Hi
ADVANCED ENCRYPTION STANDARD ALGORITHM
The increasing need for protecting
data communication in computer networks has led to development of several
cryptography algorithms. The Advanced Encryption Standard (AES) is a computer security
standard issued by the National Institute of Standards and Technology (NIST)
intended for protecting electronic data. Its specification is defined in
Federal Information Processing Standards (FIPS) Publication 197.
Features of AES algorithm
·
The AES cryptography algorithm can be used to
encrypt/decrypt blocks of 128 bits and is capable of using cipher keys of 128,
196 or 256 bits wide (AES128, AES196, and AES256).
·
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 identical. Each
round of processing includes one single-byte based substitution 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.
·
A 128-bit block as consisting of a 4 × 4 matrix
of bytes, arranged as follows
Therefore,
the first four bytes of a 128-bit input block occupy the first column in the 4
× 4 matrix of bytes. The next four bytes occupy the second column, and so on.
·
The 4 × 4 matrix of bytes is referred to as the
state array.
·
AES also has the notion of a word. A word
consists of four bytes, that is 32 bits. Therefore, each column of the state
array is a word, as is each row.
·
Each round of processing works on the input
state array and produces an output state array.
·
The output state array produced by the last
round is rearranged into a 128-bit output block.
·
AES is a subset and slightly modified form of
Rijndael algorithm.
·
AES requires the block size to be 128 bits, the
original Rijndael cipher works with any block size that is a multiple of 32 as long as it
exceeds 128. The state array for the different block sizes still has only four
rows in the Rijndael cipher.
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. 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.
Expansion of encryption
into 44- 4 Byte word
Complete
Structure of AES
·
The number of rounds are 10, is for the case
when the encryption key is 128 bit long.
·
Before any round-based processing for encryption
can begin, the input state array is do ne XOR with the first four words of the
key schedule. The same thing happens during decryption — except that now we XOR
the ciphertext state array with the last four words of the key schedule.
·
For encryption, each round consists of the
following four steps
1) Substitute bytes
This step consists of
using a 16 × 16 lookup table to find a
replacement byte
for a given byte in the input state array. The entries in the lookup table are created by using the notions of
multiplicative inverses in GF (28) and bit scrambling
to destroy the bit-level correlations inside each byte.
2) Shift rows
For shifting the rows of
the state array during the forward process
3) Mix columns
For mixing up of the
bytes in each column separately during the forward process
4) Add round key.
For adding the round key
to the output of the previous step during the forward
process
·
The last step consists of XOR the output of the previous
three steps with four words from the key schedule.
·
For decryption, each round consists of the
following four steps
1) Inverse shift rows
2) Inverse substitute bytes
3) Add round key
4) Inverse mix columns.
·
The third step consists of XOR the output of the
previous two steps with four words from the key schedule.
·
The last round for encryption does not involve
the “Mix columns” step. The last round for decryption does not involve the
“Inverse mix columns” step.
Thank You
