Subject: Information and Network Security
Semester: BE Computer Sem-7
Question: Advanced Encryption Standard (AES)
Answer:
For AES defined a cipher in which the block length and the key length can be independently specified to be 128, 192, or 256 bits.
2. The third transformation is missing in the last round.
1) Add Round Key
Here each byte of the input state is combined with corresponding byte of the subkey by doing bitwise XOR.
Input State ⊕ Cipher Key
Ex : 3 2 0011 0010
⊕ 2 b 0010 1011
1 9 0001 1001
2) Subbyte transformation
The value in the state is substituted by another value according to a lookup table S-box.
The leftmost 4-bit of the byte are used as row number and rightmost 4-bit are used as column number.
Now row and column number serves as index into the S-Box to select unique 8-bit output value.
For Ex hexadecimal value 19 is refered to row 1 and column 9 and value in table at that position is D4 so byte value 19 is replaced with D4.
The first row of state is not altered(changed).
For the second row a 1-byte circular left shift is performed.
For the Third row a 2-byte circular left shift is performed.
For the fourth row a 3-byte circular left shift is performed.
4) Mix Columns
In this step four byte of each column is replace by doing multiplication with a fixed matrix show below.
