A Novel Multi Modification in AES Block Cipher Algorithm for Complexity Improvement

Abstract

Advanced Encryption System (AES) is a symmetric block cipher that is intended to replace Data Encryption System (DES) as the approved standard for a wide range of applications and it processes data blocks of 128 bits. In this paper three functions have been modified in original AES to achieve higher complexity while keeping the required time for encryption and decryption processes approximately similar. A dual keys SubByte transformation has been created. The main advantage of such approach is that an enormous number of S-boxes can be generated. Normally, in standard AES the ShiftRow transformation uses a single key for encryption and decryption processes, while the modified AES in this research uses five different keys in each round for both operations. In AES, Mix Column Transformation is the most expensive operation where input matrix is multiplied (Over GFⁿ).  In this paper, the proposed algorithm to improve mix column transformation uses four keys with dimension 2*2, which led to increasing the complexity without increasing the time delay for encryption and decryption processes as in AES standard. Comparing the performance results between the original AES and the modified AES algorithm by applying both algorithms to audio files at different lengths, it is found that there is no much difference in time delay between the two schemes which is a novel achievement due to a high increasing of complexity of the code while the time delay is still similar.

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