The main difference between the two incompatible formats lies in the copyright protection system (CPS). The HD DVD camp has chosen to use the AACS technology only, compared to the AACS BD-ROM technology which also uses two additional layers of security, the BD-ROM Mark and the BD+.
Below you can an overview of the BD-ROM format and a comparison with the corresponding HD DVD's proposal.
The AACS technology
The Advanced Access Content System provides content protection for BD-ROM AV data. Generally, it defines the rules how to handle such protected content. AACS uses an advanced encryption system with 128-bit keys, as well as a next generation media key block. The technology offers precise revocation of compromised devices and enhanced drive authentication. The technology also supports the managed copy feature, which enables authorized movement of content onto home network and portable devices.
The Image Constrain Token (ICT) is also a feature of the AACS which will be applied to analog video outputs. The technology will downgrade the HD video signal which comes out from analogue outputs of a HD player to a maximum resolution of 960 x 540. Image Constrain Token will be optionally employed by the video content owner so until now, early adopters of the HD technology (HD DVD and Blu-Ray) have not experienced it. Note that any the BD-ROM release that supports the ICT technology should clearly notify it on the disc package.
Another analogue protection supported by the AACS is the Digital Only Token technology. It restricts video content from flowing out of analogue ports of a player, but it is addressed to be used in new business models, which are materially different than the traditional video sell-through. Such video releases would be theatrical versions or HD movie previews. Digital Only Token will be permitted under the terms of the final AACS agreement, expected to be available in Q2 2006.
Audio Watermark is also included in AACS. It is an inaudible signal that a content provider may choose to insert into the audio track of their content. The feature is not supported by current (interim) devices and it is expected to be part of the final AACS agreement.
Last, both HD DVD and Blu-Ray camps have decided to phase-out any analog broadcasting in US and Japan by 2013. The so-called "Analog Sunset" is expected to begin in 2010, where analog output will be limited to SD Interlace Modes only (i.e., Composite, S-Video, 480i component).
All the above features are supported by both HD DVD and Blu-Ray formats. As mentioned before, the BDA has added two additional layers of protection to BD-ROM, the ROM-Mark and the BD+.
The BD-ROM Mark
The ROM-Mark is another physical layer technology to prevent stamping of unauthorized titles by professional pirates. It includes special invisible data on a BD-ROM disc. The BD+ provides a capability of Title-by Title countermeasure against hack on BD players, which are already released in the market field. It is a renewability system based on the Security Virtual Machine.
The ROM-Mark is expected to prevent the casual copy from BD-ROM to recordable media It is an analog level mechanism of bit-by-bit copy protection. The ROM-Mark requires a special machinery in the disc mastering process in order to be inserted on disc and thus, it prevents malicious replications.
In addition, broadcast discs will be analyzed before broadcasting in order to identify the inclusion of the BD-ROM Mark.
The BD+ Technology
The BD-ROM players include a small Virtual Machine (VM, BD+ Content Code interpreter) to provide a basic processing environment for BD+ Content Code. In addition, the VM allows Studios to optionally include Title-specific BD+ Content Code on the Disc.
The Virtual Machine is a small BD+ Content Code interpreter that includes 100 lines of code and 60 instructions, so the impact on player system resources is minimal.
During reproduction of a BD-ROM title, the VM will run the Content Code to apply the Security Check and enable content playback when the player is legitimate. A security code runs continuously during playback in order to correct the corrupted stream and produce viewable content. Media Transform is processed in real time block by using the output from Security VM:
The BD+ deployment includes three phases:
- Transform code (can be included on any title)
It swaps a part of AV data with separately prepared AV data. For example, such a part of AV data on the disc may be corrupted and will not be useful without corrections with BD+ content code running in the BD+ security VM. This same process can be used for forensic marking purpose, which may be used to identify the source of content that has been illegally distributed.
- Basic Countermeasure (when a hack has been confirmed)
When a hack is suspected, content provider can enter into a hack study. Once a hack is confirmed by the manufacturer of suspected Player, then Content Provider can have developed and release BD+ Content Protection code that detects and responds to the hack.
- Advanced Countermeasure (when basic countermeasure code does not work
BD+ includes the ability to load native code (code that runs directly on the player's host process). It is allowed to deploy it only after it is proven that basic countermeasure code cannot address the hack.
According to the BDA, to successfully attack the BD+ system, pirates would have to overcome the AV content security system (e.g. extract AACS keys) and also overcome title-specific security code (e.g. reverse engineer security code).
As a last note, the BD+ Content Code works only while Disc with the code is loaded. After its ejection, the BD player will return to its state before the code is loaded.
In addition, the BD+ Key is available to cryptographically differentiate the target, hacked player from non-hacked, legitimate players so that BD+ Content Code can work on such hacked player identified with BD+ Key.
The BDA Association believes that the BD+ content code should be included by Studios on a title-by-title basis.