Faster Evaluation of SBoxes via Common Shares

We describe a new technique for improving the efficiency of the masking countermeasure against side-channel attacks. Our technique is based on using common shares between secret variables, in order to reduce the number of finite field multiplications. Our algorithms are proven secure in the ISW probing model with n > t + 1 shares against t probes. For AES, we get an equivalent of 2.8 nonlinear multiplications for every SBox evaluation, instead of 4 in the Rivain-Prouff countermeasure. We obtain similar improvements for other block-ciphers. Our technique is easy to implement and performs relatively well in practice, with roughly a 20% speed-up compared to existing algorithms.