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Posted by Jeff Rife on 02/27/06 04:27
David (sorry@toomuchspam.com) wrote in alt.video.dvd:
> Well, I know they CAN be copied with the approrpiate software, but I am
> just curious why that is even necessary. As far as I can tell, CSS just
> encrypts the data, so that my copy (like the original) can only be played
> on authorized DVD players. But when I try to do a whole disc copy with
> Nero, it tells me "This DVD is copy-protected and canot be read". The only
> thing I can think of is that Nero checks for the copy protection, and
> willingly refuses to copy it. Or am I missing something?
A simplified explanation:
A DVD-ROM (which is what a DVD-Video disc is based on, just with a
specific place to put the files) is like any other disc in that it has
sectors of data. And, like any other disc, in addition to the data,
there is extra space in each sector for error correction. The error
correction on a DVD (or a CD, for that matter) is a form of cyclic
redundancy check (CRC)...basically, a checksum of the data. There are
other error correction methods used on DVDs as well, and you can do more
research if you want find out more.
What CSS does is take the *entire* sector that was on the master data
(including the error correction) and encrypts it. This data is then
what ends up on the master DVD, and gets pressed to every copy. The
trouble is that the CRC is no longer valid, and every time the drive
reads it, it thinks that the sector is bad.
So, unless you decyrpt the data, you can't know if the data is accurate,
and many DVD drives won't even return the "error-filled" data unless the
programmer is doing some very low-level calls. Even then, there is
still no way to know if the data is accurate without decryption, and
since there is a "soft" error (that is, one that can be corrected by the
drive because of the extra data on the disc) on about 1 in 10 sectors,
you basically end up with garbage without error correction.
As for how Nero tells it is copy-protected, I suspect there is just a
byte in one of the files that says it is. I haven't looked that deep
into that part of CSS.
But, even if this wasn't set, Nero (or any other program) couldn't copy
the disc without some sort of CSS decrypter.
--
Jeff Rife | "The old guy was leading a 'Simon Says' game
| when he collapsed. On the way down he yelled
| 'call an ambulance!', but no one moved."
| -- Wings
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