cosmian_kyber: AVX2 backend skips Fujisaki-Okamoto implicit rejection, enabling chosen-ciphertext key recovery

Summary

## Summary

`cosmian_kyber` is a fork of `Argyle-Software/kyber` and inherits a broken constant-time conditional-move routine `cmov` in its AVX2 backend. When built with the opt-in `avx2` feature on x86_64, `cmov` is a no-op: it never performs the copy. That `cmov` is the step that applies the Fujisaki-Okamoto (FO) implicit rejection during decapsulation. With it disabled, decapsulating an *invalid* ciphertext no longer returns a key-independent pseudorandom value; it returns a value that depends on the decrypted plaintext of the attacker-chosen ciphertext. This restores the chosen-ciphertext decryption oracle that the FO transform exists to remove, and yields full recovery of the static secret key.

The attack requires no timing measurement, no side channel, and no faults; only the ability to submit ciphertexts to a decapsulation operation under a reused key pair and observe the resulting shared secret. See the linked pull request for the root cause and the reachable call path.

## Affected configuration

Reachable only in builds with `features = ["avx2"]` on an x86_64 target. The default (reference) backend and all non-x86_64 targets are unaffected.

## Impact

Complete IND-CCA break. The plaintext-checking oracle drives a standard chosen-ciphertext key-recovery attack and extracts the entire static secret key. Any protocol reusing a Kyber key pair across decapsulations is exposed; ephemeral-only key shares are not. The identical defect in the parent crate was verified end to end on ML-KEM-768 (full key recovered in 4,272 decapsulation queries).

## Maintenance status

`cosmian_kyber` is a stale fork (last published 0.1.0, January 2023) of an unmaintained upstream, and no fixed release is expected.

## Mitigation

- If remaining on this crate, do not enable the `avx2` feature; the default reference backend performs implicit rejection correctly. - Do not reuse a Kyber key pair across decapsulations.

## Credit

Reported by 007bsd.