It's also worth noting that while many DRM systems do use cryptography, cryptography is not required to implement DRM and cryptography alone is insufficient to implement DRM. Cryptography only keeps data away from people who don't have the decryption key - it can't provide a way for people to read the data without copying it. But that is the goal of DRM. Because reading and copying data are fundamentally the same thing, the only way to achieve that particular goal is to modify the operating system so that it lies to the user. That is called a rootkit.