APPARATUS AND METHOD FOR EVALUATING A CIPHER STRUCTURE'S RESISTANCE TO CRYPTANALYSIS

Disclosed is a method for evaluating resistance to cryptanalysis of a cipher structure having a diffusion element including a linear transformation placed between differently-sized confusion elements at an input and an output of the diffusion element. A generalized minimum number of non-zero symbols at the diffusion element's input and output is determined. The diffusion element's input is divided into subset inputs, each having a size corresponding to the size of each confusion element at the diffusion element input. For each subset input, a subset number of non-zero symbols at the subset input and the diffusion element output is determined. Each subset number is summed to generate a summed subset number. The summed subset number is subtracted from the generalized minimum number to generate a worst-case number. An upper bound of a maximum differential characteristic probability is calculated and used to evaluate the cipher structure.