ITPEC FE Morning April 2019 Question 38

Testing every key in turn against a known plaintext and ciphertext pair — that is what makes an attack brute force.

A brute force (exhaustive key search) attack uses no weakness of the algorithm. Given a plaintext and its ciphertext, the attacker encrypts the plaintext with each possible key in sequence until the output matches the ciphertext. Its cost depends only on the key length: every extra key bit doubles the number of keys to try.

Answer (b)

Why not others:

• •(a) — altering the plaintext by a chosen difference and studying how the ciphertext changes is differential cryptanalysis
• •(c) — solving equations that express the relationship between plaintext, ciphertext and key is an algebraic attack
• •(d) — exploiting statistical correlation between some plaintext bits and some ciphertext bits is linear cryptanalysis

Key rule: Brute force exploits nothing but the size of the key space, so the defence is a longer key; analytical attacks exploit structure in the cipher itself.

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