One-way function

A """one way function""" is a computable bijective function f with the following properties:

The computation of $f(x)$ is tractable (which generally means that a polynomial algorithm for the computation is known, see complexity theory).

The computation of the inverse function $f^{-1}(x)$ is not tractable (i.e. no polynomial algorithm is known or exists).

An example of a one way function is the discrete lograithm.

One way functions are useful in cryptography, e.g. for digital signatures.