Optimizing the Decoding Process of a Post-Quantum Cryptographic Algorithm

Abstract

QcBits is a state-of-the-art constant-time implementation of a code-based encryption scheme for post-quantum public key cryptography. This paper presents an optimized version of its decoding process, which is used for message decryption. Our implementation leverages SSE and AVX instructions extensions and performs 3.6 to 4.8 times faster than the original version, while preserving the 80-bit security level and constant time execution. We also provide experimental data that indicates a further 1.4-factor speedup supposing the existence of instructions for vectorial conditional moves and 256-bit register shifts. Finally, we implemented countermeasures for side-channel security and showed that they do not affect the overall performance.