Some embodiments provide an independent authentication system for authenticating entities that have registered accounts across different online service providers on behalf of the service providers. The authentication system maintains a database of previously verified entity information. A service provider requests authentication by providing the authentication system with unverified and basic identifying information used by an entity when registering with the service provider. The authentication system attempts to match the registration information against previously verified information for a known entity. When a match is found, the authentication system generates a series of challenge questions. The questions are submitted to the entity through the service provider and answers are processed in order to authenticate the entity. Authentication is thus provided on behalf of the service provider without the service provider accessing the previously verified information and without the service provider independently verifying the information within the generated challenge questions.