An authentication system for authenticating the identity of a requester of access by an unauthorized service client to a secured resource. The system has a messaging gateway having a first set of instructions operable to receive from a requester purporting to be an authorized user of a secured resource a request for limited access to the secured resource by an unauthorized service client for a particular transaction; a server having a second set of instructions operable to determine a key string known to both the secured resource and the authorized user the requestor purports to be, the key string being adapted to provide a basis for authenticating the identity of the requester as the authorized account holder; and a service user interface having a third set of instructions embodied in a computer readable medium operable to receive transaction specific information input into the request by the unauthorized service client.