13. [5 points] WEP is vulnerable to active attacks that allow an active attacker to flip bits in the ciphertext and thereby cause unauthorized modifications to the message received by the recipient. What would be the best defense against this kind of attack? Provide a brief justification for your answer. a) Use a different key for each direction and for each wireless device b) Protect the ciphertext using a MAC c Encrypt using AES in Cipher Block Chaining (CBC) mode d) Prepend a random 32-bit nonce to the packet before applying the CRC and encrypting it