Meet-in-the-Middle Attack
The meet-in-the-middle attack targets double encryption E2(E1(P)) by enumerating keys from both ends and meeting at the middle value, cutting O(2^(2k)) brute force down to O(2^k) time and storage.
Purpose
It proved that double DES (2DES) is no more secure than single DES, motivating the 3DES design. It also serves in round-key recovery of block ciphers.
Mechanism
For the middle value M: enumerate K1 from the plaintext side and store M = E1(P, K1) in a table; enumerate K2 from the ciphertext side, compute M = E2^-1(C, K2), and look for matches. Matching (K1, K2) pairs are key candidates.
Use cases
- 2DES security analysis
- Design rationale for 3DES
- Block-cipher key recovery