Efficient Polynomial Multiplication for HQC on ARM Cortex-M4

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In this paper, we propose the Hybrid FAFFT-CRT method for accelerating HQC polynomial multiplication on the ARM Cortex-M4. The method uses the Chinese Remainder Theorem to map the polynomial ring to a product of an FAFFT-friendly ring of size $2^{d+1}$ and a small-degree residual ring, reducing the FAFFT transform length by half compared to the state-of-the-art. We also present the Hybrid Karatsuba-FAFFT method as an alternative hybrid based on a 2-way Karatsuba split, and apply radix-16 multipl