How does iTunes DRM work?

[asboy]

So I read the code that allows VLC to play stuff from the iTunes Music Store because I've always been curious about exactly how it all works. Unfortunately, I couldn't really figure it out from the code. All I got was that the actual audio data was AES encoded, but I'm not really clear on how the key and the IV are derived from the SC Info.sidb file.

Might someone be so kind as to explain this? Also, how the hell did DVD Jon actually figure all this out?!

[pronerd]

From The Register:

"Johansen deduced that the system key which locks the locked music to a single Windows computer is derived from four factors: the serial number of the C: drive, the system BIOS version, the CPU name and the Windows Product ID. "

http://www.theregister.co.uk/content/6/34712.html

[asboy]

the serial number of the C: drive, the system BIOS version, the CPU name and the Windows Product ID.

Well, okay, but I want to know the details. So there's this "system key" derived from all those factors. But HOW is it derived from those paramters (i.e. via what cryptographic operations)? And what is it a key to? (It can't be the key to decrypting the music; the music can work on 3 computers and infinity ipods, so the system key must somehow be used to decrypt the real key.) Obviously all these details are contained in the code, but I was hoping someone could give a nice English-language overview of what's going on.

[the Fiddler]

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