A system and method whereby the identity of a person, entity, device or the like attempting to gain access to a secured resource may be securely authenticated includes a means for receiving from a service client (such as a retail store, service station, on-line service provider or merchandiser, healthcare provider, medical insurer, information consumer or the like) a request for access to a secured resource, where the request for access was previously submitted to the service client by a requester purporting to be an authorized user of said secured resource; means for generating and communicating to the purported authorized user a challenge string adapted to provide a basis for authenticating the identity of the requester; a means for receiving from the service client a response string corresponding to the challenge string; and a means for evaluating the response string to authenticate the identity of the requester.