a. Let K = 111... ... 111 be the DES key consisting of all 1's. Show that if Ex(P) = C, then = EK(C) P, meaning encrypting twice with this key returns the plaintext. Note: Ex denotes Encryption function parameterized by a key, K. P denotes plaintext and C denotes ciphertext. b. Find another key that has the same property as K in part (a). [15 marks] [5 marks]