# tether

Phone ↔ laptop clipboard relay. **v0.3 — WebRTC P2P working.**

Today: an HTTP+SSE broadcast bus that also bootstraps WebRTC DataChannels
between participants. Once paired, clipboard text flows direct
peer-to-peer with DTLS encryption — the server never sees the payload.

The roadmap is what makes it interesting: symmetric presence chirps for
self-healing mesh discovery, Sign in with Apple for cross-device
identity, mDNS for zero-config same-LAN discovery, file drop, system
tray UX.

```
phone (web UI)              tether-server                tether-client
─────────────               ─────────────                ──────────────
type/paste                  HTTP+SSE relay               Linux/Mac/Win
  │                              │                              │
  └─── POST /api/send ──────────▶│                              │
                                 ├──── event: clipboard ───────▶│
                                 │                              ▼
                                 │                          stdout / OS
                                 │                          clipboard
                                 │◀──── POST /api/send ─────────┘
                                 ▼
                          web UI shows it
```

## Quick start

```bash
go run ./server
# in another terminal
go run ./client -server http://localhost:8765

# send a one-shot message from CLI:
go run ./client -server http://localhost:8765 -send "hello"
```

Then open `http://localhost:8765/` on your phone (same network) and try
the buttons.

## Pieces

- `server/` — single Go binary. Embedded HTML page. Exposes:
  - `GET /` — phone UI
  - `POST /api/send` — accept a message
  - `GET /api/stream` — SSE feed of every published message
  - `GET /healthz`
- `client/` — CLI client. Subscribes to `/api/stream`, prints received
  messages to stdout. `-send` for one-shot send.
- `server/web/index.html` — phone UI (paste, send, live feed of incoming).

## Roadmap

| Phase | What | Why |
|---|---|---|
| ✅ v0.1 | HTTP+SSE relay, single broadcast bus | Prove the shape end-to-end |
| ✅ v0.2 | `/metrics` endpoint, server-side observability | Latency/throughput visibility |
| ✅ v0.3 | WebRTC DataChannel (Pion ↔ browser RTCPeerConnection), peer chirps offer until paired | True P2P with mandatory DTLS encryption |
| **v0.4 (next)** | **Symmetric presence chirps** — every participant (peer, browser, future Mac) broadcasts `{type:"presence", from, role, capabilities}` every 10-15s. Lets peers/browsers self-discover, auto-upgrade SSE→RTC, detect dead participants via heartbeat timeout. Pairs N-way (mesh, not star). | Self-healing discovery without the "happen to be open at the right moment" timing trap of v0.3's single-direction chirp |
| v0.5 | mDNS service advertisement + QR pairing | Zero-config same-LAN discovery |
| v0.6 | Sign in with Apple OAuth → stable `sub` ties multiple devices to one trust circle | Identity without account/password |
| v0.7 | OS clipboard hook on client (read + write) — phone copy → laptop paste appears automatically | The actual Universal-Clipboard UX (current v0.3 has *write* on receive; missing the *read* on local change) |
| v0.8 | File drop (large blob over WebRTC), encrypted at-rest history | Snapdrop-like UX bundled in |
| v0.9 | Mouse/keyboard handoff (Synergy-style) using the same authenticated peer set + DataChannel | Universal Control for the rest of us |
| v1.0 | macOS / Windows tray UX, push notifications when off-network, packaged installers | Product |

### Note on v0.4 presence chirps

The current peer's "chirp every 5s while unpaired" only solves
one-direction late-joining. The full pattern is **symmetric**:

```
Every participant emits:
    {type: "presence", from: <peerID>, role: "peer" | "browser",
     capabilities: ["clipboard", "filedrop", "cursor"], ts: …}
every 10-15s on the bus.

Maintains  Map<peerID, lastSeen> table on each side.
- New peer presence seen by browser → wait for offer.
- New browser presence seen by peer → post targeted offer.
- After RTC opens → drop chirp rate to 60s "alive" heartbeats.
- lastSeen > 30s → mark dead, restart discovery.
```

Once that's in, the system becomes properly mesh-shaped — N devices can
all auto-pair, see each other in the UI, drop a file to any one, etc.

## Why E2E by default

WebRTC data channels mandate DTLS. Once we move from SSE relay (v0.1) to
P2P data channels (v0.3+), the server only ever sees encrypted bytes
(and only during signaling — not for the data itself). That's free
end-to-end encryption, modeled on Apple's Continuity but using
standardized protocols.

## Why Sign in with Apple

~80% of Windows users also own an iPhone. SiwA gives us a free,
privacy-respecting identity provider that returns a stable per-app
subject ID. Devices that authenticate to the same `sub` are in the same
trust circle automatically. No passwords, no per-app account.

## What this isn't (yet)

- Not a Snapdrop clone — no file drop (v0.8 roadmap)
- Not KDE Connect — no system-wide integration on the laptop side beyond clipboard (v0.7+)
- Not Pushbullet — server never sees data, no persistence
- Not Universal Control — but the mouse/keyboard handoff is on the roadmap (v0.9)

The foundation is right: bus → signaling → WebRTC → mesh → identity.

## License

MIT
