Polynomial-Time Key-Recovery Attack on the Specification of
River Moreira Ferreira, Sorbonne University
Ludovic Perret, Sorbonne University
Abstract
In this paper, we present an efficient attack against , a recent variant of the popular Unbalanced Oil and Vinegar () multivariate signature scheme, that has been submitted to the ongoing standardization process for additional post-quantum signature schemes. A notable feature of is its proof of security, namely, existential unforgeability under a chosen-message attack (), assuming the hardness of solving the system formed by the public-key non-linear equations.
We present a polynomial-time key-recovery attack against the first specification of (v). To do so, we remark that a small fraction of the secret-key is leaked during the signature process. Adapting and extending previous works on basic , we show that the entire secret-key can be then recovered from such a small fraction in polynomial-time. This leads to an efficient attack against that we validated in practice. For all the security parameters suggested in by the authors of , our attack recovers the secret-key in at most seconds. We conclude the paper by discussing the apparent mismatch between such a practical attack and the theoretical security claimed by designers. Our attack is not structural but exploits that the current specification of differs from the required security model.
A simple countermeasure makes immune against the attack presented here and led the designers to update the specification of (v).