Question 4

The following describes a toy block cipher. The block size is 8 bits and the key size is 16 bits. A

round will consist of the following.

1. The input is XORed with the rst 8 bits of the round subkey

2. The input, considered as an element of GF(28) with irreducible P(x) = x8 + x4 + x3 + x + 1, is squared.

3. The left 4 bits of the block are swapped with the right 4 bits of the block. For example 0x4A becomes 0xA4.

There are two rounds. The key schedule is the following:

1. During the rst round, the subkey is the rst 8 bits of the key.

2. During the second round, the subkey is the second 8 bits of the key.

Question 4 part a

Use the above cipher with key k = 0x1111 to encrypt the block B = 0x88.

Answer here

Question 4 part b

Use this block cipher to encrypt the plaintext x =0x4A01 in OFB mode with IV =0x22 and key

k =0x4444. Please show all intermediate steps.