Methods and systems for single sign-on while protecting user privacy

A method of enabling applications to reference user information is provided, including receiving a request for a user identifier that references a user of the application and sending a second request for the user identifier to a server. The second request may include a second user identifier that references the user and a second authentication token for the second user identifier. Furthermore, the second user identifier and the second authentication token are not accessible by the user. The method includes receiving the user identifier and an authentication token for the first user identifier. The user identifier corresponds to the second identifier; and providing the user identifier and authentication token to the application. A method of enabling an application to identify users associated with a user of the application is provided; the method may include receiving, from the server, user identifiers that reference one or more users scoped to the application.