# Trezor Safekeeping Flow

This flow adds hardware-backed custody controls without replacing the current payment model. The backend never stores private keys. Trezor support starts as a single hardware signer and is designed to upgrade to multisig later.

Default mode: optional. Existing release/refund flows do not require Trezor proof unless `TREZOR_SAFEKEEPING_REQUIRED=true`.

## Goals

- Generate a fresh receive address per user/payment from a registered Trezor xpub.
- Require a Trezor-produced signature before release/refund confirmation when safekeeping enforcement is enabled.
- Keep the Request Network payment adapter and legacy provider abstractions intact while adding custody controls.
- Preserve the existing `Payment` model and orchestration surface.

## Registration

1. User connects a Trezor in the frontend and exports an Ethereum account xpub, for example `m/44'/60'/0'`.
2. Backend builds a registration challenge:
   - `GET /api/trezor/registration-message?xpub=...&registrationAddress=...`
3. The registration address must be the first derived address from the xpub:
   - `m/44'/60'/0'/0/0`
4. User signs the challenge with that Trezor address.
5. Frontend submits:
   - `POST /api/trezor/register`
   - `xpub`
   - `registrationAddress`
   - `proofMessage`
   - `proofSignature`
   - optional `basePath`, `deviceLabel`
6. Backend verifies:
   - xpub is public, not private.
   - registration address matches xpub-derived index `0`.
   - signature recovers the registration address.
7. Backend stores only:
   - `userId`
   - xpub fingerprint
   - xpub
   - base derivation path
   - registration address
   - next address index
   - issued address records

## Address Generation

To issue the next payment address:

```http
POST /api/trezor/addresses/next
{
  "purpose": "deposit",
  "paymentId": "..."
}
```

The backend derives non-hardened receive addresses from the registered xpub:

```text
m/44'/60'/0'/0/{index}
```

If a `paymentId` already has an address, the endpoint returns the same address instead of incrementing the index.

## Transaction Approval

Before a release/refund confirmation, the admin asks the backend for the exact operation message:

```http
POST /api/trezor/operation-message
{
  "operation": "release",
  "paymentId": "...",
  "transactionHash": "0x...",
  "amount": 100,
  "currency": "USDT",
  "provider": "request.network"
}
```

The Trezor signs that message. Release/refund confirmation then includes:

```json
{
  "txHash": "0x...",
  "trezor": {
    "message": "Amanat escrow Trezor transaction approval\n...",
    "signature": "0x..."
  }
}
```

When `TREZOR_SAFEKEEPING_REQUIRED=true`, `confirmReleaseRefundInstruction` verifies the signature before calling the payment adapter confirmation path.

## Enforcement Flag

```env
TREZOR_SAFEKEEPING_REQUIRED=false
```

Default is permissive so existing Request Network release/refund flows continue to work. Set it to `true` only after registering the operating admin's Trezor account and testing the signing path. Any value other than the literal string `true` is treated as disabled.

## Safety Rules

- Never store Trezor seed words, private keys, or xprv/tprv values.
- Reject private extended keys at registration.
- Verify every signature locally before accepting it.
- Use exact transaction-intent messages; do not accept free-form signatures.
- Treat generated deposit addresses as public routing metadata, not as proof of payment.
- Keep ledger availability checks enabled for release/refund accounting.

## Upgrade Path To Multisig

The current design stores a single `trezor-eoa` signer. The recommended production path is to replace the signer policy with:

- `addressType: safe-multisig`
- a Safe address per tenant/admin group
- threshold policy, such as `2-of-3`
- Trezor owners as Safe signers
- release/refund flow creates a Safe transaction and records collected signatures before execution

The payment orchestration API should stay the same: build instruction, collect hardware-backed approval, confirm release/refund, append ledger entry. See [[PRD - Decentralized Custody and Smart-Contract Escrow Roadmap]] for the staged Safe-first path before any custom escrow contract.