SPHINCSLET: An Area-Efficient Accelerator for the Full SPHINCS+ Digital Signature Algorithm

This work presents SPHINCSLET, the first fully standard-compliant and area-efficient hardware implementation of the SLH-DSA algorithm, formerly known as SPHINCS+, a post-quantum digital signature scheme. SPHINCSLET is designed to be parameterizable across different security levels and hash functions, offering a balanced tradeoff between area efficiency and performance. Existing hardware implementations either feature a large area footprint to achieve fast signing and verification or adopt a coprocessor-based approach that significantly slows down these operations. SPHINCSLET addresses this gap by delivering a 4.7× reduction in area compared to high-speed designs while achieving a 2.5× to 5× improvement in signing time over the most efficient coprocessor-based designs for a SHAKE256-based SPHINCS+ implementation. The SHAKE256-based SPHINCS+ FPGA implementation targeting the AMD Artix-7 requires fewer than 10.8K LUTs for any security level of SLH-DSA. Furthermore, the SHA-2-based SPHINCS+ implementation achieves a 2× to 4× speedup in signature generation across various security levels compared to existing SLH-DSA hardware, all while maintaining a compact area footprint of 6K to 15K LUTs. This makes it the fastest SHA-2-based SLH-DSA implementation to date. With an optimized balance of area and performance, SPHINCSLET can assist resource-constrained devices in transitioning to post-quantum cryptography.