Content providers are faced with massive intellectual properly piracy and resort to technical protection measures encapsulated in Digital Rights Management Systems (DRMS). Given the cost-beneflt trade-off of varying levels of content protection there is n need to implement a selected level of content protection. Based on a functional refkrence model of a DRMS, this paper proposes an access-usage-control-matrix to heuristically determine the protective strength of a given DRMS installation. The underlying assumption is that the combined robustness of the preventive security functions access control and usage control determines overall strength. The robustness of both matrix dimensions is refined with concrete authentication and client tamper-resistance methods.