# clipsync Protocol v1 (MVP)

This document defines an implementation-friendly first protocol for clipboard sync.

## Transport

- Discovery: UDP broadcast on port `45821`
- Session: TCP on port `45821`
- Payload encoding: UTF-8 JSON frames
- Frame format on TCP: 4-byte big-endian length prefix + JSON bytes

## Versioning

Each message includes:

- `version`: protocol version string, initially `"1"`
- `type`: message type
- `request_id`: unique string per request/response pair where relevant
- `device_id`: sender stable ID
- `timestamp_ms`: unix epoch milliseconds

## Discovery message (UDP)

`DISCOVERY_ANNOUNCE`

```json
{
  "version": "1",
  "type": "DISCOVERY_ANNOUNCE",
  "device_id": "desktop-uuid",
  "device_name": "workstation",
  "platform": "linux|windows",
  "tcp_port": 45821,
  "capabilities": ["clipboard:text"],
  "timestamp_ms": 0
}
```

Android listens for these packets and shows peers.

## Pairing flow (TCP)

### 1) HELLO

Both peers exchange:

- protocol version
- device info
- ephemeral nonce

### 2) PAIR_REQUEST

Unpaired side sends pairing request including:

- public key
- pairing method (`"pin"` or `"qr"`)

### 3) PAIR_CHALLENGE

Responder sends short code metadata (or QR payload hash) and challenge nonce.

### 4) PAIR_CONFIRM

Requester returns challenge response.

### 5) PAIR_ACCEPT

Responder confirms trust and stores remote public key.

After `PAIR_ACCEPT`, both devices persist:

- `peer_device_id`
- `peer_public_key`
- `paired_at`

## Session messages

### `CLIP_UPDATE`

```json
{
  "version": "1",
  "type": "CLIP_UPDATE",
  "request_id": "req-123",
  "device_id": "android-uuid",
  "timestamp_ms": 0,
  "origin_device_id": "android-uuid",
  "content_type": "text/plain",
  "encoding": "utf-8",
  "content_hash": "sha256:...",
  "content": "clipboard text",
  "sequence": 42
}
```

Rules:

- Only sync `text/plain` in MVP.
- Ignore messages with known `content_hash` from recent history.
- Ignore message if `origin_device_id` equals local device id (echo prevention).

### `ACK`

Acknowledges accepted `request_id`.

### `ERROR`

Contains error code and message (`UNPAIRED`, `UNSUPPORTED_VERSION`, `INVALID_MESSAGE`, `RATE_LIMITED`).

### `PING` / `PONG`

Keepalive every 15 seconds. Drop and reconnect after timeout.

## Security baseline

- Reject non-paired peers for clipboard writes.
- Encrypt session channel (TLS or Noise).
- Enforce max payload size (e.g., 1 MiB for MVP text).
- Apply rate limits per peer.

## Current daemon behavior (implemented)

- Server sends `HELLO` immediately on TCP connect.
- First client message must be `HELLO` (else `ERROR: INVALID_MESSAGE`).
- After `HELLO`, server accepts either:
  - `PAIR_REQUEST` and responds with `PAIR_ACCEPT` (while persisting peer trust), or
  - `CLIP_UPDATE` for already-paired peers.
- `CLIP_UPDATE` from unknown peers is rejected with `ERROR: UNPAIRED`.
- While connected, server polls local clipboard and emits `CLIP_UPDATE` to the connected paired peer when local text changes.
- Incoming paired `CLIP_UPDATE` (`content_type = text/plain`) is applied to local OS clipboard.
- Inbound `CLIP_UPDATE` messages are deduplicated using `content_hash` (with a fallback hash for missing values).
- Inbound `CLIP_UPDATE` messages are rate-limited per peer; over-limit updates return `ERROR: RATE_LIMITED`.
- Keepalive is implemented: server sends periodic `PING`, responds to inbound `PING` with `PONG`, and drops idle sessions after timeout.