Masked NTT-Multiplication with Fresh Randomness on ML-KEM Decapsulation to Hinder Profiling-based Attacks for Cortex-M4

Abstract

NIST standardized CRYSTALS-Kyber as an ML-KEM, a post-quantum key encapsulation mechanism to resist attacks using quantum computers. However, side-channel analysis, which exploits analog leakage from electronic devices, has already broken several implementations. In particular, the Fujisaki-Okamoto transform requires decryption computations that involve the secret key, making counter-measures essential. Although masking-based countermeasures have been proposed, prior work has shown that key recovery may remain possible with a small number of traces, particularly via profiling-based attacks. In this paper, we propose an instruction-level hiding technique for masked multiplication in the NTT domain, referred to as basemul. The method computes a fresh random multiplication within the same instruction as the secret-dependent multiplication, introducing a secret-independent component into the physical leakage without increasing the masking order. Through power analysis experiments on a Cortex-M4 target, we show that the proposed method reduces the tested first-order correlation. Our model-based analysis further estimates a reduction in the signal-to-noise ratio, suggesting that the method may increase the effort required for profiling-based attacks. We further discuss the scheme's limitations against higher-order non-profiling attacks, which remain effective against the proposed method.