[libdvdcss-devel] bit perfect copies of CSS DVDs

[Christoph Anton Mitterer]

Hey.

I was looking into making ideally bit-perfect copies of some ageing
DVDs after some of them started to fail.

Some questions, where some of you could possibly help me:


1) I've always thought that CSS was operating just on the file level,
but apparently when reading out a DVD with dd, one gets gazillions of
block errors and it seems that these aren't just intentional error
blocks, cause when one reads the disc with ddrescue its contents can't
still be played afterwards.

I've noticed, though, that when one plays the disc with some player
that uses libdvdcss, dd works just fine.

So is there actually some drive level encryption that happens before
the block layer, and libdvdcss initialises the drive with the correct
keys?
Right?


2) I further noticed that even when first playing the DVD and then
creating an image with e.g. dd (which works fine then), the contents of
the UDF fs within that image differ binary from what e.g. tools like
dvdbackup would dump me to disk (i.e. VOBs, IFOs, etc. but without the
UDF fs around).

So it seems that in addition to the drive level encryption/protection,
which is defeated in (1), there's still another file level encryption?
Right?


3) Interestingly, when I create two images of the same DVD (without
ejecting it in the meantime or clearing up ~/.dvdcss/) than these
images still always differ in 10 bytes or so, always at the same
location.
Any idea why?


Thanks,
Chris.
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