Which of the following best defines a hash function?

• A. A method for digital signatures
• B. Reversible encoding
• C. An algorithm for symmetric encryption
• D. A one-way function producing fixed-length output

Answer: (D)

A hash function is a one-way function that maps input data of any size to a fixed-length output. It is designed so you can compute the digest quickly, but you cannot practically reverse the process to recover the original data. The fixed-length output is important because it provides a consistent size for comparisons and integrity checks, regardless of how large the input is. Hashes are deterministic—the same input always yields the same output—and they are not reversible or keyed, unlike encryption methods. While digital signatures use hashes as part of the signing process, the hash function itself is not a signing method.