A system, method, and computer-readable medium for challenge-response authentication are provided. A plurality of codes is received over a communication network based on input provided by way of a user interface displaying a plurality of images. An alphanumeric string is generated based on the received plurality of codes and based on a table that associates each one of the plurality of codes with a respective one of the plurality of images and with a respective one of a plurality of alphanumeric characters. A determination is made as to whether to grant authorization based on whether the generated alphanumeric string matches an alphanumeric user identifier stored in a memory device in association with a user.