Non-constant time Authentication Tag Check in AES-GCM Decryption

Summary

AES-GCM decryption used an implementation for checking the provided authentication tag against the recomputed authentication tag that was intended to be constant-time, but resulted in non-constant-time code generation in certain circumstances. Note that `libcrux-aesgcm` does not give guarantees on constant-time code generation and possible mitigations must be considered on a best-effort basis.

## Impact Users of `libcrux-aesgcm` that expose a decryption oracle to an attacker, which allows repeatedly querying for the same ciphertext were at risk from timing side-channel attacks, depending on their compilation environment. In the worst case, this could lead to recovery of the recomputed authentication tag by the attacker, which enables ciphertext forgery.

## Mitigation Starting from version `0.0.9` (published as `libcrux-aes@v0.0.9`), AES-GCM decryption uses a different, best-effort constant-time implementation of the tag check, which has been checked to reliably lead to constant time code generation, at the moment.