In a multimedia recording and playback network for receiving from a content server a service package of multimedia content, the network including a media server in communication with the content server, a method of securely transmitting a master encryption key from the content server to the media server,...http://www.google.de/patents/US7231516?utm_source=gb-gplus-sharePatent US7231516 - Networked digital video recording system with copy protection and random access playback
