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 intercepts communications between two parties in transit and is a network-layer attack, not a cryptanalytic technique targeting double encryption key spaces.

The meet-in-the-middle attack works by encrypting the known plaintext forward with all possible first keys and decrypting the ciphertext backward with all possible second keys, then finding matches in the middle. This reduces the effective key space from 2^112 to 2^57, making Double DES only marginally stronger than single DES. It is a known-plaintext attack because the attacker must possess a plaintext-ciphertext pair to perform the matching step.

A replay attack involves capturing and retransmitting valid authentication or session data to impersonate a user, and does not involve cryptanalysis of encryption key combinations.

Traffic analysis examines patterns, timing, and volume of network communications to infer information without decrypting content, and does not evaluate the strength of multi-key encryption schemes.