A Five-Round Algebraic Property of the Advanced Encryption Standard

Abstract. This paper presents a five-round algebraic property of the Advanced Encryption Standard (AES). In the proposed property, we modify twenty bytes from five intermediate values at some fixed locations in five consecutive rounds, and we show that after five rounds of operations, such modifications do not change the intermediate result and finally still produce the same ciphertext. We introduce an algorithm named δ, and the algorithm accepts a plaintext and a key as two inputs and outputs twenty bytes, which are used in the five-round property. We demonstrate that the δ algorithm has 20 variants for AES-128, 28 variants for AES-192 and 36 variants for AES-256. By employing the δ algorithm, we define a modified version of the AES algorithm, the δAES. The δAES calls the δ algorithm to generate twenty bytes, and uses these twenty bytes to modify the AES round keys. The δAES employs the same key scheduling algorithm, constants and round function as the AES. For a plaintext and a key, the AES and the δAES produce the same ciphertext.