International Association
for Cryptologic Research

Safeguarding cryptographic implementations against the increasing threat of Side-Channel Analysis (SCA) attacks is essential. Masking, a countermeasure that randomizes intermediate values, is a cornerstone of such defenses. In particular, SCA-secure implementation of AES, the most-widely used encryption standard, can employ Boolean masking as well as multiplicative masking due to its underlying Galois field operations. However, multiplicative masking is susceptible to vulnerabilities, including the zero-value problem, which has been identified right after theintroduction of multiplicative masking. At CHES 2018, De Meyer et al. proposed a hardware-based approach to manage these challenges and implemented multiplicative masking for AES, incorporating a Kronecker delta function and randomness optimization. In this work, we evaluate their design using the PROLEAD evaluation tool under the glitch- and transition-extended probing model. Our findings reveal a critical vulnerability in their first- and second-order implementation of the Kronecker delta function, stemming from the employed randomness optimization. This leakage compromises the security of their presented masked AES Sbox. After pinpointing the source of such a leakage, we propose an alternative randomness optimization for the first-order design to address this issue, and demonstrate its effectiveness through rigorous evaluations by means of PROLEAD.