Hardware Reference
In-Depth Information
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 usu-
ally called
VTS_xx_yy.VOB
(video object), where
xx
represents the title number and
yy
rep-
resents 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.
ThisencryptionisnotaproblemifyouuseaCSS-licensedplayer(eitherhardwareorsoft-
ware) 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
unlockkeyassignedtothem.EveryDVDvideodischas400ofthese5-bytekeysstamped
ontothediscinthelead-inarea(whichisnotusuallyaccessiblebyprograms)ontheDVD
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 very 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
theCSSprotectiononanyDVDvideotitleandsaveunencryptedVOBfilestoaharddisk
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 onthe Web. Do
a search on DeCSS for some interesting legal reading.
Asifthatweren'tenough,inMarch2001,twoMITstudentspublishedanincrediblyshort
(only seven lines long!) and simple program that can unscramble CSS so quickly that a
movie can essentially be unscrambled in real time while it is playing. They wrote and
demonstrated the code as part of a two-day seminar they conducted on the controversial
Digital Millennium Copyright Act, illustrating how trivial the CSS protection really is.
