Which scenario would indicate vulnerability to meet-in-the-middle attacks?

• A. A hash function with salt.
• B. Public-key encryption.
• C. A cryptographic scenario that relies on multiple encryption operations in sequence.
• D. A single DES operation.

Answer: (C)

Meet-in-the-middle attacks target systems that perform encryption in more than one stage with separate keys. The attacker uses plaintext and ciphertext to split the problem in half: they enumerate all possible first-keys by encrypting the known plaintext and store the resulting intermediate values. Separately, they enumerate all possible second-keys by decrypting the known ciphertext and check if any of those results match one of the stored intermediates. A match reveals a valid pair of intermediate states, effectively exposing both keys with far less effort than trying all key combinations. This is why a scheme that relies on multiple encryption operations in sequence is vulnerable to this attack—the two-stage process creates a bridge in the middle that MITM can exploit. A single DES operation isn’t subject to this particular vulnerability, and hashing with a salt or using public-key encryption isn’t framed here as a meet-in-the-middle risk. In practice, double DES would leak security benefits to around 57-bit strength rather than the full 112-bit strength due to MITM.