HILA5 Pindakaas: On the CCA security of lattice-based encryption with error correction.

We show that the NISTPQC submission HILA5 is not secure against chosen-ciphertext attacks. Specifically, we demonstrate a key-recovery attack on HILA5 using an active attack on reused keys. The attack works around the error correction in HILA5. The attack applies to the HILA5 key-encapsulation mechanism (KEM), and also to the public-key encryption mechanism (PKE) obtained by NIST’s procedure for combining the KEM with authenticated encryption. This contradicts the most natural interpretation of the IND-CCA security claim for HILA5.