A system, apparatus, method, and machine readable medium are described for a hosted authentication service. For example, one embodiment of a system comprises: a hosted authentication service to provide authentication services for relying parties, the hosted authentication service registering a relying party by sharing a key with the relying party; a first program code component inserted into an application hosted by the relying party, the first program code component causing a client device accessing the application to be redirected to the hosted authentication service for authentication-related functions; and the hosted authentication service transmitting one or more assertions to the relying party specifying authentication-related events occurring between the client device and the hosted authentication service, the relying party validating the assertions using the key.