1. A method for challenge-response authentication, the method comprising: receiving, from an external terminal over a communication network, a request for access to a service; presenting to a user, via a display, a plurality of objects; receiving, over the communication network, a plurality of codes, each of the plurality of codes corresponding to one of the plurality of objects; matching the plurality of codes to a plurality of alphanumeric characters according to a predetermined table; generating an alphanumeric string from the plurality of alphanumeric characters; comparing the alphanumeric string to a user identifier stored in a database; and determining whether to grant the user access to the service based on a result of the comparing.