In the context of cryptographic hash functions, what is a collision?

• A. Two distinct inputs producing the same hash
• B. A single input producing two different hashes
• C. Hash outputs being too long
• D. Hash function being reversible

Answer: (A)

A collision in cryptographic hash functions is when two different inputs end up producing the same hash value. This can happen because the hash output is fixed-length while the input can be any size, so multiple inputs can map to the same digest. Cryptographic hash functions strive to make finding such pairs computationally hard, which is known as collision resistance. The fact that two distinct pieces of data yield the same digest is precisely what defines a collision and what auditors and attackers watch for.

Two other ideas don’t describe a collision. If a single input produced two different hashes, that would break the function’s determinism—the same input should always yield the same output. Hash outputs being too long isn’t about collisions; it concerns the size of the digest. A reversible hash would allow recovering the input from the output, which is a different property and undesirable, but it does not describe a collision.