Which attack exploits hash collision probability, often associated with hash functions?

• A. Exploiting hash collision probability
• B. Attacker obtains known plaintext
• C. Brute-force on keys
• D. Replay attack

Answer: (A)

Hash collision attacks exploit the fact that a hash function compresses input data into a fixed-size output, so two different inputs can produce the same hash. By searching for two distinct messages that hash to the same value, an attacker can substitute a legitimate message with a forged one that has an identical hash. This undermines integrity checks and digital signatures that rely on the hash value being unique to a specific message. The chance of finding a collision grows with the number of inputs tested, and the math behind it is described by the birthday bound, which is why longer hash outputs offer stronger resistance. In practice, this type of attack targets the collision probability rather than recovering plaintext, brute-forcing a key, or replaying previously captured data, which are different kinds of threats.