Trapdoor function
![The idea of trapdoor function. A trapdoor function f with its trapdoor t can be generated by an algorithm Gen. f can be efficiently computed, i.e., in probabilistic polynomial time. However, the computation of the inverse of f is generally hard, unless the trapdoor t is given. [1]](/Images/godic/202502/17/Trapdoor_permutation.svg3230.png")
A trapdoor function is a function that is easy to compute in one direction, yet difficult to compute in the opposite direction (finding its inverse) without special information, called the "trapdoor". Trapdoor functions are widely used in cryptography.