1. A transfer function f which is relatively easy to compute, but for which, for a general value, y , within the selected range, it is computationally difficult to find a value x within the expected domain such that f (x) = y. There may be several values for y , for which finding x is not computationally difficult. 2. A function which is easy to compute but whose inverse is computationally intractable. 3. A (mathematical) function which is easy to compute but whose inverse is computationally intractable.