Paper 2025/548 Original: eprint.iacr.org/2025/548 ↗

Breaking HuFu with 0 Leakage: A Side-Channel Analysis

Abstract

HuFu is an unstructured lattice-based signature scheme proposed during the NIST PQC standardization process. In this work, we present a side-channel analysis of HuFu's reference implementation. We first exploit the multiplications involving its two main secret matrices, recovering approximately half of their entries through a non-profiled power analysis with a few hundred traces. Using these coefficients, we reduce the dimension of the underlying LWE problem, enabling full secret key recovery with calls to a small block-sized BKZ. To mitigate this attack, we propose a countermeasure that replaces sensitive computations involving a secret matrix with equivalent operations derived solely from public elements, eliminating approximately half of the identified leakage and rendering the attack unfeasible. Finally, we perform a non-profiled power analysis targeting HuFu's Gaussian sampling procedure, recovering around 75\% of the remaining secret matrix's entries in a few hundred traces. While full key recovery remains computationally intensive, we demonstrate that partial knowledge of the secret significantly improves the efficiency of signature forgery.