# std.crypto: make ghash faster, esp\. for small messages \(\#13464\) · gitcafe/zig

[View on GitCafe](https://git.cafe/gitcafe/zig/commit/7d48cb11389f5dfe73d25e59c1038398a8ba9ca9)

Repository: [gitcafe/zig](https://git.cafe/gitcafe/zig)

Visibility: public

Requested revision: 7d48cb11389f5dfe73d25e59c1038398a8ba9ca9

Requested commit: 7d48cb11389f5dfe73d25e59c1038398a8ba9ca9

Commit: 7d48cb11389f5dfe73d25e59c1038398a8ba9ca9

Tree: 7a06b19b14b978ac1ed6fd0a6afb642017d1c982

Author: Frank Denis

Committer: GitHub

## Message

```
std.crypto: make ghash faster, esp. for small messages (#13464)

* std.crypto: make ghash faster, esp. for small messages

Aggregated reduction requires 5 additional multiplications (to
precompute the powers of H), in order to save 2 multiplications
per batch.

So, only use large batches when it's actually interesting to do so.

For the last blocks, reuse the precomputations in order to perform
a single reduction.

Also, even in .ReleaseSmall, allow 2-block aggregation.
The speedup is worth it, and the code increase is reasonable.

And in .ReleaseFast, bump the upper batch size up to 16.

Leverage comptime by the way instead of duplicating code.

std/crypto/benchmark.zig on Apple M1:

    Zig 0.10.0: 2769 MiB/s
        Before: 6014 MiB/s
         After: 7334 MiB/s

Normalize function names by the way.

* Change clmul() to accept the half to be processed

This avoids a bunch of truncate() calls.

* Add more ghash tests to check all code paths
```

## Parents

- [88d2e4f66a988dabca45c9add992c9f029c2176f](https://git.cafe/gitcafe/zig/commit/88d2e4f66a988dabca45c9add992c9f029c2176f?format=markdown)

[Source at this commit](https://git.cafe/gitcafe/zig/tree/7d48cb11389f5dfe73d25e59c1038398a8ba9ca9?format=markdown)