AES Key Expansion Algorithm in Block Cipher - Data Encryption Course
Learn the AES key expansion algorithm step by step for key length of 128 bits, essential for data encryption in block cipher. Dr. Fatimah Al-Ubaidy explains the key generation process and the significance of round constants in the encryption process.
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Mustansiriyah University Engineering College AES Block Cipher Computer Engineering Dep. Key Generation Algorithm Lecturer: Dr. Fatimah Al-Ubaidy Block Cipher Class: Third Stage Course name: Data Encryption AES Key Expansion (for key length of 128 bits) The AES key expansion algorithm takes as input a 4-word (16-byte) key and produces a linear array of 44 words (176 bytes). This is sufficient to provide a 4-word round key for the initial AddRoundKey stage and each of the 10 rounds of the cipher. The following pseudocode describes the expansion: for (i = 0 ; i < 4 ; i++) [key[4*i] , key[4*i+1] , key[4*i+2] , key[4*i+3]] for (i = 4; i < 44 ; i++) temp = w[i 1]; if (i mod 4 = 0) temp = SubWord ( RotWord ( temp ) ) Rcon[i / 4]; w[i] = w[i 4] temp Where *RotWord performs a one-byte circular left shift on a word. *SubWord performs a byte substitution on each byte of its input word using the S-box. *Rcon[j] is a round constant. 1
Mustansiriyah University Engineering College AES Block Cipher Computer Engineering Dep. Key Generation Algorithm Lecturer: Dr. Fatimah Al-Ubaidy Block Cipher Class: Third Stage Course name: Data Encryption AES Key Expansion The round constant is different for each round and is defined as Rcon[j] = (RC[j], 0, 0, 0), with RC[1] = 1, RC[j] = 2 RC[j - 1]. The values of RC[j] in hexadecimal are: For example: Suppose that the round key for round 8 is EA D2 73 21 B5 8D BA D2 31 2B F5 60 7F 8D 29 2F Then the first 4 bytes (first column) of the round key for round 9 are calculated as follows: 2
Mustansiriyah University Engineering College AES Block Cipher Computer Engineering Dep. Key Generation Algorithm Lecturer: Dr. Fatimah Al-Ubaidy Block Cipher Class: Third Stage Course name: Data Encryption The powerful of the expansion key algorithm: Knowledge of a part of the cipher key or round key does not enable calculation of many other round-key bits. An invertible transformation [i.e., knowledge of any N consecutive words of the expanded key enables regeneration the entire expanded key] (N is key size in words). Speed on a wide range of processors. Usage of round constants to eliminate symmetries. Diffusion of cipher key differences into the round keys; that is, each key bit affects many round key bits. Enough nonlinearity to prohibit the full determination of round key differences from cipher key differences only. Simplicity of description. 3
Mustansiriyah University Engineering College AES Block Cipher Computer Engineering Dep. Key Generation Algorithm Lecturer: Dr. Fatimah Al-Ubaidy Block Cipher Class: Third Stage Course name: Data Encryption 4
