What is the known plaintext attack used against DES which gives the result that encrypting plaintext with one DES key followed by encrypting it with a second DES key is no more secure than using a sin

A man-in-the-middle attack is a network interception attack where an attacker positions themselves between two communicating parties, not a cryptanalytic technique applied to symmetric key exhaustion.

The meet-in-the-middle attack works by encrypting all plaintexts with every possible first key to build a table of intermediate values, then decrypting the ciphertext with every possible second key and comparing results for matches. This reduces the effective key search space from 2^112 to approximately 2^57, making 2DES nearly as weak as single DES. It is a known-plaintext attack because the attacker must possess at least one plaintext-ciphertext pair to execute it.

A replay attack involves capturing a legitimate transmission and retransmitting it to gain unauthorized access, which is unrelated to symmetric cipher key space analysis.

Traffic analysis is a passive attack focused on inferring information from communication patterns and metadata, not on breaking symmetric encryption keys through plaintext-ciphertext pairs.