International Association
for Cryptologic Research

HQC (Hamming Quasi-Cyclic) was selected as the fifth algorithm in the NIST suite of post-quantum cryptographic (PQC) standards. As the only code-based algorithm currently standardized by NIST, HQC offers a good balance between security assurance, performance, and implementation simplicity. Most existing power analyses against HQC are of the SPA style: they can recover secrets with a small number of traces, but can only tolerate limited noise. In this paper, we develop a chosen-ciphertext DPA-style attack methodology against HQC. We formalize a dedicated chosen-ciphertext setting in which the adversary selects $(\mathbf{u},\mathbf{v})$ to target the intermediate value $\mathbf{v}\oplus(\mathbf{u}\mathbf{y})$ over $\mathbb{F}_2[x]/(x^n-1)$. We further optimize the attack by reducing its computational complexity and generalizing it to target masked HQC implementations. The proposed approach is validated through both simulation and practical experiments. In noiseless simulations, full-key recovery is achieved with just \(10\) traces, and the required number of traces increases linearly with 1/SNR. In practical evaluations on an STM32F4 microprocessor, the secret key can be recovered with \(45\) traces without profiling and \(10\) traces with profiling. When first-order masking is applied, key recovery on the same hardware target remains feasible by exploiting second-order features, requiring approximately \(7{,}500\) traces without profiling. Our results establish a direct and analyzable connection between leakage on \(\mathbf{v}\oplus \mathbf{u}\mathbf{y}\) and end-to-end key recovery, emphasizing the necessity of higher-order masking countermeasures for HQC implementations.