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Hack exposes AACS 'hole'

Keys handed over without authentication

Hackers appear to have figured out how to access one of the crucial HD DVD encryption keys without having to authorise the data - potentially rendering the latest attempt to block such activity useless.

The crack, posted on the Xboxhacker website, uses a standard, unmodified Microsoft's Xbox 360 HD DVD drive to hand over a disc's Volume ID without ensuring AACS has authenticated it first. Since no authentication is performed, even if the Volume ID has been rejected by the AACS Licensing Authority - the organisation that maintains the copy-protection system - it can still be used.

The Volume ID is just one of a number of keys used by AACS to encrypt a disc's content. It's used with the Processing Key held by the playback software to create the Volume Unique Key (VUK) used to decrypt the content.

The crack doesn't facilitate copying per se, but it does appear to make it possible to play back copies as if they were the real thing. Until now, this has required the drive's firmware to be patched.

Decrypting a disc to access the content - say, to burn a AACS-free copy, or to transcode the content into another format - remains difficult, not least because the Processing Key can be controlled by the AACS LA by demanding it be changed, typically by releasing a new version of the playback app.

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