History:
 

The DES algorithm was developed in 1975 and was considered fairly strong and superior for that time and computation power at that time but with newer developments in computer systems and increasing computational power it became breakable faster and the process became relatively easy. It was cracked for the first time in 1997, so a more robust algorithm had to be developed. Triple DES was created to fix the problem but was relatively slower so was not adapted by most. Hence, the AES was developed, also known as the Rijnadael algorithm, is a symmetric block cipher algorithm with block size of 128-bits. Keys of 128, 192 and 256 bits are used to convert these individual blocks, once encrypted the blocks are joined together to form the ciphertext.

 

Following are the steps to perform AES algorithm:

 

Steps for key expansion:

1. For the first four words w₀, w₁, w₂ and w₃, consider the cipher key as an array of 16 bytes (k₀ to k₁₅). Then, w₀= k₀ to k₃, w₁= k₄ to k₇, w₂= k₈ to k₁₁ and w₃= k₁₂ to k₁₅.
2. For remaining words i.e., w₄ to w₄₃ the procedure to follow is:
   1. If i mod 4 ≠ 0, w_i= w_i-1 ⊕ w_i-4.
   2. If i mod 4 = 0, w_i = t ⊕ w_i-4, where

t = SubWord (RotWord (w_i-1)) ⊕ RCon_i/4

RotWord is similar to ShiftRows transformation, but is applied to only one row i.e., it takes a word (4 bytes) and shifts each byte to left with wrapping.

SubWord is similar to SubBytes transformation, but is applied only to four bytes i.e., it takes each byte in the word and substitutes another byte for it from a predefined 16 x 16 (0 to F x 0 to F) matrix.

Round Constants or RCon, is a 4-byte value with three rightmost bytes as zero. The table below shows values for AES-128 with 10 rounds.

Round	RCon	Round	RCon
1	(01 00 00 00)16	6	(20 00 00 00)16
2	(02 00 00 00)16	7	(40 00 00 00)16
3	(04 00 00 00)16	8	(80 00 00 00)16
4	(08 00 00 00)16	9	(1B 00 00 00)16
5	(10 00 00 00)16	10	(36 00 00 00)16

 

Encryption:

      1. Shift Byte Transformation

• ‘sbox’ is a 16 x 16 matrix consisting of total 256 entries of 8 bit each.
• Each byte of the state array is split in two hexadecimal values.
• These are row and column values for to sbox to generate the new value for final state array.

      2. Shift Row Transform

• Shifts elements of a row to left depending on the row they are present in.
• First row remains unaffected.
• Second row elements are shifted one place to the left.
• Third row elements are shifted two places to left.
• Fourth row elements are shifted three places to the left.

      3. Mixed Column Transform

• A constant matrix is multiplied to each column of state array.
• It is important that the constant matrix is same for all columns and this step is skipped for the last round.

      4. Add Round Key Transform

• The respective key for the round is XOR’d with state array generated in previous round.
• The resulting array is then passed as the new step array for the next round.
• The resulting array of last round becomes the ciphertext for the block.