Paper 2025/830 Original: eprint.iacr.org/2025/830 ↗

Simple Power Analysis Attack on SQIsign

Abstract

The isogeny-based post-quantum digital signature algorithm SQIsign offers the most compact key and signature sizes among all candidates in the ongoing NIST call for additional post-quantum signature algorithms. To the best of our knowledge, we present the first Simple Power Analysis (SPA) side-channel attack on SQIsign, demonstrating its feasibility for key recovery. Our attack specifically targets secret-dependent computations within Cornacchia's algorithm, a fundamental component of SQIsign's quaternion module. At the core of this algorithm, a secret-derived yet ephemeral exponent is used in a modular exponentiation subroutine. By performing SPA on the modular exponentiation, we successfully recover this ephemeral exponent. We then develop a method to show how this leaked exponent can be exploited to ultimately reconstruct the secret signing key of SQIsign. Our findings emphasize the critical need for side-channel-resistant implementations of SQIsign, highlighting previously unexplored vulnerabilities in its design.