ARMed SPHINCS : computing a 41KB signature in 16KB of RAM

This paper shows that it is feasible to implement the stateless hash-based signature scheme SPHINCS-256 on a "very small device" with memory even smaller than a signature and limited computing power. We demonstrate that it is possible to generate and verify the 41\,KB signature on an ARM Cortex M3 that only has 16\,KB of memory available. We provide benchmarks for our implementation which show that this can be used in practice. To analyze the costs of using the stateless SPHINCS scheme instead of its stateful alternatives, we also implement XMSS^{MT} on this platform and give a comparison. Keywords: post-quantum cryptography, hash-based signature schemes, microcontroller, resource-constrained devices, ARM Cortex M3, SPHINCS-256, XMSS^{MT}