Hardware Reference
In-Depth Information
• Cinevia
• BD-ROM Mark
Caution
The Digital Millennium Copyright Act (DMCA) signed into law in 1998 prohibits the breaking
of copy-protection schemes or the distribution of information (such as tools, website links, and
so forth) on how to break these schemes.
CSS
The CSS provides the main protection for DVD-Video discs. It wasn't until this protection was
implemented that the Motion Picture Association of America (MPAA) agreed to release movies in the
DVD format, which is the main reason the rollout of DVD had been significantly delayed.
CSS originally was developed by Matsushita (Panasonic) and is used to digitally scramble and
encrypt the audio and video data on a DVD-Video disc. Descrambling requires a pair of 40-bit (5-
byte) keys (numeric codes). One of the keys is unique to the disc, whereas the other is unique to the
video title set (VTS file) being descrambled. The disc and title keys are stored in the lead-in area of
the disc in an encrypted form. The CSS scrambling and key writing are carried out during the glass
mastering procedure, which is part of the disc manufacturing process.
You can see this encryption in action if you put a DVD disc into a DVD-ROM drive on a PC, copy the
files to your hard drive, and then try to view the files. The files are usually called
VTS_xx_yy.VOB
(video object), where
xx
represents the title number and
yy
represents the section number.
Typically, all the files for a given movie have the same title number and the movie is spread out
among several 1GB or smaller files with different section numbers. These VOB files contain both the
encrypted video and audio streams for the movie interleaved together. Other files with an IFO
extension contain information used by the DVD player to decode the video and audio streams in the
VOB files. If you copy the VOB and IFO files onto your hard drive and try to click or play the VOB
files directly, you either see and hear scrambled video and audio or receive an error message about
playing copy-protected files.
This encryption is not a problem if you use a CSS-licensed player (either hardware or software) and
play the files directly from the DVD disc. All DVD players, whether they are consumer standalone
units or software players on your PC, have their own unique CSS unlock key assigned to them. Every
DVD video disc has 400 of these 5-byte keys stamped onto the disc in the lead-in area (which is not
usually accessible by programs) on the DVD in encrypted form. The decryption routine in the player
uses its unique code to retrieve and unencrypt the disc key, which is then used to retrieve and
unencrypt the title keys. CSS is essentially a three-level encryption that originally was thought to be
secure but has proven otherwise.
In October 1999, a 16-year-old Norwegian programmer was able to extract the first key from one of
the commercial PC-based players, which allowed him to very easily decrypt disc and title keys. A
now-famous program called DeCSS was then written that can break the CSS protection on any DVD
video title and save unencrypted VOB files to a hard disk that can be played by any MPEG-2 decoder
program. Needless to say, this utility (and others based on it) has been the cause of much concern in
the movie industry and has caused many legal battles over the distribution and even links to this code
on the Web. Do a search on DeCSS for some interesting legal reading.
