In mathematical terms, if "f" is a trapdoor function, then there exists some secret information "t", such that given "f"("x") and "t", it is easy to compute "x". Consider a padlock and its key. It is trivial to change the padlock from open to closed without using the key, by pushing the shackle into the lock mechanism. Opening the padlock easily, however, requires the key to be used. Here the key is the trapdoor and the padlock is the trapdoor function.