One of the most common methods of user authentication utilizes passwords. This technique is widely spread and well known. Its main drawback is that it stresses human memory to allow for successful authentication and still remains in the domain which is called weak authentication. This paper presents a solution to this problem by introducing a form of two factor authentication by utilization of smartphones. The idea of such system is presented as well as a sample implementation, called MOTH (an acronym for MObile auTHentication), which allows for user authentication. The discussion of the system's security and its limitations is also presented.