Ramblings & ephemera

How DVD encryption (CSS) works … or doesn’t

From Nate Anderson’s “Hacking Digital Rights Management” (Ars Technica: 18 July 2006):

DVD players are factory-built with a set of keys. When a DVD is inserted, the player runs through every key it knows until one unlocks the disc. Once this disc key is known, the player uses it to retrieve a title key from the disc. This title key actually allows the player to unscramble the disc’s contents.

The decryption process might have been formidable when first drawn up, but it had begun to look weak even by 1999. Frank Stevenson, who published a good breakdown of the technology, estimated at that time that a 450Mhz Pentium III could crack the code in only 18 seconds – and that’s without even having a player key in the first place. In other, words a simple brute force attack could crack the code at runtime, assuming that users were patient enough to wait up to 18 seconds. With today’s technology, of course, the same crack would be trivial.

Once the code was cracked, the genie was out of the bottle. CSS descramblers proliferated …

Because the CSS system could not be updated once in the field, the entire system was all but broken. Attempts to patch the system (such as Macrovision’s “RipGuard”) met with limited success, and DVDs today remain easy to copy using a multitude of freely available tools.

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