Codeword Masking Can Be Harmful Under Replay Attacks on HQC

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HQC, selected by NIST for standardization in 2025, was recently shown vulnerable to a replay attack recovering $\mathbf{v} - \mathbf{u}\cdot \mathbf{y}$, from which the long-term secret $\mathbf{y}$ was shown to be recoverable, and against which \emph{codeword masking} was proposed as the countermeasure. We revisit both and demonstrate that the countermeasure yields an opposite effect: on the target build, the unmasked replay attack is ineffective, whereas introducing codeword masking paradoxic