () => sock.receive(),
    );

    // 6) Exchange PAIR_IDENTITY — initiator (browser) sends FIRST, then reads.
    const ourFrame: PairingFrame = {
      type: "PAIR_IDENTITY",
      device_id: identity.deviceId,
      device_name: identity.deviceName,
      device_type: identity.deviceType,
      long_term_public_key_hex: identity.longTermPublicKeyHex,
      signing_public_key_hex: identity.signingPublicKeyHex,
      ws_port: identity.wsPort,
      vault_id: identity.vaultId, // Change 1
    };
    sock.send(session.encrypt(encodeJson(ourFrame)));

    const theirCipher = await sock.receive();
    const theirFrame = decodePairIdentity(session.decrypt(theirCipher));

    // 7) Derive the SAS (REUSED — same value the helper computes from the
    //    shared handshake hash) and surface it for user confirmation.
    const sas = derivePairingSAS(session.handshakeHash);
    cb.onSasReady(sas);

    const confirmed = await confirm(sas);
    if (!confirmed) {
      cb.onError("sas_mismatch");
      return;
    }

    // 8) Persist. deviceType "desktop"; ws_port from the helper frame,
    //    defaulting to 8765 (the helper always announces 8765, but guard it).
    const device: PairedDevice = {
      deviceId: theirFrame.device_id,
      deviceName: theirFrame.device_name,
      deviceType: (theirFrame.device_type as DeviceType) || "desktop",
      longTermPublicKeyHex: theirFrame.long_term_public_key_hex,
      signingPublicKeyHex: theirFrame.signing_public_key_hex,
      trustSource: "direct",
      wsPort: typeof theirFrame.ws_port === "number" ? theirFrame.ws_port : DEFAULT_SYNC_PORT,
      pairedAt: Date.now(),
    };
    await deps.persistDevice(device);
    if (deps.cacheLastKnownIp && device.longTermPublicKeyHex) {
      await deps.cacheLastKnownIp(device.longTermPublicKeyHex, host).catch(() => undefined);
    }
    cb.onComplete(device);
  } catch {
    cb.onError("handshake_failed");
  } finally {
    sock.close();
  }
}
```

### Production deps factory (sketch — lives alongside or in the wiring layer)

```ts
import { upsertPairedDevice, setLastKnownIp } from "./paired-devices";
import { buildLocalIdentity } from "./chirp-identity";
import { computeVaultId } from "../shared/sync-identity";
import { getRootKeyHex, isUnlocked } from "./vault";
import { probeSubnetForPort } from "./desktop-pairing-probe"; // Change 5
import { getLocalSubnet } from "../shared/local-network";

export function productionDesktopPairingDeps(): DesktopPairingDeps {
  return {
    async findHost(port) {
      const subnet = await getLocalSubnet();
      if (!subnet) return null;
      return probeSubnetForPort(subnet, port);
    },
    async openSocket(ip, port) {
      return openBinaryWebSocket(ip, port); // small wrapper around WebSocket
    },
    async getLocalIdentity() {
      if (!isUnlocked()) throw new Error("vault_locked");
      const masterKeyHex = getRootKeyHex();
      if (!masterKeyHex) throw new Error("vault_locked");
      const masterKey = hexToBytesLocal(masterKeyHex);
      try {
        const base = await buildLocalIdentity(masterKey);   // chirp-identity.ts:79
        const vaultId = await computeVaultId(masterKey);     // sync-identity.ts:75
        return { ...base, vaultId };                          // LocalIdentity + Change-1 field
      } finally {
        masterKey.fill(0);
      }
    },
    persistDevice: (d) => upsertPairedDevice(d),
    cacheLastKnownIp: (pubkeyHex, ip) => setLastKnownIp(pubkeyHex, ip),
  };
}
```

---

## Threading note — where new state comes from (mirrors Change 1)

`LocalIdentity.vaultId` (added in Change 1) is **not** populated by
`buildLocalIdentity` (`chirp-identity.ts:79` returns no `vaultId`). The
production deps factory above threads it in:

- `masterKey` is obtained the same way `CHIRP_GET_LOCAL_IDENTITY` does it
  (`messages.ts:1324-1326`): `getRootKeyHex()` → `unhexBytes` → `masterKey.fill(0)`
  in `finally`. **Use the stable ROOT key, not the password-derived key**
  (SEV-2-B), so pairing survives a master-password change.
- `vaultId = await computeVaultId(masterKey)` (`sync-identity.ts:75`).
- Both are zeroed after use.

**INTEGRATOR TODO:** `buildLocalIdentity` returns `LocalChirpIdentity`, whose
shape matches `LocalIdentity` minus `vaultId`/`wsPort`. Confirm the two types
are assignable, or add `vaultId?` to `LocalChirpIdentity` directly so the
spread is type-clean. The extension has no `wsPort` (it is never a sync host),
so leaving `wsPort` undefined is correct.

---

## Wiring — popup ⇄ background

The handshake needs `getUserMedia`-free crypto only, so it CAN run entirely
in the background service worker (unlike the audio chirp flow, which must run
in the popup). Recommended split: **the background runs the whole pairing**,
the popup drives the UI and the SAS confirmation round-trip.

### New message types (propose)

| Direction | `type` | Payload | Response |
| --- | --- | --- | --- |
| popup → bg | `PAIR_WITH_DESKTOP` | `{ code: string }` | `{ ok: true }` (started) or `{ ok:false, error }` |
| bg → popup | `DESKTOP_PAIR_SAS` | `{ sas: string }` | — (push via `chrome.runtime.sendMessage`) |
| popup → bg | `DESKTOP_PAIR_CONFIRM` | `{ confirmed: boolean }` | `{ ok: true }` |
| bg → popup | `DESKTOP_PAIR_RESULT` | `{ status:"complete", device } \| { status:"error", error }` | — (push) |

### SAS / confirm round-trip

Because `pairWithDesktopHelper` takes a `confirm(sas) => Promise<boolean>`
callback that must block on the user, the background handler bridges it to a
message round-trip:

1. `PAIR_WITH_DESKTOP` handler calls `pairWithDesktopHelper(code, cb, deps, confirm)`.
2. `cb.onSasReady(sas)` and the `confirm` callback both fire `DESKTOP_PAIR_SAS`
   to the popup and then **await a deferred Promise** that a later
   `DESKTOP_PAIR_CONFIRM` message resolves.
3. The popup shows the SAS, the user compares it to the helper's display,
   taps confirm/reject, and the popup sends `DESKTOP_PAIR_CONFIRM { confirmed }`.
4. The handler resolves the deferred Promise with `confirmed`, completing the
   `confirm()` call inside `pairWithDesktopHelper`.
5. `cb.onComplete`/`cb.onError` push `DESKTOP_PAIR_RESULT` to the popup.

Keep one in-flight pairing per worker (store the pending `confirm` resolver
in a module-level variable, cleared on completion/error). This mirrors how
`chirp-pair.ts`'s `confirmSAS(sas): Promise<boolean>` dep gates persistence
(`chirp-pair.ts` `confirmSAS` doc, lines ~62-71), just relocated to the
background since no microphone is involved.

**INTEGRATOR TODO (handler placement):** add the three `case` blocks to the
`switch` in `messages.ts` (alongside `CHIRP_*` at lines 1318-1384). Gate on
`vault.isUnlocked()` and `requirePro("sync")` exactly like
`CHIRP_GET_LOCAL_IDENTITY` (`messages.ts:1319-1321`). Validate the persisted
device with the same shape check `CHIRP_COMPLETE_PAIRING` uses
(`messages.ts:1344-1360`) — except here the background builds the
`PairedDevice` itself, so the validation can be skipped (the device comes from
our own decode path, not from the popup).

---

## Open integration TODOs

1. **`openBinaryWebSocket` helper** — a small adapter producing a
   `DesktopPairingSocket` from `WebSocket` (set `binaryType="arraybuffer"`,
   queue inbound frames, expose `closedBeforeFrame()`). Pattern already exists
   in `chirp-sync-transport.ts:58-121` (inbox/readers + onclose); extract a
   trimmed version. Critically, `closedBeforeFrame()` must distinguish "socket
   closed with no ACK" from "ACK received" — this is the `invalid_code` signal.
2. **`LocalChirpIdentity` vs `LocalIdentity`** type reconciliation (see
   threading note).
3. **Pro gating + lock checks** in the new `messages.ts` cases.
4. **Single-flight guard** for the pending-confirm resolver in the background.
5. **Timeout** — wrap `findHost`/`receive` in a ceiling (e.g. 10s probe, 5s
   per receive) so a stalled helper surfaces `no_host_found`/`handshake_failed`
   instead of hanging the popup. `connectWebSocket` in
   `chirp-sync-transport.ts:174` already shows the timeout idiom.