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. The modified overlay image comprises a plurality of numbers. 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, in sequence, with two or more numbers from the overlay image that equal a pre-selected algebraic result when one or more algebraic operator is apply to the numbers.