What is the known plaintext attack against DES that suggests using two keys is no more secure than one?

• A. Meet-in-the-middle attack.
• B. Brute-force attack.
• C. Differential cryptanalysis.
• D. Side-channel attack.

Answer: (A)

This question tests the idea that simply applying DES twice with two keys doesn’t necessarily give you double the security. In a known-plaintext meet-in-the-middle attack on double DES, you start with a known plaintext P and its ciphertext C = E_K2(E_K1(P)). You compute and store all values of E_K1(P) for every possible K1. Then you walk through all possible K2 by computing D_K2(C) and look for matches with the stored E_K1(P) values. A match reveals a candidate pair of keys (K1, K2). The work factor ends up around 2^57, not 2^112, showing that two-key DES doesn’t provide the expected security boost over single DES.

That’s why this option is the best answer: it’s a specific known-plaintext attack that demonstrates the limited security gain from using two DES keys. The other choices describe different attack categories that don’t capture the phenomenon in question—brute-force is a generic exhaustive search, differential cryptanalysis is a cryptanalytic technique, and a side-channel attack exploits physical leakage.