> For the complete documentation index, see [llms.txt](https://docs.nullmask.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.nullmask.io/compliance/guard-service.md).

# 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:

```solidity
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

```bash
cd web
pnpm --filter guard dev
```

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


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters:

```
GET https://docs.nullmask.io/compliance/guard-service.md?ask=<question>&goal=<user_goal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `build a script that syncs our docs to a CMS` lets GitBook tailor the answer to that use case.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.