Paper 2021/1495

Veri

Mahmoud Yehia, Riham AlTawy, and T. Aaron Gulliver

Abstract

SPHINCS+ is a stateless hash-based digital signature scheme and an alternate candidate in round 3 of the NIST Post- Quantum Cryptography standardization competition. Although not considered as a finalist because of its performance, SPHINCS+ may be considered for standardization by NIST after another round of evaluations. In this paper, we propose a Verfi able Obtained Random Subsets (v-ORS) generation mechanism which with one extra hash computation binds the message with the signing FORS instance (the underlying few-time signature algorithm). This enables SPHINCS+ to off er more security against generic attacks because the proposed modi cation restricts the ORS generation to use a hash key from the utilized signing FORS instance. Consequently, such a modi cation enables the exploration of di erent parameter sets for FORS to achieve better performance at the same security level. For instance, when using v-ORS, one parameter set for SPHINCS+-256s provides 82.9% reduction in the computation cost of FORS which leads to around 27% reduction in the number of hash calls of the signing procedure. Given that NIST has identfi ed the performance of SPHINCS+ as its main drawback, these results are a step forward in the path to standardization.