Two inputs producing the same hash is an example of which type of attack?

• A. Collision attack
• B. Attacker has plaintext and ciphertext
• C. Birthday paradox
• D. Known-plaintext attack

Answer: (A)

Collision attacks focus on hash functions by trying to find two different inputs that produce the same hash value. A hash function takes arbitrary data and outputs a fixed-size hash; because there are more possible inputs than possible hash outputs, collisions exist. A successful collision attack demonstrates that the hash function is not collision resistant, meaning an attacker can produce two distinct messages that hash to the same value. In practice, this could allow substituting one message for another without changing the hash, which undermines data integrity and can enable forgeries in certain contexts.

The other options don’t fit this scenario. Having plaintext and ciphertext describes a situation for breaking or analyzing encryption keys, not finding a hash collision. The birthday paradox is a probability concept used to estimate how many attempts are needed to expect a collision, not a type of attack itself. A known-plaintext attack involves using known pairs of plaintext and ciphertext to deduce a key or decrypt other data, not finding collisions in a hash function.