What property of a hashing algorithm makes it useful for verifying data integrity?

• A. It is reversible
• B. It produces a fixed-size output that changes if input changes
• C. It encrypts data with a key
• D. It compresses data

Answer: (B)

A hash is used for data integrity because it creates a compact fingerprint of the data that responds to any change in the input. The essential idea is that the hash function outputs a fixed-length digest, and even a tiny modification to the data produces a different digest. This makes it easy to verify integrity: you store or transmit the original digest, and later recompute the hash of the data and compare. If the digests match, the data is highly likely to be unchanged; if they differ, something altered the data or a transmission error occurred. The fixed-size output helps with storage and comparison, while the sensitivity to input changes (the avalanche effect) ensures tampering is detectable. Hashes are designed to be one-way and collision-resistant, so you can’t reasonably recover the original data from the digest or find two different inputs that produce the same digest, which reinforces reliable integrity checks.