An attacker sniffs encrypted traffic from the network and is subsequently able to decrypt it. The attacker can now use which cryptanalytic technique to attempt to discover the encryption key?

A birthday attack exploits probability collisions in hash functions to find two inputs producing the same hash, and is unrelated to recovering an encryption key from decrypted traffic.

A known-plaintext attack (often loosely called 'plaintext attack') uses existing plaintext-ciphertext pairs to deduce the key, but does not capture the active ability to choose and submit specific ciphertexts for decryption.

A meet-in-the-middle attack targets double-encryption schemes such as 2DES by attacking each encryption layer independently, and is not applicable to decrypting sniffed traffic to find a single key.

In a chosen ciphertext attack, the adversary has the ability to obtain decryptions of chosen ciphertexts and uses the resulting plaintexts to deduce the key. Because the attacker in this scenario has already decrypted captured traffic, they are in a position to select and submit additional ciphertexts for decryption to systematically narrow down and recover the key. This is the attack model that best fits having both the ciphertext and its decrypted plaintext available.