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

Andrea Giuliano a.giuliano at
Fri Jan 20 17:24:55 UTC 2006

Okay, I think I almost got it, now: since some sectors can only be 
decrypted at hardware level, an attempt to read them with the 
appropriate system calls would give an I/O error, right?

This explains also the following: I had right here another system with a 
different drive, but the same OS (Breezy), and I don't need libdvdcss to 
copy the same video DVDs with dd. I can remove libdvdcss it completely, 
eject and reload the disc, or even another one, and dd works fine till 
the end of the disc. I guess this means that this particular drive is 
able to give the OS access to encrypted sectors without the need of 
libdvdcss, right?

But please, lead me to some good documentation, I don't want to bother 
the other users on the list with this issue any more, nor you yourself, 
except you explicitly tell me you don't mind to carry on the discussion 
a bit between us.

Best regards.

Phillip Susi wrote:
> Andrea Giuliano wrote:
>> 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:
> Only the parts of the disc that contain the vob files are encrypted, but 
> it is most definitely done at the hardware level.  The player itself 
> must decrypt those sectors and it can not ( because it has not been 
> unlocked ) it fails requests to read them.  This is why dd can not read 
> the disc.
> <snip>
>> 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.
> Again, only the sectors that contain the vob file are encrypted, which 
> is why you can mount the disc and see the filesystem directory 
> structure.  This is also why dd manages to read the first couple of 
> sectors, then fails when it hits the ones containing the vob files.
> If only the file data itself were encrypted, then you would easily be 
> able to copy the disc, and that's exactly what the MPAA wants to prevent 
> you from doing.
>> 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

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