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

[Phillip Susi]

Andrea Giuliano wrote:
> I believe in what you say, but actually I can mount a video DVD both as 
> UDF and ISO9660. As you say, Nautilus and "mount" unlock the DVD before 
> mount it, that's why I always can mount it.
> 

What do you mean you can mount it as iso9660?  If the disk is UDF you 
certainly can't mount it as iso9660.  Usually your /etc/fstab will 
specify udf,iso9660, which means EITHER filesystem is acceptable, and 
both will be tried until the correct one is found.

> Yet, I don't understand why dd should not copy a stream of byte from a 
> device if they represent an encrypted video DVD. I mean, whatever the 
> bytes represents, dd should dump them. At most, you should get a big, 
> useless file, but that file should be exactly what was written on the 
> media.
> 

Because the encryption is done at the hardware level, the drive will 
refuse to read those sectors unless it can correctly decrypt them.  The 
firmware in the drive will return an error rather than the encrypted data.

> But probably there's something I don't know. You said that mplayer (and 
> obviously xine, vlc and so on) unlocks the drive, and I guess you mean 
> "at the hardware level". If it's so, every tools like dd, tar, cpio and 
> such should fail reading the media.
> 
> Am I right?
> 


Yes, all those tools will fail if the drive has not been unlocked, and 
will work if it has.


> Another question: is there something one can do before invoking dd to 
> accomplish the same without using mplayer? I have nothing against 
> mplayer, it's just curiousity.
> 

I'm sure there is some other way to unlock the disk, but I don't know of 
one.  It's easy enough though to just fire up mplayer for a second ;)