An Efficient and Reconfigurable Post-Quantum Crypto-Processor for SPHINCS+

SPHINCS+ is the sole hash-based digital signature scheme among the selected post-quantum cryptography (PQC) in 2022. This algorithm possesses the ability to resist attacks from both classical and quantum computers. Due to the extensive computations and different data widths for various parameters, its hardware implementation faces the weakness of long operation time, large area requirement, and low flexibility. This paper presents an efficient and reconfigurable SPHINCS+ processor. The proposed on-the-fly WOTS+ public key generation scheme with unified chain address generator accelerated the most time-consuming operations. This optimization achieves efficient resource utilization. A security switch mechanism resolves the bit misalignment among different data widths with resource reduction. Finally, we introduce a grouped subtree and segmented signature streaming scheme. They reduce the memory to 16k bytes. The processor consumes 29410 LUTs, 14090 FFs, 4 BRAMs on Artix-7 FPGA and achieves $1.04\times/2.41\times $ ATPs (area-time-product) optimizations in Sign/Verify with the advantage of supporting all security levels of SPHINCS+.