GHSA-753p-wrj5-g8fj: PQClean has a correctness error in HQC decapsulation

A correctness error has been identified in the reference implementation of the HQC key encapsulation mechanism. Due to an indexing error, part of the secret key is incorrectly treated as non-secret data. This results in an incorrect shared secret value being returned when the decapsulation function is called with a malformed ciphertext.

No concrete attack exploiting the error has been identified at this point. However, the error involves mishandling of the secret key, and in principle this presents a security vulnerability.