1. Consider a symmetric cryptosystem (where a shared key is used). Bob has a message that he claims came from Alice, and to prove it he shows both the cleartext message and the ciphertext message. The ciphertext corresponds to the plaintext enciphered under the secret key that Alice and Bob share. a) Explain why this does not satisfy the requirements of nonrepudiation of origin. 5pt (b) How might you modify a symmetric cryptosystem to provide nonrepudiation? 5pt.] Hint for part (b): suggesting to use public key cryptosystem is not the