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736 Chapter <strong>13</strong> Optical Storage<br />
CSS originally was developed by Matsushita (Panasonic) and is used to digitally scramble and encrypt<br />
the audio and video data on a DVD-Video disc. Descrambling requires a pair of 40-bit (5-byte) keys<br />
(numeric codes). One of the keys is unique to the disc, whereas the other is unique to the video title<br />
set (VTS file) being descrambled. The disc and title keys are stored in the lead-in area of the disc in an<br />
encrypted form. The CSS scrambling and key writing are carried out during the glass mastering procedure,<br />
which is part of the disc manufacturing process.<br />
You can see this encryption in action if you put a DVD disc into a DVD-ROM drive on a PC, copy the<br />
files to your hard drive, and then try to view the files. The files are usually called VTS_xx_yy.VOB<br />
(video object), where the xx represents the title number and the yy represents the section number.<br />
Usually, all the files for a given movie have the same title number, and the movie is spread out among<br />
several 1GB or smaller files with different section numbers. These .VOB files contain both the<br />
encrypted video and audio streams for the movie interleaved together. Other files with an .IFO extention<br />
contain information used by the DVD player to decode the video and audio streams in the .VOB<br />
files. If you copy the .VOB and .IFO files onto your hard drive and try to click or play the .VOB files<br />
directly, you either see and hear scrambled video and audio or receive an error message about playing<br />
copy protected files.<br />
This encryption is not a problem if you use a CSS-licensed player (either hardware or software) and<br />
play the files directly from the DVD disc. All DVD players, whether they are consumer standalone<br />
units or software players on your PC, have their own unique CSS unlock key assigned to them. Every<br />
DVD video disc has 400 of these 5-byte keys stamped onto the disc in the lead-in area (which is not<br />
usually accessible by programs) on the DVD in encrypted form. The decryption routine in the player<br />
uses its unique code to retrieve and unencrypt the disc key, which is then used to retrieve and unencrypt<br />
the title keys. CSS is essentially a three-level encryption that originally was thought to be very<br />
secure, but has proven otherwise.<br />
In October 1999, a 16-year-old Norwegian programmer was able to extract the first key from one of<br />
the commercial PC-based players, which allowed him to very easily decrypt disc and title keys. A now<br />
famous program called DeCSS was then written that can break the CSS protection on any DVD video<br />
title and save unencrypted .VOB files to a hard disk that can be played by any MPEG-2 decoder program.<br />
Needless to say, this utility (and others based on it) has been the cause of much concern in the<br />
movie industry and has caused many legal battles over the distribution and even links to this code on<br />
the Web. Do a search on DeCSS for some interesting legal reading.<br />
As if that wasn’t enough, in March 2001, two MIT students published an incredibly short (only seven<br />
lines long!) and simple program that can unscramble CSS so quickly that a movie can essentially be<br />
unscrambled in real-time while it is playing. They wrote and demonstrated the code as part of a twoday<br />
seminar they conducted on the controversial Digital Millenium Copyright Act, illustrating how<br />
trivial the CSS protection really is.<br />
Because of the failure of CSS, the DVD forum is now actively looking into other means of protection,<br />
especially including digital watermarks, which consists essentially of digital noise buried into the data<br />
stream, which is supposed to be invisible to normal viewing. Unfortunately, when similar technology<br />
was applied to DIVX (the discontinued proprietary DVD standard), these watermarks caused slight<br />
impairment of the image—a raindrop or bullet-hole effect could be seen by some in the picture.<br />
Watermarks also might require new equipment to play the discs.<br />
Analog Protection System<br />
APS (also called CopyGuard by Macrovision) is an analog protection system developed by<br />
Macrovision and is designed to prevent making VCR tapes of DVD-Video discs. APS requires codes to<br />
be added to the disc, as well as special modifications in the player. APS starts with the creation or<br />
mastering of a DVD, where APS is enabled by setting predefined control codes in the recording.
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