Hardware Implementation of dynamic S-BOX to use in AES Cryptosystem.

One of the major cipher symmetric algorithms is AES. Its main feature is to use S-BOX step, which is the only non-linear part of this standard possessing fixed structure. During the previous studies, it was shown that AES standard security was increased by changing the design concepts of S-BOX and production of dynamic S-BOX. In this paper, a change of AES standard security is studied by production of dynamic and key-dependent S-BOX. Also the LFSR random number generation hardware algorithm is applied in order to produce the dynamic S-BOX. In order to produce a dynamic and key-dependent S-BOX, the field bits of key are divided into separated bits at first and then a byte is selected by LFSR algorithm randomly. The number of selected bit is considered as the repeating number of LFSR algorithm and is applied in order to produce dynamic S-BOX. In the evaluation step, we compared the proposed model with fixed S-BOX model in the original AES algorithm. It was shown that the proposed implementation could increase the security as about 0.2%, 0.017% and 0.19%, 0.04 in the case of avalanche effect, output bits dependence criteria, compared with the strict avalanche criteria and in the case of linear criteria, respectively.