Which statement correctly describes a hash function?

• A. It is reversible and used for encryption
• B. It is a one-way function that produces a fixed-length output
• C. It requires a secret key
• D. It cannot be implemented efficiently

Answer: (B)

Hash functions map input data of any size to a fixed-length output in a deterministic way. They are designed to be one-way, meaning you can compute the hash easily, but recovering the original input from the hash is infeasible in practice. They do not require a secret key; the same input always yields the same digest, and there’s no hidden key involved. The fixed-length output property holds regardless of how large or small the input is, which makes hashes efficient to compute and compare. These features—one-way behavior, fixed-length output, and lack of a secret key—best describe how hash functions operate, setting them apart from encryption, which is reversible and typically uses keys.