One cryptanalysis method that is used to defeat a multi-step encryption process uses both the original clear text to work forward toward an intermediate value, and the ending cipher text to work…

A brute-force attack tries every possible key sequentially without leveraging intermediate values or the structure of multi-step encryption.

The meet-in-the-middle attack works against multi-step encryption such as Double DES by encrypting the plaintext forward through the first key to produce an intermediate value, then decrypting the ciphertext backward through every possible second key, comparing results for a match. This approach reduces the effective key space from 2^112 to 2^56 for Double DES, making it computationally feasible. The attacker exploits the intermediate state rather than exhaustively trying all combined key pairs.

A ciphertext-only attack is a scenario where the attacker possesses only the encrypted output and has no access to known plaintext to work forward from.

A birthday attack exploits the birthday paradox to find collisions in hash functions, not to reduce the key space of a multi-step block cipher.