[libdvdcss-devel] bit perfect copies of CSS DVDs

[Christoph Anton Mitterer]

Hey.


On Tue, 2015-06-30 at 22:16 +0200, Jean-Baptiste Kempf wrote:
> Right. CSS is both at the file level and at the disk level. You
> cannot
> dd directly and have something working.
> Therefore, when doing copies, you will always remove some part of CSS
> encryption.
I see, but even after one has basically "initialised" the device with
the correct key (by first playing the media via libdvdcss), then the
UDF still contains files with further CSS encryption, which is why my
dd dump, when mounted and played afterwards, still shows libdvdcss in
working, right?

btw: I noticed the following
When I play the real physical DVD, and dvdcss creates the cache entry
it's named e.g.:
SOME_TITLE-2000102812240400-17a487205e/

but when playing the dd image, it will create:
SOME_TITLE-2000102812240400-0000000000/

Is this kinda expected?

> > 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?
> 
> Well, yes, but I don't know how is that related to your 2).
Well in (1) "we found out", that there's
a) encryption at the drive level, which would give me the block level
errors when the drive isn't sent the key
b) encryption at the file level, which is still in place after (a) has
been done
Right?

So when I dd the disk and mount the UDF image afterwards as loopback
device, then (a) is gone, but (b) is still present and will need to be
decrypted.


When I "backup" the disk however with e.g. dvdbackup, then I don't get
an UDF image but rather normal files VIDEO_TS/*.
And those are (by name) the same as in the dd'ed UDF image, but the
binary content of both differs largely.

So I assumed, that dvdbackup would also do away with (b) and that's why
the files differ.


> Are you referring about ARCCOS system?
Not sure, is there a way to find out whether ARccOS is used?


> > 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?
> 
> No. That seems like a bug.

hmm strange... any idea where the bug could be? hardware issue?
libdvdcss?

Just to make things clear:
The difference is somewhere in the UDF between the two images of the
same disc.
But when I loop mount those two dd'ed images and diff the files within
them - those are the same.

That's why I thought maybe that's part of CSS, i.e. that the drive
kinda watermarks the UDF.


Thanks,
Chris.
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