Datasheet

13.2.2 AES Algorithm
The AES algorithm generates block ciphers. The AES block size is 16 bytes. The AES key(s) can
be coded on 128, 192, or 256 bits. The larger key sizes provide a higher level of security, but at the
cost of a moderate decrease in throughput.
For the AES algorithm:
■ The length of the input and output blocks is 128 bits, which is represented by Nb = 4, which
reflects the number of 32-bit words.
■ 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.
■ 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).
Table 13-1 on page 967 lists the combinations of keys, blocks, and rounds.
Table 13-1. Key-Block-Round Combinations
Number of Rounds (Nr)Block Size (Nb)Key Length (Nk)Key
1044128 bits
1246192 bits
1448256 bits
The AES algorithm for cipher and inverse cipher uses a round function composed of four different
byte-oriented transformations:
■ Byte substitution using a substitution table (S-Box)
This transformation is a non-linear byte substitution that operates independently on each byte
of the state (the state is an intermediate processed block of 128 bits inside the AES; the state
is arranged as an array of [4 x Nk] bytes) using an S-Box. This S-Box transformation is reversible.
■ Shifting rows of the state array by different offsets
In this transformation, the bytes in the last three rows of the state are cyclically shifted over
different numbers of bytes (offsets). The first row (r = 0) is not shifted.
■ Mixing the data within each column of the state array
This transformation operates on the state column-by-column, treating each column as a four-term
polynomial. The columns are considered polynomials over GF(2
8
) and multiplied modulo x
4
+
1 with a fixed polynomial a(x).
■ Adding a round key to the state
In this transformation, a round key is added to the state by a simple bitwise XOR operation. Each
round key consists of Nb words from the key schedule.
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 x (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 [w
i
], with i in the range 0 ≤ i < Nb
x (Nr + 1).
967June 18, 2014
Texas Instruments-Production Data
Tiva
™
TM4C129ENCPDT Microcontroller