Finding balanced S-boxes with high nonlinearity and low transparency order is a difficult problem. The property of transparency order is important since it specifies the resilience of an S-box against differential power analysis. Better values for transparency order and hence improved side-channel security often imply less in terms of nonlinearity. Therefore, it is impossible to find an S-box with all optimal values. Currently, there are no algebraic procedures that can give the preferred and complete set of properties for an S-box. In this paper, we employ evolutionary algorithms to find S-boxes with desired cryptographic properties. Specifically, we conduct experiments for the 8×8 S-box case as used in the AES standard. The results of our experiments proved the feasibility of finding S-boxes with the desired properties in the case of AES. In addition, we show preliminary results of side-channel experiments on different versions of "improved" S-boxes.
['Stjepan Picek', 'Baris Ege', 'Lejla Batina', 'Domagoj Jakobovic', 'Łukasz Chmielewski', 'Marin Golub']
On using genetic algorithms for intrinsic side-channel resistance: the case of AES S-box