Kimon is a cryptanalyst and is working on breaking a cryptographic algorithm's secret key. He has a copy of an intercepted message that is encrypted, and he also has a copy of the decrypted version of that message. He wants to use both the encrypted message and its decrypted plaintext to retrieve the secret key for use in decrypting other messages. What type of attack is Kimon engaging in?

In a known plaintext attack, the attacker has a copy of the encrypted message along with the plaintext message used to generate that ciphertext. In a chosen plaintext attack, the attacker has the ability to choose the plaintext to be encrypted. In a chosen ciphertext attack, the attacker can choose the ciphertext output. In a brute-force attack, the attacker simply tries all possible key combinations.