What best defines a meet-in-the-middle attack (MITM)?

• A. A cryptographic attack against encryption schemes that rely on multiple encryption operations in sequence, primarily affecting Double DES and Triple DES.
• B. A denial-of-service method.
• C. A hashing collision method.
• D. A brute-force password attack.

Answer: (A)

Meet-in-the-middle attacks target encryption schemes that apply more than one operation in sequence. The idea is to split the process into two halves and look for a meeting point in the middle, rather than trying every combination of all stages at once.

In practice, for a scheme that does E_k1 followed by E_k2 (two sequential encryptions), you would first guess and compute the intermediate value after the first encryption for all possible first keys, storing the results. Then you would take the final ciphertext and decrypt with all possible second keys, producing potential middle values. If a middle value from the second pass matches a stored middle value from the first pass, you’ve found the correct pair of keys. This reduces the overall work from trying all combinations of both keys to roughly the sum of the two separate key-space efforts, which is substantially smaller when the keys are each of moderate length. It’s a well-known vulnerability for two-key encryption schemes such as Double DES (and variants of two-key 3DES).

It’s not a denial-of-service method, not a hashing collision method, and not a brute-force password attack.