Paper 2020/128

LegRoast: Efficient post-quantum signatures from the Legendre PRF

Ward Beullens and Cyprien Delpech de Saint Guilhem

Abstract

We introduce an efficient post-quantum signature scheme that relies on the one-wayness of the Legendre PRF. This "LEGendRe One-wAyness SignaTure" (LegRoast) builds upon the MPC-in-the-head technique to construct an efficient zero-knowledge proof, which is then turned into a signature scheme with the Fiat-Shamir transform. Unlike many other Fiat-Shamir signatures, the security of LegRoast can be proven without using the forking lemma, and this leads to a tight (classical) ROM proof. We also introduce a generalization that relies on the one-wayness of higher-power residue characters; the "POwer Residue ChaRacter One-wAyness SignaTure" (PorcRoast). LegRoast outperforms existing MPC-in-the-head-based signatures (most notably Picnic/Picnic2) in terms of signature size and speed. Moreover, PorcRoast outperforms LegRoast by a factor of 2 in both signature size and signing time. For example, one of our parameter sets targeting NIST security level I results in a signature size of 7.2 KB and a signing time of 2.8ms. This makes PorcRoast the most efficient signature scheme based on symmetric primitives in terms of signature size and signing time.