Which function takes streams of data and reduces them to a fixed size using a one-way operation?

• A. Secure hash function
• B. Encrypt-then-decrypt
• C. Symmetric cipher
• D. Public key cryptography

Answer: (A)

A secure hash function is designed to take input of any length and produce a fixed-size output, a digest. The crucial point is that this operation is one-way: you can compute the digest from the data easily, but you cannot realistically recover the original data from the digest. It’s also built to minimize the chance that two different inputs yield the same digest. This makes it ideal for tasks like integrity verification and digital signatures, where a compact, unique representation of the data is needed regardless of how large the input is. The other options describe reversible cryptographic processes or systems that rely on keys and don’t produce a fixed-size digest in a one-way fashion, so they don’t fit the requirement of reducing streams of data to a fixed size with a one-way operation.