A system for challenge-response authentication is provided by receiving, from an external terminal over a communication network, a request for access to a service. A plurality of objects is presented to a user via a display. A plurality of codes is received over the communication network, each of the plurality of codes corresponding to one of the plurality of objects. The plurality of codes are matched to a plurality of alphanumeric characters according to a predetermined table. An alphanumeric string is generated from the plurality of alphanumeric characters and the alphanumeric string is compared to a user identifier stored in a database. Based on the comparing, a determination is made as to whether to grant the user access to the service.