# Hardware Wallet Integration Architecture

## Overview

Archipelago supports hardware wallets for secure Bitcoin transaction signing via PSBT (Partially Signed Bitcoin Transactions). This document covers integration with ColdCard, Trezor, and Ledger hardware wallets.

## Supported Devices

| Device | Connection | PSBT Support | DID Signing | Detection |
|--------|-----------|-------------|-------------|-----------|
| ColdCard Mk4 | USB / MicroSD / NFC | Native PSBT | No (Bitcoin-only) | USB VID `0xd13e` |
| Trezor Model T/Safe 3 | USB / WebUSB | Via trezorctl | No | USB VID `0x534c` (SatoshiLabs) |
| Ledger Nano S/X/Plus | USB / Bluetooth | Via HWI | No | USB VID `0x2c97` |

## Architecture

```
┌─────────────────────────────────────────────────────────────────┐
│  Archipelago Node                                               │
│                                                                 │
│  ┌──────────┐    ┌────────────┐    ┌──────────────────────────┐ │
│  │ Web UI   │───▸│ RPC Server │───▸│ LND (gRPC)              │ │
│  │          │    │            │    │  - FundPsbt              │ │
│  │ QR code  │    │ lnd.create │    │  - SignPsbt (partial)    │ │
│  │ display  │    │   -psbt    │    │  - FinalizePsbt          │ │
│  │          │    │            │    │  - PublishTransaction    │ │
│  │ File     │    │ lnd.final  │    └──────────────────────────┘ │
│  │ upload   │    │   ize-psbt │                                 │
│  └──────────┘    └────────────┘                                 │
│       ▲                                                         │
│       │ PSBT (base64)                                           │
│       ▼                                                         │
│  ┌──────────────────────┐                                       │
│  │ Hardware Wallet      │                                       │
│  │  - USB direct        │                                       │
│  │  - QR code scan      │                                       │
│  │  - MicroSD (CC)      │                                       │
│  └──────────────────────┘                                       │
└─────────────────────────────────────────────────────────────────┘
```

## PSBT Signing Flow

### 1. Create Unsigned PSBT

The user initiates a transaction (send coins, open channel, close channel). Instead of LND signing automatically, we create an unsigned PSBT.

**RPC endpoint**: `lnd.create-psbt`

```json
{
  "method": "lnd.create-psbt",
  "params": {
    "outputs": [{"address": "bc1q...", "amount_sats": 50000}],
    "fee_rate_sat_per_vbyte": 10,
    "change_address": "bc1q..."
  }
}
```

**Response**:
```json
{
  "psbt_base64": "cHNidP8BAH...",
  "psbt_hex": "70736274ff...",
  "estimated_fee_sats": 1420,
  "inputs": [{"txid": "abc...", "vout": 0, "amount_sats": 100000}],
  "outputs": [{"address": "bc1q...", "amount_sats": 50000}, {"address": "bc1q...", "amount_sats": 48580}]
}
```

**LND gRPC mapping**: Uses `WalletKit.FundPsbt` to select UTXOs and create the PSBT template.

### 2. Sign with Hardware Wallet

Three transfer methods supported:

#### QR Code (ColdCard NFC, Trezor via companion app)
- Display PSBT as animated QR code (BBQr format for large PSBTs)
- User scans with hardware wallet
- Hardware wallet displays transaction details for verification
- User confirms on device
- Signed PSBT returned as QR code — user scans with camera or uploads screenshot

#### USB Direct (Trezor, Ledger)
- Detect hardware wallet USB device
- Pass PSBT via USB HID protocol
- User confirms on device
- Signed PSBT returned via USB

#### MicroSD (ColdCard)
- Export PSBT file for download
- User transfers to ColdCard via MicroSD
- ColdCard signs and saves signed PSBT to MicroSD
- User uploads signed PSBT file back to Archipelago

### 3. Finalize and Broadcast

**RPC endpoint**: `lnd.finalize-psbt`

```json
{
  "method": "lnd.finalize-psbt",
  "params": {
    "signed_psbt_base64": "cHNidP8BAH..."
  }
}
```

**Response**:
```json
{
  "txid": "abc123...",
  "raw_tx_hex": "0200000001...",
  "broadcast": true
}
```

**LND gRPC mapping**: Uses `WalletKit.FinalizePsbt` then `WalletKit.PublishTransaction`.

## USB Device Detection

**RPC endpoint**: `system.detect-usb-devices`

Scans `/sys/bus/usb/devices/` or uses `lsusb` to detect known hardware wallet vendor IDs:

| Vendor | VID | Product IDs |
|--------|-----|-------------|
| ColdCard (Coinkite) | `0xd13e` | `0xcc10` (Mk4), `0xcc20` (Q) |
| Trezor (SatoshiLabs) | `0x534c` | `0x0001` (One), `0x0002` (T) |
| Ledger | `0x2c97` | `0x0001` (Nano S), `0x0004` (Nano X), `0x0005` (Nano S+) |

Implementation approach:
```rust
// Read from /sys/bus/usb/devices/*/idVendor and idProduct
async fn detect_usb_devices(known_vids: &[(u16, &str)]) -> Vec<DetectedDevice> {
    // Parse /sys/bus/usb/devices/X-Y/idVendor
    // Match against known VIDs
    // Return device list with type identification
}
```

The detection runs server-side since the hardware wallet is plugged into the Archipelago node (not the browser).

## UI Integration Points

### Send Coins View
- Add "Sign with Hardware Wallet" toggle/option
- When enabled: create unsigned PSBT → show QR/download → accept signed PSBT → finalize

### Channel Management
- Open Channel: PSBT funding option
- Close Channel: Cooperative close via PSBT

### Hardware Wallet Status
- Show notification banner when USB device detected
- Display device type and connection status
- Auto-detect on the Server/Dashboard page

## Security Considerations

1. **PSBT verification**: Display transaction details (amounts, addresses, fees) before and after hardware signing — user should verify they match
2. **No private keys on node**: When using hardware wallet flow, LND's internal wallet creates watch-only inputs; the hardware wallet holds the actual signing keys
3. **PSBT size limits**: QR codes can handle ~2KB; larger PSBTs need animated QR (BBQr) or file transfer
4. **USB permissions**: The `archipelago` user needs access to USB HID devices (`udev` rules)

## Implementation Priority

1. **Phase 1** (HW-02): PSBT create/finalize RPC endpoints via LND gRPC
2. **Phase 2** (HW-03): QR code display + file upload UI
3. **Phase 3** (HW-04): USB device detection and notification
4. **Future**: Direct USB HID communication (trezorlib, ledger-transport)

## Dependencies

- LND v0.18+ (PSBT API via WalletKit)
- `qrcode` npm package (QR generation in UI)
- `lsusb` or `/sys/bus/usb/` access (device detection)
- No external hardware wallet libraries needed for Phase 1-3 (PSBT is a standard format)