Abstract: To explain the function of each part of the algorithm we make a detailed analysis of the structure and implementation of AES algorithm. Researching the details of the algorithm, we summarized the characteristics of it, optimized and merged the steps of each round of the algorithm. In order toreduce the complexity of decryption algorithm and make it have the same structure with the encryption algorithm, we changed the order of the decryption algorithm and proved the correctness of this change.

Key words: advanced encryption standard, subkey, encryption algorithm