One of the major stumbling blocks in achieving interoperability in DRM systems is due to the variety of different user authentication systems utilised by DRM systems. For example, in [6], the authors detailed how Microsoft's Rights Management System fails in fulfilling its requirements mainly because of a lack of its user identity system. The authors discussed how, because one DRM system cannot authenticate users from another DRM system, it cannot offer interoperability, even if they shared the same data formats. Furthermore, interoperability for user authentication in DRM systems is further hampered by the wide range of devices that need to support DRM enabled data, but do not necessarily offer the same features.
Decoupling of user identity from the main DRM system also reduces the chances of correlating users' access patterns of protected works. This improves the privacy of users of DRM systems, another major criticisms of current DRM systems. In this paper we discuss the requirements for user identity in a DRM system and then introduce a Kerberos like reusable ticket based user identity system. This system allows multiple systems to be authenticated by the use of time limited authentication tickets, without requiring online authentication. Tickets can be stored at a central controlling point, which is also responsible for acquiring tickets from authentication servers and redistributing tickets to the devices that need the tickets. In our experience, our approach fulfils all the requirements and is a more scalable and inter-operable approach when compared to existing DRM systems
