Consider Alice sending Bob a cipher message C1 with plaintext M1. Message is encrypted using AES in Counter mode (CRT Mode), i.e., Kj = E(K, Tj) Cj = Mj Kj

where 'K' is the encryption key, 'Tj' is a counter (nonce) corresponding to block j, 'Mj' is the plaintext block j, and 'Cj' is the ciphertext corresponding to 'Mj'.

a. An adversary Charlie can intercept and change messages. Assume Charlie somehow knows plaintext M1 for a particular ciphered message m = C1. Explain how he can modify the message as M1 to fool Bob (Bob receives C1 as the ciphertext of M1).

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b. Are there any way to prevent Charlie from doing this?

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