Symmetry structured SPN block cipher algorithm

Feistel and SPN are the two main structures in designing a block cipher algorithm. Unlike Feistel, an SPN has an asymmetric structure in encryption and decryption. In this paper we propose an SPN algorithm which has a symmetric structure in encryption and decryption. The whole operations in our SPN algorithm are composed of the even numbers of N rounds where the first half of them, 1 to N/2, applies function and the last half of them, (N+1)/2 to N, employs inverse function. Symmetry layer is executed to create a symmetry block in between function layer and inverse function layer. AES encryption and decryption algorithm, whose safety is already proved, are exploited for function and inverse function, respectively. Proposed symmetry layer is the simple logical operation, because this doesn´t effect at the whole process time of encryption and decryption. The proposed algorithm can be easily applied to the algorithm which has different encryption and decryption and make it same, and it can be good idea to be used to design a new block cipher algorithm.