A method enables website providers to prohibit non-human users, such as programmed agents, from viewing their websites. According to a method for user authentication according to an embodiment, an authentication challenge is generated by a computing device, the authentication challenge including a number of challenge elements. The authentication challenge is presented to a user of a computing device and the user responds to the authentication challenge by selecting one or more challenge elements in accordance with an instruction provided to the user on how to complete the authentication challenge. The user is then notified whether they have successfully chosen the correct challenge element/s.