Consider a Feistel cipher with four rounds. Start with a message of 2n bits. Divide it into two blocks of length n (a left halfand a right) MoM. The key K consists of k bits for some integer k. There is a function fK, M-KMX, where X is a constant. One round of encryption starts with a pair of MM. The output is the pair M. M, , where M1-sKThis is done for five rounds thus the ciphertext is MsM a. Express ciphertext in terms of M and K b. If you have a machine that does the m-round encryption. If you would use the same machine to decrypt the ciphertext MsM (using the same key K). Prove that your decryption works. (4+4)