AES - Advanced Encryption standard

Rounds

1. SubBytes—a non-linear substitution step where each byte is replaced with another according to a lookup table.
2. ShiftRows—a transposition step where each row of the state is shifted cyclically a certain number of steps.
3. MixColumns—a mixing operation which operates on the columns of the state, combining the four bytes in each column.
4. AddRoundKey.

AES has 10 rounds in which state the following transformations (called `layers’):

                        Byte substitution (1 S-box used on every byte)

                        Shift rows (permute bytes between groups/columns)

                        Mix columns (uses matrix multiplication in GF(256))

                                    Add round key (XOR state with round key)

 

[1]ByteSub:

 

Each byte of state is replaced by byte indexed by row (left 4-bits) & column (right 4-bits)

Example:  byte {95} is replaced by byte in row 9  column 5 which has value {2A}

[2] ShiftRows

 

 Circular byte shift in each each

1st row is unchanged

2nd row does 1 byte circular shift to left

3rd row does 2 byte circular shift to left

4th row does 3 byte circular shift to left

Decrypt inverts using shifts to right.

Since state is processed by columns, this step permutes bytes between the columns. 

[3]MixColumn

 

 

The MixColumns stage is a substitution that makes use of arithmetic over GF(2^8). Each byte of a column is mapped into a new value that is a function of all four bytes in that column. It is designed as a matrix multiplication where each byte is treated as a polynomial in GF(28). The inverse used for decryption involves a different set of constants.

[4]AddRoundKey 

Add Round Key stage which is a simple bitwise XOR of the current block with a portion of the expanded key. 


AES Key Scheduling:

Takes 128-bit (16-byte) key and expands into array of 44 32-bit words.

AES Key Expansion:

 

AES Decryption:

AES decryption is not identical to encryption since steps done in reverse.

AES can be implemented very efficiently on an 8-bit processor.

AddRoundKey is a bytewise XOR operation.

ShiftRows is a simple byte shifting operation.

SubBytes operates at the byte level and only requires a lookup of a 256 byte table S.

MixColumns (matrix multiply) can be implemented as byte XOR’s.