The Design And Analysis Of Block Cipher

In this paper we introduce the concept of the block cipher and its position in the communication systems. According to the AES and NESSIE projects, we propose the structures, some design principles and some analysis methods of the block cipher. Our main work is on Twofish and IDEA and we get some interesting results.Twofish is a Feistel cipher,and the length of the key is 128 192 or 256 bits, and the length of the block is 128 bits. The S-boxes are the key-dependent S-boxes that are derived from the key. We show that the longer the key of Twofish is, the smaller the differential probabilities of the key-dependent S-boxes are. And we propose a new method to construct the key-dependent S-boxes, and get that the modified S-boxes are better than the original S-boxes in xor differential and modular addition differential properties from the theory and simulation results.IDEA is a good block cipher, and it has no obvious weakness in the safety. It went into the phase II of the NESSIE project successfully. To analyze the algorithm, we simplify it. And we refer to the simplified algorithm as Variant IDEA. We break the Variant IDEA - IDEA-X14 of 14 rounds and IDEA-M8 of 8 rounds- by a new analysis method called Multiplicative Differentials. Fortunately, the original IDEA is still safe to the Multiplicative Differentials.