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 a request for access to a 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. The secured resource has a common identifier by which it may be generally identified outside of the authentication system, but the request for access lacks sufficient information content for the service client to be able to determine the common identifier.