If people were able to create DVD+/-RW with a standard CSS on it
(acceptable to DVD-ROM readers) this would allow 'home users' to make
true DVD clones which become indistinguishable from the orignal DVD to
average DVD players. This is a strong argument for the fact that
content protection system was designed such that this DVD cloning is
not possible.
Let us first look at how CSS works in off-the-shelf (pressed) DVD's:
Each CSS licensee is given a key from a master set of 400 keys stored
on every CSS-encrypted disc. The theory was to allow a license to be
revoked by removing its key from future discs. The CSS decryption
algorithm exchanges keys with the drive unit to generate an encryption
key that is then used to obfuscate the exchange of disc keys and title
keys that are needed to decrypt data from the disc. DVD players have
CSS circuitry that decrypts the data before it's decoded and
displayed, and computer DVD decoder hardware and software must include
a CSS decryption module. All DVD-ROM drives have extra firmware to
exchange authentication and decryption keys with the CSS module in the
computer.
(Excerpt from
http://www.dvddemystified.com/dvdfaq.html#1.11 )
The critical question is: Where are those 400 keys stored on the disk?
If you can not store them in the same place on a DVD+/-RW that would
be a simple way to prevent the creation of 'cloned' copies.
There is a dedicated 2048 byte block on the DVD, which holds encrypted
disk keys. The host requests this block from the drive, and the drive
will only deliver it if a prior authentication of the host has taken
place. (see
http://www.extremetech.com/article2/0,1558,1230030,00.asp for the
simplified protocol) Similarly, encrypted title keys are stored in
sector headers. It is important to note that the keys are not part of
the UTF file system payload, but stored as meta-data to it. Even if
you can write title keys (assuming disks are not pre-sectored) you
will also have to write to the hidden disk key area.
Now, depending on the actual media, this area may already have been
pre-written, or be unwritable. In that case. You can not create a CSS
protected disk, since you can not provide an encrypted disk key
(region code seems to be stored in the same place?) for the drive and
player to read.
The relevant media specifications appear to be ECMA-267, -337, and
-338 for DVD-ROM, DVD+RW and DVD-RW respectively. Unfortunately, they
go into physical, optical, and encoding characteristics only, and do
not specifically address our question of the 'hidden sector'.
However, reading some drive specs, the DeCSS source code, and the
cdrom/dvd driver of the linux kernel lead me to believe that the
'hidden sector' is in fact part of the control data zone (section 26.5
in ECMA-267 DVD-ROM). Please understand that this is a speculation on
my part, altough educated speculation. The same control data zone in
DVD-RW (ECMA-338, 26.1.6) is explicitly stated to be embossed, i.e.
not to be writable by the end-user device. Something similar holds
true for DVD+RW (ECMA-337, 17.11). In fact, there is a very explicit
statement to be found there:
Let me summarize my longish answer, going directly to the points of your
question:
Q: Is it technically impossible to apply CSS on DVD-RW?
A: Yes.
The relevant areas on the media are embossed, and can not be recorded on.
Q: Is it technically impossible to apply CSS on DVD+RW?
A: No, just very hard.
The relevant area is in fact recordable, and the drive is supposed to make
sure that the area is always set to 0. That said, one could speculate that
a modification of the drive firmware might enable one to create custom
content in this area.
