DVD Demystified

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Chapter 4
back encrypted content. The DVD Forum does not specify copy protection
technologies. The industry’s CPTWG solicits proposals and makes recommendations.
The DVD Forum Working Group 9 is responsible for coordinating
with the CPTWG. WG9 reviews copy protection systems and submits
them to the DVD Forum for approval. Once a copy protection system is
approved, the various working groups of the DVD Forum amend specifications
as needed to support the requirements of the copy protection system.
Over time, it was recognized that an overall framework was needed for
security and access control across the entire DVD family and beyond. The
4C entity (that is, Intel, IBM, Matsushita, and Toshiba), in cooperation with
the CPTWG and the Secure Digital Music Initiative (SDMI), developed the
Content Protection System Architecture (CPSA). CPSA covers encryption,
watermarking, playback control, protection of analog and digital outputs,
and so on. It is broadly defined to include physical and electronic distribution
of analog and digital audio and video in consumer electronics and computer
systems.
The CPSA creates a structure that defines the content protection obligations
of compliant modules. It defines how content management information
(CMI) and copy control information (CCI) are carried and verified
throughout the playback chain. CMI, also known as usage rules, specifies
the conditions under which the content can be used. It also may contain
other information such as triggers telling the player when and how to protect
audio and video outputs. CCI, a subset of CMI, constrains how the content
can be copied.
Eight forms of content protection apply to DVD. Each is explained below.
Technical details, particularly as they apply to computers, are covered in
Chapter 11.
Content Scrambling (CSS) Because of the potential for perfect digital
copies, worried movie studios forced a deeper copy protection requirement
into the DVD-Video standard. The Content Scrambling System (CSS) is a
data encryption and authentication scheme intended to prevent copying
video files directly from the disc. Occasional sectors containing A/V data
(audio, video, or subpicture) are scrambled in such a way that the data cannot
be used to recreate a valid signal. Scrambled sectors are encrypted with
a combination of a title key and a disc key. The title key is stored in the sector
header, which is normally not readable from a DVD-ROM drive or other
DVD reader. Each video title set (VTS) on the disc has a separate key. The
disc key is hidden in the control area of the disc, which is also not directly
accessible.
Use of CSS is strictly controlled by licensing. Each CSS licensee is given
a player key from a master set of 400 keys that are stored on every CSS-