# PR#74: Fingerprints of 128 Bits

> **WARNING:** This document describes only the change applied in this PR.
> It may already be outdated once the next PR is merged.
> Historic PR documentation is not maintained along with new PRs; treat it as a snapshot.

## Related Links

- [PR#73](https://git.javagil.de/mi/werkrevisor/pulls/73): an approval survives a rebase.

## The Problem

PR#73 was merged at the head that the user approved, while its last review was still running.
That review found that a fingerprint of 64 bits is too short:
the PR author writes both the approved and the later block,
so a birthday attack finds two blocks with the same fingerprint after about 2^32 tries.
The fix and the removal of a Werkbaum node that belongs to Werkregie came after the merge.

The merge also shows a gap in the gate:
a human approval that came before the review ended unlocked an ORANGE whose finding the human had not seen.

## Non-Goals

- No fix of the gap in the gate; the Werkbaum plans it as `#gate.after`.

## The Scenarios

### Feature: a fingerprint cannot be forged

#### Scenario#74.01: a fingerprint has 128 bits

So that a PR author cannot make a harmful block look approved.

- **Given** a human approved a head with a harmless block
- **And** a later head has a harmful block instead
- **When** werkrevisor compares the two heads
- **Then** it counts the harmful block as new
- **And** a block made to look the same would cost the author about 2^64 tries

##### Verified by

- [test_carry_over.BlocksTest.test_a_fingerprint_has_128_bits](../../tests/test_carry_over.py)

## The Solution

- A fingerprint has 32 hex characters, so a collision costs about 2^64 hashes.
- A PR keeps fingerprints for at most 1000 blocks, so that the result comment stays small.
- Version 0.32.1.

## Additional Changes

- The Werkbaum loses the node `#werkregie.pwa`: it belongs to the Werkbaum of Werkregie.
- The Werkbaum plans `#gate.after`: an approval counts only when it came after the result comment of its head.