What type of function is easy to compute but hard to reverse?

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

Answer: (A)

This tests the idea of a one-way function: it’s quick to compute the function for any input, but given the result, finding the original input is computationally hard. This asymmetry is central to many cryptographic constructions because you can generate a value easily, yet you cannot easily reverse to recover the input without substantial effort or additional information. A concrete example is taking a large exponent modulo a prime: computing g^x mod p is fast, but solving for x given g^x mod p (the discrete log) is hard; that’s the essence of a one-way function.

The other options don’t capture that hardness in the ordinary sense. A trapdoor function is easy to compute in one direction but becomes easy to invert only with a secret key; without that key it’s not guaranteed to be hard to reverse. The Caesar cipher and the transposition cipher are traditional, symmetric methods where encryption and decryption are both straightforward once you know the method, so reversing the operation isn’t hard.