Which cryptographic primitive is a one-way function that produces a fixed-length output?

• A. Hash Function
• B. Initialization Vector (IV)
• C. MAC (Message Authentication Code)
• D. HTTPS

Answer: (A)

A fixed-length-output, one-way function is a hash function. It takes input of any length and produces a digest of a predetermined size (for example, 256 bits with SHA-256). It’s easy to compute the hash for any input, but infeasible to reverse the process and recover the original data from the digest, and small changes to the input yield vastly different outputs. This combination of fixed output length and one-way, preimage-resistant behavior is what makes hash functions the go-to primitive for integrity checks, digital signatures, and password storage.

The other options don’t fit as neatly: an Initialization Vector is simply a random value used to seed encryption modes, not a digesting function. A MAC outputs a fixed-length tag but is a keyed function designed for authentication, not a general one-way digest. HTTPS is a protocol that uses such primitives but isn’t itself a single one-way function that produces a fixed-length output.