HD DVD Requires Digital Imprimatur to Authorise Playback
Monday, July 25, 2005

A while back there was a mention that the next generation DVD format, HD DVD will use the sophisticated content protection system AACS. According to the AACS Pre-recorded Video Book Specification, in order for media and players to be AACS compliant, a disc can only be published if it contains a digital imprimatur and players for the media must reject any work that lacks a digital imprimatur. An imprimatur is the latin for "let it be printed", which means that a work cannot be published without first being approved by some central authority.

Every disc that is published must contain an up-to-date list of revoked works and when ever a disc is loaded, the player must update its revocation list and prevent the playback of any disc that contains a revoked digital imprimatur.

If the HD DVD format becomes the main format for movies, the AACS organisation will likely decide on who handles the digital imprimaturs. The AACS organisation is founded by several members from both the HD DVD and Blu-ray sides such as Sony, Toshiba, Microsoft, Disney, Intel and so on. Unfortunately, as there will likely only be one single central organisation reponsible for issuing digital imprimaturs, they may be subject to anticompetitive abuses as this organisation would be reponsible for the authorising of every single HD DVD disc for publishing.

HD-DVD, one of the two candidates for the next-gen DVD format, uses a "content protection" technology called AACS. And AACS, it turns out, requires a digital imprimatur on any content before it can be published.

(The imprimatur - the term is Latin for "let it be printed" - was an early technology of censorship. The original imprimatur was a stamp of approval granted by a Catholic bishop to certify that a work was free from doctrinal or moral error. In some times and places, it was illegal to print a work that didn't have an imprimatur. Today, the term refers to any system in which a central entity must approve works before they can be published.)

The technical details are in the AACS Pre-recorded Video Book Specification. The digital imprimatur is called a "content certificate" (see p. 5 for overview), and is created "at a secure facility operated by [the AACS organization]" (p. 8 ). It is forbidden to publish any work without an imprimatur, and player devices are forbidden to play any work that lacks an imprimatur.

From what I can see, if a HD DVD player is forced to reject HD DVD media that lacks a digital imprimatur, this means that blank media must also contain a digital imprimatur in order for a HD DVD player to allow playback. This likely means that should someone succeed in copying HD DVD movies and illegally resells them, then it is possible revoke all (or at least the majority) of media that the pirate has used. If this is true, this is likely one method the movie industry will use to tackle piracy in the future.



*Article courtesy o fSeán Byrne and www.cdfreaks.com