International Association
for Cryptologic Research

Probing physical bits in hardware has compromised cryptographic systems. This work investigates how to instantiate Shamir's secret sharing so that the physical probes into its shares reveal statistically insignificant information about the secret. Over prime fields, Maji, Nguyen, Paskin-Cherniavsky, Suad, and Wang (EUROCRYPT 2021) proved that choosing random evaluation places achieves this objective with high probability. Our work extends their randomized construction to composite order fields -- particularly for fields with characteristic 2. Next, this work presents an algorithm to classify evaluation places as secure or vulnerable against physical-bit probes for some specific cases. Our security analysis of the randomized construction is Fourier-analytic, and the classification techniques are combinatorial. Our analysis relies on (1) contemporary Bezout-theorem-type algebraic complexity results that bound the number of simultaneous zeroes of a system of polynomial equations over composite order fields and (2) characterization of the zeroes of an appropriate generalized Vandermonde determinant.

Motivated by leakage-resilient secure computation of circuits with addition and multiplication gates, this work studies the leakage-resilience of linear secret-sharing schemes with a small reconstruction threshold against any {\em bounded-size} family of joint leakage attacks, \ie, the leakage function can leak {\em global} information from all secret shares. We first prove that, with high probability, the Massey secret-sharing scheme corresponding to a random linear code over a finite field $F$ is leakage-resilient against any $\ell$-bit joint leakage family of size at most $\abs{F}^{k-2.01}/8^\ell $, where $k$ is the reconstruction threshold. Our result (1) bypasses the bottleneck due to the existing Fourier-analytic approach, (2) enables secure multiplication of secrets, and (3) is near-optimal. We use combinatorial and second-moment techniques to prove the result. Next, we show that the Shamir secret-sharing scheme over a prime-order field $F$ with randomly chosen evaluation places and with threshold $k$ is leakage-resilient to any $\ell$-bit joint leakage family of size at most $\abs{F}^{2k-n-2.01}/(k!\cdot 8^\ell)$ with high probability. We prove this result by marrying our proof techniques for the first result with the existing Fourier analytical approach. Moreover, it is unlikely that one can extend this result beyond $k/n\leq0.5$ due to the technical hurdle of the Fourier-analytic approach.