How do standalone players get the encryption key

[artronic]

I guess the key must be on the disk, somewhere, right?
Or do they do their own decryption?
;)

[LIGHTNING UK!]

pretty much the same way the rippers do!

[artronic]

That is so cool!

Thanks for your reply, Lightning.
And, most importantly, Thank you very much for DVDDecrypter!

If I may, I would like to share an observation of mine: it appears that the key repeats for more VOB's on the same disk. Would it make sense for the decryption routine to try the previous code first and in case it fails, decrypt again?

If it already does that, forgive me.
Come to think of it, that may not be a big time saver.
Anyway...

Thanks again.

[LIGHTNING UK!]

Using proper key obtaining methods (i.e. not brute force), it takes a fraction of a second to get the key on a per file basis and is more reliable than using the same one for each file in a VTS set.

[artronic]

Got it.

That's why I said it may not be a time saver. It takes DVDDecrypter a blink of an eye to get the key for a file.

Thanks again.

[ParticleMan]

Here's a more detailed explanation of getting the decryption key:

http://www-2.cs.cmu.edu/~dst/DeCSS/Gallery/plain-english.html


Edit: and to Lightning Uk, what player key does DVD Decrypter use?

[Nic]

Being nosey...like I am ;)

Looking through DVD Decrypter with a disassembler and hex editor, DVD Decrypter does contain a list of player keys.

(For those that are interested upx -d DVDDecrypter and then look at offset 0x001C04E8 for the keys)

There appear to be 31 different player keys DVDDecrypter could use to decrypt the Disk Key (so that it could then decrypt the title keys for the sets of VOB files).

However, DVD Decrypter probably contains disk key decryption routines so that it could decrypt a disk key without the need of any player keys. (although, I'm not certain as I didn't spot that bit of code...)

Cheers,
-Nic

[LIGHTNING UK!]

For nosey Nic, ;-)

Yes, it can decrypt the disc key without needing player keys. It takes a while longer though.

[ParticleMan]

Originally posted by LIGHTNING UK!
For nosey Nic, ;-)

Yes, it can decrypt the disc key without needing player keys. It takes a while longer though.

Via brute forcing, I'm assuming.

[Nic]

The method of cracking the disk key was developed by Dr Frank Stevenson....It's a kind of bruteforce (it's a reverse hash where the key is tested against all combinations)

http://developers.videolan.org/cgi-bin/viewcvs.cgi/libdvdcss/src/css.c?rev=1.29&cvsroot=VideoLAN&content-type=text/vnd.viewcvs-markup

If you're really interested go there and look for the CrackDiscKey function...

or read about how it was created from the man himself:
http://www-2.cs.cmu.edu/~dst/DeCSS/FrankStevenson/mail3.txt


-Nic