Guard Service

The guard is a backend service that monitors and approves deposits into the Nullmask privacy pool.

Responsibilities

1. Monitor DepositPending events on the Nullmask contract

2. Screen depositor addresses using chain analysis tools

3. Approve legitimate deposits by calling approveDeposit()

4. Reject flagged deposits by calling rejectDeposit()

5. Publish revocation keys with each approved deposit

Guard Address

The guard is a privileged address set in the contract's state. Only the guard can call approveDeposit() and rejectDeposit().

The guard address can be changed by:

• The current guard (via setGuard())

• The upgrade controller admin (via setGuard())

Approval Flow

When approving a deposit, the guard provides:

function approveDeposit(
    uint256 index,
    address recipient,
    address token,
    uint256 amount,
    address depositor,
    uint256 gasEscrow,
    uint256 revocationPublicKeyX,    // Compliance key
    uint256 revocationPublicKeyY,    // Compliance key
    uint256 revocationPublicKeyHash  // Used as value_trapdoor
) external

The contract validates the deposit data against the stored hash to prevent the guard from approving modified deposit parameters.

Gas Escrow

The guard receives the gas escrow from each deposit it processes (whether approved or rejected). This covers the guard's gas costs for the on-chain transaction.

Running the Guard

cd web
pnpm --filter guard dev

The guard service runs as a Fastify application and connects to the Nullmask contract to monitor events.