SPRUJB6B November 2024 – May 2025 AM2612
The AES algorithm generates block ciphers. Block ciphers, as opposed to stream ciphers, operate on blocks of plaintext and cipher text. The AES block size is 16-byte. The AES keys can be coded on 128, 192, or 256 bits. The larger key sizes provide a higher level of security, at the cost of a moderate decrease in throughput.
For the AES algorithm, the length of the input block, the output block is 128 bits. This is represented by Nb = 4, which reflects the number of 32-bit words.
For the AES algorithm, the length of the Cipher Key, K, is 128, 192, or 256 bits. The key length is represented by Nk = 4, 6, or 8, which reflects the number of 32-bit words in the Cipher Key. For the AES algorithm, the number of rounds to be performed during the execution of the algorithm is dependent on the key size. The number of rounds is represented by Nr, where Nr = 10 when Nk = 4(128-bit key), Nr = 12 when Nk = 6(192-bit key), and Nr = 14 when Nk = 8(256-bit key).
| Key Size | Key Length (Nk words) | Block Size (Nb words) | Number of Rounds (Nr) |
|---|---|---|---|
| 128 bits | 4 | 4 | 10 |
| 192 bits | 6 | 4 | 12 |
| 256 bits | 8 | 4 | 14 |
For both its Cipher and Inverse Cipher, the AES algorithm uses a round function that is composed of four different byte-oriented transformations:
Key expansion :
The AES algorithm takes the Cipher Key, K, and performs a Key Expansion routine to generate a key schedule. The Key Expansion generates a total of Nb * (Nr + 1) words: the algorithm requires an initial set of Nb words, and each of the Nr rounds requires Nb words of key data.
The resulting key schedule consists of a linear array of 4-byte words, denoted [wi], with i in the range 0 ≤ i <Nb * (Nr + 1).
Inverse Cipher:
Each of the basic transformations can be inverted and then implemented in reverse order to produce a straightforward Inverse Cipher for the AES algorithm.
Keying Restrictions:
Unlike the DES, no weak or semi-weak keys have been identified for the AES algorithm, and there is no restriction on key selection.