Patent ID: 8422684

Claim:
A method of enforcing a hierarchy of security classes defined in a common media key block, each of said classes being associated with a particular format of media content, and said media key block being commonly used by media devices that play media content formats associated with different security classes, the method comprising: in response to a request for a media device to decrypt an encrypted media file, said media device plays media content of a highest security class of said hierarchy and security classes lower in said hierarchy than said highest security class, the media file being stored on a storage medium that includes said media key block and at least one media file comprising media content being of a particular format, said device: reading said media key block; calculating a first media key precursor by processing said media key block, said first precursor allows said device to decrypt said media file for media content associated with a first security class being of a highest security class of said hierarchy; and if said media file is for media content associated with a second security class being of a second highest security class of said hierarchy: (i) calculating a media key by executing at least one predefined one-way function at least once on said first precursor and (ii) using said media key to decrypt said media file for said media content associated with said second security class.