17-04-2013, 04:34 PM
AES: The Advanced Encryption Standard
AES The Advanced.pdf (Size: 162.72 KB / Downloads: 25)
SALIENT FEATURES OF AES
• AES is a block cipher with a block length of 128 bits.
• AES allows for three different key lengths: 128, 192, or 256 bits.
Most of our discussion will assume that the key length is 128
bits. [With regard to using a key length other than 128 bits,
the main thing that changes in AES is how you generate the key
schedule from the key — an issue I address at the end of Section
8.8.1. The notion of key schedule in AES is explained in Sections
8.2 and 8.8.]
• Encryption consists of 10 rounds of processing for 128-bit keys,
12 rounds for 192-bit keys, and 14 rounds for 256-bit keys.
• Except for the last round in each case, all other rounds are iden-
tical.
• Each round of processing includes one single-byte based substi-
tution step, a row-wise permutation step, a column-wise mixing
step, and the addition of the round key. The order in which these
four steps are executed is different for encryption and decryption.
THE ENCRYPTION KEY AND ITS
EXPANSION
• Assuming a 128-bit key, the key is also arranged in the form of
a matrix of 4 × 4 bytes. As with the input block, the first word
from the key fills the first column of the matrix, and so on.
• The four column words of the key matrix are expanded into a
schedule of 44 words. (As to how exactly this is done, we will
explain that later.) Each round consumes four words from the
key schedule.
• The figure below depicts the arrangement of the encryption key
in the form of 4-byte words and the expansion of the key into a
key schedule consisting of 44 4-byte words.