It is well known that the algebraic expression of ASS S-box is very simple and only 9 terms are involved. Hence, AES security is suspected although there is no vulnerability on it so far. To eliminate the weakness of extremely small terms in the algebraic expression of AES S-box, one improved AES S-box is proposed, which preserves the algebraic degree invariable but significantly increases the number of its algebraic expression terms from 9 to 255. At the same time, Boolean function has good characters in balance and strict avalanche criterion (SAC), etc. Finally, it is proved that the improved AES S-box scheme is secure gainst the powerful known differential and linear cryptanalysis.
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