SPHINCS⁺

SPHINCS⁺

SPHINCS⁺ is a stateless hash-based signature scheme, which was submitted to the NIST post-quantum crypto project. The design advances the SPHINCS signature scheme, which was presented at EUROCRYPT 2015. It incorporates multiple improvements, specifically aimed at reducing signature size. For a quick overview of the changes from SPHINCS to SPHINCS⁺ see the blog post by Andreas Hülsing. The submission proposes three different signature schemes:

• SPHINCS⁺-SHAKE256
• SPHINCS⁺-SHA-256
• SPHINCS⁺-Haraka

These signature schemes are obtained by instantiating the SPHINCS⁺ construction with SHAKE256, SHA-256, and Haraka, respectively.

The second round submission of SPHINCS⁺ introduces a split of the above three signature schemes into a simple and a robust variant for each choice of hash function. The robust variant is exactly the SPHINCS⁺ version from the first round submission and comes with all the conservative security guarantees given before. The simple variants are pure random oracle instantiations. These instantiations achieve about a factor three speed-up compared to the robust counterparts. This comes at the cost of a purely heuristic security argument.