Paper 2025/1202

t-Probing (In-)Security - Pitfalls on Noise Assumptions

Abstract

The ongoing transition to post-quantum cryptography has led to a surge of research in side-channel countermeasures tailored to these schemes. A prominent method to prove security in the context of side-channel analysis is the utilization of the well-established t-probing model. However, recent studies by Hermelink et al. at CCS 2024 demonstrate a simple and practical attack on a provably secure implementation of the Fujisaki-Okamoto transform. In this paper, we present an unsupervised single-trace side-channel attack on a tenth-order masked implementation of fixed-weight polynomial sampling. Notably, this masking scheme has also been proven to be secure in the t-probing model. Both attacks reveal a mismatch between the correct, well-understood theory of the t-probing model and its practical application – the security proofs are valid, yet the attacks still succeed at high noise levels. Therefore, we take a closer look at the underlying causes and the assumptions made for transferring t-probing security to practice. In particular, we investigate the amount of noise required for this transfer. We find that, depending on the design decisions, the additional noise required can be substantial and difficult to achieve. Consequently, we examine the factors that impact the required amount of additional noise and which need to be considered for practically secure implementations. In particular, non-uniformly distributed shares – a setting that is increasingly encountered in post-quantum cryptographic algorithms – can lead to an increased noise requirement, and thus could reduce the security level of the masked implementation. Our analysis allows us to provide practical guidelines for masking scheme designers, thereby facilitating the development of theoretically and practically secure designs.