AES

Question 1

Before describing the operations, describe the encryption algorithm for block size 128 bits and key size 128 bits.

Answer 1

Each plaintext block is arranged in a 4x4 array of bytes, called the state. Successive bytes of plaintext are written column-wise in the array.
The state then goes through 10 rounds of encryption for 128-bit key where each round consists of four operations.

Question 2

Describe the subBytes operation.

Answer 2

In the subBytes operation, a substitution (S-box) is applied to each byte. The table for the substitution is fixed in the standard. The byte is replaced by the found in the s-box on the row determined by the first 4 bits of the original byte and the column determined by the last 4 bits.

Question 3

Describe the shiftRows operation.

Answer 3

In the shiftRows operation, each row is cyclically shifted to the left. Row 0 is not shifted;Row 1 is shifted by one byte;Row 2 is shifted by two bytes;Row 3 is shifted by three bytes.

Question 4

Describe the mixColumns operation.

Answer 4

In the mixcolumns operation, each individual byte will be replaced by a new value, which is computed from the old values of the 4 bytes of that column.

Question 5

Describe the addRoundKey operation.

Answer 5

In the addRoundKey operation, each round uses a new 128-bit key, called the round key. The round key, written column-wise, is XORed to the current block.