What is a One-Way Function?

A one-way function is a mathematical function that is significantly easier to perform in one direction (the forward direction) than in the opposite direction (the inverse direction). It might be possible, for example, to compute the function in seconds but to compute its inverse could take months or years. A trap-door one-way function is a one-way function where the inverse direction is easy given a certain piece of information (the trap door), but difficult otherwise.