Modes of Operation for AES and Symmetric Algorithms

Modern encryption requires fast symmetric algorithms with relatively short keys. Several such ciphers are in use, notably the Advanced Encryption Standard (AES) algorithm. This chapter briefly discusses some modes of operation for general block encryption algorithms, i.e. block ciphers. The AES code is based on properties of the finite field of 256 elements. The first step in a round of the code is called the ByteSub Transformation (BS), which is a nonlinear transformation and is therefore resistant to linear and differential attacks. The second step is called the ShiftRow Transformation (SR). The third step is called MixColumn Transformation (MC). This step creates high diffusion between the columns over multiple rounds of the code. The final step is referred to as the RoundKey Addition (ARK) step. The decryption of the AES code is based on the fact that each of the steps BS, SR, MC, and ARK are invertible.