Design and Implementation of Hardware-Software Architecture Based on Hashes for SPHINCS

Advances in quantum computing have posed a future threat to today's cryptography. With the advent of these quantum computers, security could be compromised. Therefore, the National Institute of Standards and Technology (NIST) has issued a request for proposals to standardize algorithms for post-quantum cryptography (PQC), which is considered difficult to solve for both classical and quantum computers. Among the proposed technologies, the most popular choices are lattice-based (shortest vector problem) and hash-based approaches. Other important categories are public key cryptography (PKE) and digital signatures. Within the realm of digital signatures lies SPHINCS+. However, there are few implementations of this scheme in hardware architectures. In this article, we present a hardware-software architecture for the SPHINCS+ scheme. We utilized a free RISC-V (Reduced Instruction Set Computer) processor synthesized on a Field Programmable Gate Array (FPGA), primarily integrating two accelerator modules for Keccak-1600 and the Haraka hash function. Additionally, modifications were made to the processor to accommodate the execution of these added modules. Our implementation yielded a 15-fold increase in performance with the SHAKE-256 function and nearly 90-fold improvement when using Haraka, compared to the reference software. Moreover, it is more compact compared to related works. This implementation was realized on a Xilinx FPGA Arty S7: Spartan-7.