Which cryptographic function produces a fixed-length output from input data and is not reversible?

• A. Hashing
• B. Encryption
• C. Scrambling
• D. Ciphering

Answer: (A)

Hashing takes input data of any size and outputs a fixed-length value called a digest. This fixed length is the same no matter how big or small the input is (for example, SHA-256 always produces 256 bits). The key point is that hashing is one-way: there is no practical way to recover the original data from its digest. This non-reversibility is what makes hashes useful for integrity checks and verifying data without exposing the original content.

Encryption and ciphering, in contrast, are designed to be reversible with a key, producing ciphertext that can be turned back into the original plaintext. Scrambling isn’t a precise cryptographic term with defined guarantees like fixed output length or non-reversibility. So the function that best matches the description—producing a fixed-length output and not being reversible—is hashing.