Advance Encryption Standard

Advance Encryption Standard (Rijndael)

The Advanced Encryption Standard (Rijndael) is a symmetric-key encryption standard. It encrypts a 128-bit plaintext into 128-bit cipher text using variable key either of 128 bits, 192 bits and 256 bits, this difference of key lengths in AES is referred to: AES-128, AES-192 and AES 256.[pic 1]

[pic 2]

[pic 3]

Let,

Plain Text: 12 34 56 78 90 AB CD EF 12 34 56 78 90 AB CD EF

Key            : 12 90 12 90 34 AB 34 AB 56 CD 56 CD 78 EF 78 EF

We will write plain text and key in state matrix form column wise as shown below.

P.T. =                         Key =          [pic 4][pic 5]

AES Key Expansion:

AES key expansions can be understand easily with the help of the following diagram.

[pic 6][pic 7]

In above figure there is one unknown element which is ‘g’. Working of ‘g’ is as follow:

1. 8 Bit Left Shift/Rotate
2. Substitution BOX
3. XOR with round constant

According to figure 2:

W4 = W0 ⊕ g (W3)

W5 = W4 ⊕ W1

W6 = W5 ⊕ W2

W7 = W6 ⊕ W3

Here,

W0 = 12 90 12 90

W1 = 34 AB 34 AB

W2 = 56 CD 56 CD

W3 = 78 EF 78 EF

For W4

g (W3) = g (78 EF 78 EF )

                = EF 78 EF 78                 after         8 Bit Left Circular Shift