Authorization Services require that Servers be registered with them because the Access Tokens can only be used with a single Server. This can introduce a lot of complexity for administrators. One way to reduce this complexity is to leverage the Server information that is already managed by a Global Discovery Service (GDS) described in OPC 10000-12. In this model the user identities are still managed by a central Authorization Service. The interactions are shown in Figure 25.
Figure 25 – Direct Handshake with an Identity Provider
The UserTokenPolicy returned from the Server provides the URL of the Authorization Service and the identity provider. If the Application Authorization Service is linked with the GDS, it knows of all Servers which have been issued Certificates. The ApplicationUri is used as the identifier for the Server passed to the AS. The identity provider is responsible for managing users known to the system. It validates the credentials provided by the Client and returns an Identity Access Token which identifies the user. The Identity Access Token is passed to the Application Authorization Service which validates the Client and Server applications and creates a new Access Token that can be used to access the Server.