FWIW, Sun Labs came up with an "open" DRM system back in the mid 2000s called DReaM. I don't think it ever took off because of Microsoft's stranglehold on commercial DRM systems. While it's difficult to create a system that will defeat the most advanced of attackers, the requirement for DRM is probably closer to "defend the integrity of the system against attackers arms with an electron microscope, an oscilloscope and a dewar of liquid nitrogen." I'm not sure you need to make your DRM system perfect, just good enough to dissuade casual hackers.