2 (10 points). Consider the following DES-like encryption method. Start with a message of 2n bits. Divide it into two blocks of length n (a left half and a right half: MoMi. The key K consists of k bits, for some integer k. There is a function f(K, M) that takes an input of k bits and n bits and gives an output of n bits. One round of encryption starts with a pair MMj.The output is the pair M IM2 where means XOR, which is addition mod 2 on each bit). This is done for m rounds, so the ciphertext is MM m +1 If you have a machine that does the m-round encryption just described, how would you use the same machine to decrypt the ciphertext MmMm+1 (sing the same key)? Prove that your decryption method works a) b) Suppose K has bits and f(K,M)- KM, and suppose the encryption process consists of m 2 rounds. If you know only a ciphertext, can you deduce the plaintext and the key? If you know a ciphertext and the corresponding plaintext can you deduce the key? Justify your answers. cSuppose K has n bits and f( K, M)KM, and suppose the encryption process consists of m-3 rounds. Why is this system not secure