a. Let K 111... 111 be the DES key consisting of all 1's. Show that if Ex(P)C, then Ex(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. [15 marks] b. Find another key that has the same property as K in part (a). [5 marks]