Which attack uses the birthday paradox to maximize hash collisions?

• A. Collision attack
• B. Known-plaintext attack
• C. Brute-force attack
• D. Birthday attack

Answer: (D)

The idea being tested is how the birthday paradox informs when hash collisions become likely. A cryptographic hash maps many possible inputs to a fixed-size output, so collisions—two different inputs producing the same hash—are inevitable. The birthday paradox shows that with a hash of n bits, you don’t need to try 2^n inputs to expect a collision; you expect one after roughly 2^(n/2) attempts. This is the essence of a birthday attack: an approach that deliberately generates many inputs to force a collision in the hash output, exploiting that probability threshold.

That makes it the best answer because it directly ties the collision goal to the statistical phenomenon described by the birthday paradox. Other terms describe different ideas: a collision attack is a broad term for finding any two inputs with the same hash, a brute-force attack blindly exhausts possibilities, and a known-plaintext attack uses known plaintext-ciphertext pairs to infer secrets. None of those specifically hinge on the birthday paradox to maximize collisions the way the birthday attack does.