We know that AACS, the DRM shared by HD DVD and Blu-ray, is touted as being practically unbreakable and impenetrable -- much like its predecessor CSS (and we all know how well that went). Last week we saw the launch of BackupHDDVD the first tool claiming to break down AACS, but a lot of people were left wondering -- us included -- exactly what does it do? Does BackupHDDVD really actually crack AACS? Well, yes and no. With the release of version 1 of the app today, muslix64, the app's author, helped out by shedding a bit of light about exactly how this thing works. (We'll assume you know a bit about public key crypto -- if not, see here, here, and here.) One of the tenets to advanced crypto and this particular DRM system is key revocation, which for AACS means that as soon as a content publisher or industry group gets wind of some funny business, they can revoke crypto keys and push an update down the line that basically invalidates whole groups of pirated movies, hacked hardware, player software, etc.

In simple terms (ok, not that simple), muslix64 does not claim to have cracked the AACS DRM itself, but instead to expose and use each disc's hard-coded private key in order to make the hardware device run through its routine decryption process. By doing so, BackupHDDVD effectively bypasses the key revocation system which might otherwise prevent it (or your HD DVD hardware, like an Xbox 360 HD DVD drive) from being mass-invalidated as soon as the RIAA blinks. The trick here, however, is the private key extraction -- no one really knows how muslix64 exposed and extracted the private keys on the HD DVD discs needed to strip the DRM from the HD DVD discs. S/he claims it can be done with any poorly built software or hardware as the private disc keys are held in memory, but we still don't have specifics. So while we're sure to see master key lists for HD DVDs popping up on file networks in the near future, we're still hazy about how a regular user can extract his or her own keys for their own fair use purposes. We'll be sure to keep you up to date on this, though, as more information becomes available.

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What exactly does BackupHDDVD do? Oh, and version 1.0 is released.