ITPEC FE Morning April 2019 Question 38

Brute Force Attack on Private Key Cryptography — identifying the exhaustive key search method

Brute force means trying every possible key sequentially until the correct one is found. Given a known plaintext-ciphertext pair, the attacker encrypts the plaintext with each candidate key and checks if the output matches the ciphertext.

The correct answer is b): "Finding the key by testing all key combinations sequentially when a set of plaintext and ciphertext is given."

Why not the others:

• •a) Observing ciphertext changes when plaintext changes by a certain amount → this is differential cryptanalysis, which exploits input/output difference patterns
• •c) Using algebraic expressions relating plaintext, ciphertext, and key → this is an algebraic attack, which solves mathematical equations to recover the key
• •d) Using statistical correlation between parts of plaintext and ciphertext → this is linear cryptanalysis, which finds linear approximations of the cipher's behavior

Key rule: Brute force = no cleverness, just exhaustive enumeration of all possible keys. It is the simplest but most computationally expensive attack method.

AI-generated — may contain errors