Improved Differential and Linear Active S-Boxes Search Techniques for Feistel Type Ciphers

We propose improved differential and linear active S-boxes search techniques for Feistel type ciphers. We give a uniform representation of Feistel type structures which can benefit the analysis of differential propagation. By analyzing the properties of Feistel type environment, we present some important observations of differential propagation and propose a notion of equivalent state set which can narrow down the search space noticeably. We present a practical algorithm to improve the search of active S-boxes for Feistel type ciphers. It is basically a Viterbi search operating on equivalent state set and also adopts the pruning mechanism. Our experimental results show that the improved algorithm have advantages in respect to memory and time complexities, and it can be applied efficiently to Feistel cipher with large blocks. The search program can be implemented in normal PC, which will be more practical and useful for the designers and cryptanalysts.