What is the known plaintext attack used against DES which gives the result that encrypting plaintext with one DES key followed by encrypting it with a second DES key is no more secure than using a sin

Question

Options

• AMan-in-the-middle attack
• BMeet-in-the-middle attack
• CReplay attack
• DTraffic analysis attack

• A
  8% (2)
• B
  88% (22)
• C
  4% (1)

Explanation

Meet-in-the-middle attack (B) is correct because it exploits Double DES by attacking from both ends simultaneously - an attacker encrypts all possible plaintexts forward with every possible key, and decrypts the ciphertext backward with every possible key, then looks for matching values "in the middle." This reduces the effective security of Double DES from 2¹¹² to approximately 2⁵⁶ operations (the same as single DES), making it no stronger than using one key.

Option A (Man-in-the-middle) is wrong because that attack involves intercepting communications between two parties, not cryptanalysis of a cipher's key space. Option C (Replay attack) involves retransmitting captured data to trick a system, which is unrelated to DES key strength. Option D (Traffic analysis) focuses on analyzing communication patterns rather than breaking encryption algorithms.

Memory tip: Think of "meet" literally - you meet in the middle by working from both ends toward the center, just like two people walking toward each other in a hallway. If the attack meets the cipher in the middle, it's a Meet-in-the-middle attack.

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