A method of authenticating a user of a computing device is proposed, together with computing device on which the method is implemented. In the method a modified base image is overlaid with a modified overlay image on a display. At least one of the modified base image and modified overlay image is moved by the user. Positive authentication is indicated in response to the base image reference point on the modified base image being aligned, at least one of the base image reference point and the overlay image reference point having coordinates in three dimensions.