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.

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.

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.