The authentication of identities within a realm in which some identities are authenticated using direct authentication, and some identities are authenticated using federated authentication. Requests for service from valid identities in the realm that are to be authenticated by direct authentication are responded to with a direct authentication interface. Requests for service from valid identities in the realm that are to be authenticated by federated authentication are responded to with a federated authentication interface. Requests for service from invalid identities are responded to pseudo-randomly with either the direct authentication interface or the federated authentication interface.