What term describes a one-way function that can be reversed with secret information?

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

Answer: (D)

In cryptography, a trapdoor function is a one-way operation that becomes easy to reverse only when you possess a secret piece of information. The forward calculation is straightforward, but reversing it is hard without the secret. With the trapdoor (the secret), you can invert efficiently. This concept is foundational for public-key systems: anyone can apply the forward operation with a public key, but only the private key holder—the trapdoor—can invert it to recover the original input.

This differs from a plain one-way function, which is hard to invert but not designed to be easily invertible with any secret. Symmetric cryptography and simple substitution ciphers rely on shared or fixed keys for both directions and aren’t about a hidden trapdoor that makes inversion feasible only for someone who knows it.