# fast

The `fast` algorithm uses an optimized JavaScript implementation with multiple threads.
The browser solves a [proof of work](https://en.wikipedia.org/wiki/Proof_of_work) puzzle before it receives a token for the protected service.

Difficulty counts the leading hexadecimal digits that the solution must match.
Each hexadecimal digit represents four bits.

## Configuration

Use `fast` in a threshold rule:

```yaml
thresholds:
  - name: moderate-suspicion
    expression:
      all:
        - weight >= 10
        - weight < 20
    action: CHALLENGE
    challenge:
      algorithm: fast
      difficulty: 2
```

See [thresholds](/docs/configuration/thresholds.md) for rules based on request weight.