1. Block cipher security. [10 Points] As a reminder, the cipher-block chaining (CBC) mode of operation works like this: The output of the encryption is the ciphertext concatenated with the IV that was used. Plaintext Plaintext Plaintext Initialization Vector (IV) Key block cipher encryption Key block cipher encryption Key block cipher encryption Ciphertext Ciphertext Ciphertext Cipher Block Chaining (CBC) mode encryption Ciphertext Ciphertext Ciphertext Key Key block cipher decryption Initialization Vector (IV) block cipher decryption Key- block cipher decryption Plaintext Plaintext Plaintext Cipher Block Chaining (CBC) mode decryption The output of the encryption is the ciphertext concatenated with the IV that was used. (a) [4 Points] What happens if two messages are encrypted with the same key and nonce (IV)? What can the attacker learn about the two messages just by looking at their ciphertexts? (b) (2 Points) What would be different if IV is not repeated? (c) [2 Points] Suppose AES-ECB is used to encrypt an image, what might be the problem? (d) [2 Points] If AES-CBC is used to encrypt an image, would the problem still exist? Why or why not