A one-way function that can be reversed using secret information is called what?

• A. Trapdoor Function
• B. One-Way Function
• C. Caesar Cipher
• D. Substitution Cipher

Answer: (A)

A trapdoor function is a one-way function that can be reversed using secret information. The forward operation is easy to compute, but without the secret you can’t efficiently invert it; with the secret (the trapdoor) you can reverse it quickly. That’s precisely what this term describes in public-key cryptography, where the inverse becomes feasible only when you possess the secret key. The other options aren’t a match: a generic one-way function is hard to invert without any secret, and the Caesar and substitution ciphers are symmetric, easily reversible when you know the key or the mapping, so they don’t rely on a hidden trapdoor to invert.