Q18Information Security Systems
Question
Provide a detailed structure and working mechanism of the Advanced Encryption Standard (AES) algorithm. Discuss the various transformations involved in a single round.
Answer
AES is a symmetric block cipher using substitution-permutation networks over multiple rounds.
The Advanced Encryption Standard (AES) is a symmetric block cipher chosen by NIST to replace DES. It operates on fixed blocks of 128 bits and uses key sizes of 128, 192, or 256 bits, requiring 10, 12, or 14 rounds respectively. AES relies on a substitution-permutation network structure, organizing data into a 4x4 state matrix of bytes.
Each full round of AES (except the final round) consists of four distinct transformation stages:
1. SubBytes: A non-linear substitution step where each byte in the state is replaced with another byte using a predefined substitution box (S-box), providing confusion.
2. ShiftRows: A transposition step where the last three rows of the state matrix are shifted cyclically to the left by specific offsets, providing diffusion.
3. MixColumns: A mixing operation which operates on the columns of the state, combining the four bytes in each column mathematically. This provides substantial diffusion.
4. AddRoundKey: Each byte of the state is combined with a block of the round key using bitwise XOR. The round keys are derived from the master key via a key schedule.
The final round omits the MixColumns stage. The symmetric nature means the decryption process applies inverse transformations in reverse order.
