Differential Analysis and Modification of the Key-Dependent S-Boxes of Twofish

This paper proves that the longer the key of Twofish is,the smaller the differ-ential probabilities of the key-dependent S-boxes are. A new method is proposed to produce the key-dependent S-boxes. Theory and simulation results show that the XOR differential probability and modular addition differential probability of the modified S-boxes are smaller than those of the original S-boxes.