On Differential and Linear Approximation of S-box Functions

In the paper the differential and the linear approximations of two classes of S-box functions are considered. The classes are the permutations and arbitrary functions with n binary inputs and m binary outputs, where 1 ≤ n=m ≤10. For randomly chosen functions from each of the classes, the distribution of the best nonzero approximations is investigated. The based on the definitions of differential and linear approximation algorithms to compute a single element of the approximation tables, are of exponential complexity. The presented in the paper fast algorithms compute the best nonzero approximations in at worst linear time for a single element, without memory needed for storage of the whole table.