International Association
for Cryptologic Research

In this work, we study the singular locus of the varieties defined by the public keys of UOV and VOX, two multivariate quadratic signature schemes submitted to the additional NIST call for signature schemes. Singular points do not exist for generic quadratic systems, which enables us to introduce two new algebraic attacks against UOV-based schemes. We show that they can be seen as an algebraic variant of the Kipnis-Shamir attack, which can be obtained in our framework as an enumerative approach of solving a bihomogeneous modeling of the computation of singular points. This allows us to highlight some heuristics implicitly relied on by the Kipnis-Shamir attack.

We give new attacks for UOV$^{\hat +}$ and VOX targeting singular points of the public key equations. Our attacks lower the security of the schemes, both asymptotically and in number of gates, showing in particular that the parameters sets proposed for these schemes do not meet the NIST security requirements. More precisely, we show that the security of UOV$^{\hat +}$ was overestimated by factors $2^{22}, 2^{36}, 2^{59}$ for security levels $I, III, V$ respectively. We conclude the attack on VOX by showing that an attacker can perform a full key recovery from one vector obtained in the previous attacks.