Research On The Improvement Of Block Cipher Diffusion Layer

The block cipher encryption algorithms have many features better than that of other ones, such as fast encryption speed, easy to be standardized and convenient to be implemented by hardware and software, which make they play a central role in data encryption, message authentication, key management and certification in communication and information network. They have an irreplaceable position in the computer, communications and information security system field. Therefore, the research of block cipher is one of focus directions in the filed of cryptography and even the whole information security all the time.The proposition and formulation of the new encryption standard-AES, makes DES which serves more than 20 years in the field of information security be replaced in many applications. But the exquisite design and stable safety performance of DES affect the world of cryptography all the time, many scholars pay the tireless efforts for analysis and improvement of the DES algorithm. Meanwhile, researchers more focuse on the analysis of the safety performance and the encryption efficiency of the algorithm, and the improvements and implementation in a variety of security application fields to the newly proposed data encryption standard AES.First, the design principles and criteria of block cipher are analyzed and explained in this thesis and the encryption and decryption principles of DES and AES, which are two typical block cipher encryption algorithms, are described. At the same time, the attack-resistant performances of these two algorithms are described systematically. Meanwhile, the research direction and status of these two algorithms in present cryptography world are introduced. Based on these analyses, two improved algorithms aiming to diffusion layer of block cipher are presented. The study consists of two parts. The first part is about the analysis and improvement of the fixed replacement IP table. The paper proposes the algorithm which uses the key to generate nonlinear substitution tables. In this scheme the main frame is not changed and the length of the encryption key is increased without affecting the computation complexity. Then the scheme is implemented, and the analysis of anti-attack performance especially for key exhaustion attack and the differential cryptanalysis attack is also presented. The second part is mainly about the analysis and design of the diffusion layer in SPN cryptographic structure. In this part, the relationship of cyclic matrix and the optimal diffusion layer is described, and by using the theory of linear branch number the algorithm which can generate arbitrary order cyclic matrix with optimal diffusion is also constructed. Based on the analysis of all cyclic matrixes with optimal diffusion, the paper proposed that when the matrix elements meet some conditions, the number of the cyclic matrix with optimal diffusion is approximate to an even distribution. So, these results provide a good theoretical foundation to the design of diffusion layer in cryptographic algorithms.