A Stochastic Model for Differential Side Channel Cryptanalysis

This contribution presents a new approach to optimize the efficiency of differential side channel cryptanalysis against block ciphers by advanced stochastic methods. We approximate the real leakage function within a suitable vector subspace. Under appropriate conditions profiling requires only one test key. For the key extraction we present a ‘minimum principle’ that solely uses deterministic data dependencies and the ‘maximum likelihood principle’ that additionally incorporates the characterization of the noise revealed during profiling. The theoretical predictions are accompanied and confirmed by experiments. We demonstrate that the adaptation of probability densities is clearly advantageous regarding the correlation method, especially, if multiple leakage signals at different times can be jointly evaluated. Though our efficiency at key extraction is limited by template attacks profiling is much more efficient which is highly relevant if the designer of a cryptosystem is bounded by the number of measurements in the profiling step.