New parameter sets for SPHINCS⁺

SPHINCS⁺ is a state-of-the-art post-quantum hash-based signature that is a candidate for the NIST post-quantum cryptography standard. For a target bit security, SPHINCS⁺ supports many different tradeoffs between the signature size and the signing speed. SPHINCS⁺ provides 6 parameter sets: 3 parameter sets for size optimization and 3 parameter sets for speed optimization. We propose new parameter sets with better performance. Specifically, SPHINCS⁺ implementations with our parameter sets are up to 26.5% faster with slightly shorter signature sizes.