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A New Approach for Designing Key-Dependent S-Box Defined over GF (24) in AES

—In this paper a new approach for designing S-box in Advanced Encryption Standard (AES) is proposed. The proposed S-box is constructed from small S-boxes defined over GF (2 4) instead of GF (2 8) as in traditional AES. Rijndael Algorithm (RA), as one of AES standards, is modified by applying the new approach. The Modified Rijndael Algorithm (MRA) is constructed by replacing the S-box of RA by small S-boxes, and the key expansion procedure of RA is modified consequently. Each one of the small S-boxes has different equation and each equation is extracted using one of the three irreducible polynomials existing in GF (2 4). So, detecting different equations by cryptanalysts is very difficult compared to the S-box of RA which uses one equation and one irreducible polynomial. The substitution from small S-boxes is done based on the round key, so this achieves diffusion, confusion and therefore security for MRA. The MRA is tested using avalanche effect and strict avalanche criterion (SAC) to evaluate security. The performance evaluation is calculated and proved that MRA is more suitable for the applications that require security and QoS such as voice over IP (VoIP).