5. Consider applying the Hill's cipher on messages over the 26-letter English alphabet with straightforward letter encoding ('A' encoded as 0, 'B' as 1, etc.). (a) (15 points) Which of the following matrices can be used as valid keys? For every matrix that can be used as encryption key, find its corresponding decryption matrix. For every matrix that is not a valid key, explain why. 5 25 7 19 34 (b) (10 points) Let "AF" be a ciphertext produced by Hill's cipher with encryption key ). What is its corresponding plaintext message? 3 21 5 21 9 14 85