Which statement best describes a cryptographic hash and its use?

• A. A random number generator.
• B. A digital signature algorithm.
• C. A reversible encryption algorithm.
• D. A one-way function producing a fixed-size digest; used to verify data integrity.

Answer: (D)

A cryptographic hash is a one-way function that takes input of any size and produces a fixed-size digest. It’s designed to be deterministic, fast to compute, and hard to reverse or to find two inputs that yield the same digest (collision resistance). These properties make it ideal for verifying data integrity: you compute the digest of the original data and store or send it; later you compute the digest of the received data and compare. If the digests match, the data hasn’t changed; if they differ, something altered the data. The fixed-size output keeps comparisons efficient, and small changes in input produce a completely different digest (the avalanche effect). Hashes are also used as the input to digital signatures, but the hash itself isn’t a signature algorithm. The other options describe concepts that do not capture this function and use.