Study And Software Implement Of The AES Algorithm In The CTCS-3 System

CTCS-3 level Train Control System (Chinese Train Control System-3) is one of the ten major overall, strategic and forward-looking projects in "the eleventh five-year development plan for railway technology",which can improve China's railway transportation safety. Security features of DES algorithm used in th CTCS-3 system is one of the key factors to guarantee the safety communication in CTCS-3. Due to the weak key problem of DES algorithm and AES algorithm is far better than the DES algorithm in the safety aspects, so it's necessary to use AES algorithm in CTCS-3 instead of DES.First of all, the CTCS-3 safety communications is introduced briefly. And analysis of the transmission network of CTCS-3 as well as security risks of CTCS-3 is carried out and we find out that the DES algorithm is one of the most importance issues for the CTCS-3 safitey commutincation. Compared with DES algorithm, the advantages of AES algorithm are described as well as the significance of AES study.Secondly, the basic principles of AES algorithm are introduced. And we focus on the improved implement scheme of AES algorithm. Due to the relativity of the encryption and decryption, the integrated design of them is introduced. At the same time, the use of dual AES and the new SubByte transformation bases on the expansion of the limited domain can also reduce the cost of AES implement. Compare the three kinds of different ways to calculate the inversion of elements in the finite field and summarizes applicable rules of the three kinds of methods. And mapping between the finite isomorphism fields is also analyzed.Finally, the Markov feature of AES S-box is analyzed and the weakness of the iterative short cycle is pointed out. Exhaustive method and simple method to find the AES S box whose cycle is 256 are discussed. Simple method is to switch the position of some elements in the truth table. Making use of the calculation rule in the finite field, Lagrangian formula is simplified to get algebraic expression of the S-box. The avalanche testing and difference testing of the optimization S-box are carried out and its cryptography features remain the same.