Some surprise while "dd-ing" a video DVD...

[Andrea Giuliano]

Phillip Susi wrote:
> Noah Dain wrote:
> 
>> As far as encryption on dvds go, the dvd itself isn't encrypted, just
>> the .vob files on it.  So, if you can't see the VIDEO_TS directory
>> with .vob (mpeg) files in it, then the problem is not with normal dvd
>> encryption.
>>
> 
> Actually, the DVD itelf _is_ encrypted, just not the whole thing.  The 
> sectors that contain the vob files are encrypted at the hardware level, 
> which is why dd can not read them.  The drive itself performs the 
> decryption of those sectors once it has been unlocked.  If the data file 
> itself were encrypted, then you would still be able to read and copy the 
> file even though you could not understand it's contents.
> 
I must agree with Noah: just VOB files are encrypted, and not at the 
hardware level. That doesn't explain the strange dd's behaviour, but 
explains what mplayer does, described in the following two points:

1. First case: no libdvdcss2. Remove libdvdcss from your system, put 
your favourite movie into the drive, and run "mplayer dvd://" in a 
terminal. Personally, I get this output (shortened):

MPlayer dev-CVS--4.0.2 (C) 2000-2005 MPlayer Team
CPU: Intel Pentium 4/Xeon/Celeron Northwood (Family: 8, Stepping: 4)
Detected cache-line size is 64 bytes
CPUflags:  MMX: 1 MMX2: 1 3DNow: 0 3DNow2: 0 SSE: 1 SSE2: 1
Compiled for Debian.

.....

Playing dvd://.
libdvdread: Encrypted DVD support unavailable.
************************************************
**                                            **
**  No css library available. See             **
**  /usr/share/doc/libdvdread3/README.Debian  **
**  for more information.                     **
**                                            **
************************************************
Reading disc structure, please wait...
There are 13 titles on this DVD.
There are 21 chapters in this DVD title.
There are 2 angles in this DVD title.
DVD successfully opened.
Cache fill:  0.00% (0 bytes)    Encrypted VOB file! Read 
DOCS/HTML/en/dvd.html.
Encrypted VOB file! Read DOCS/HTML/en/dvd.html.
Encrypted VOB file! Read DOCS/HTML/en/dvd.html.
Encrypted VOB file! Read DOCS/HTML/en/dvd.html.
Encrypted VOB file! Read DOCS/HTML/en/dvd.html.
Encrypted VOB file! Read DOCS/HTML/en/dvd.html.

.....

The result is that you cannot watch the movie.

2. Second case: libdvdcss2 is on. Same steps as above, but here is the 
output from mplayer:

MPlayer dev-CVS--4.0.2 (C) 2000-2005 MPlayer Team
CPU: Intel Pentium 4/Xeon/Celeron Northwood (Family: 8, Stepping: 4)
Detected cache-line size is 64 bytes
CPUflags:  MMX: 1 MMX2: 1 3DNow: 0 3DNow2: 0 SSE: 1 SSE2: 1
Compiled for Debian.

.....

Playing dvd://.
libdvdread: Using libdvdcss version 1.2.5 for DVD access
Reading disc structure, please wait...
There are 13 titles on this DVD.
There are 21 chapters in this DVD title.
There are 2 angles in this DVD title.

libdvdread: Attempting to retrieve all CSS keys
libdvdread: This can take a _long_ time, please be patient

libdvdread: Get key for /VIDEO_TS/VIDEO_TS.VOB at 0x000002b7
libdvdread: Elapsed time 1
libdvdread: Get key for /VIDEO_TS/VTS_01_0.VOB at 0x0005271b
libdvdread: Elapsed time 0
libdvdread: Get key for /VIDEO_TS/VTS_01_1.VOB at 0x000954fe
libdvdread: Elapsed time 0

And there comes the movie!

This, to me, means that the disk is not encrypted, so that you can mount 
it, "cat" it, "dd" it and so on. But if you want to watch the movie, you 
need to decrypt every single VOB file. Nothing makes me think of a 
hardware level encryption.

I'd really appreciate your interpretation: I definitely am not a 
technician, as you instead maybe are.

Best regards.

-- 
Andrea Giuliano, Ph. D.
ICCU - Istituto Centrale per il Catalogo Unico
Viale Castro Pretorio 105, Rome - ITALY
Tel. +39 06 49210403, Fax +39 06 4959302