Why AACS keys will leak faster than they can be patched

Earlier this week, I blogged about a new set of AACS keys being compromised -- a set of keys that can be used to crack the anti-copying technology on HD-DVDs. The fascinating thing about this is that it came six days before the release of a new generation of HD-DVD discs that are hardened against copying using another leaked key (the AACS Licensing Authority's attempt to suppress that key was an unmitigated disaster, leading to more than a million republications of the key).

Princeton's J Alex Halderman has just posted a terrific little explanation of the logistics of all this: why it took the AACS-LA months to revoke the old key, and why it will never be able to revoke compromised keys as quickly as new keys are broken.

However, a new twist came yesterday, when SlySoft, an Antigua-based company that sells software to defeat various forms of copy protection, updated its AnyDVD product to allow it to copy the new AACS discs. Apparently, SlySoft had extracted a key from a different player and had kept the attack a secret. They waited until all the other compromised keys were blacklisted before switching to the new one.

The AACS Licensing Authority will be able to figure out which player SlySoft cracked by examining the program, and they will eventually blacklist this new key as well. However, all discs on store shelves will remain copyable for months, since disc producers must wait another ninety days before making the change.

To be successful in the long run, AACS needs to outpace such attacks. Its backers might be able to accelerate the blacklisting cycle somewhat by revising their agreements with player manufacturers, but the logistics of mastering discs and shipping them to market mean the shortest practical turnaround time will be at least several weeks. Attackers don’t even have to wait this long before they start to crack another player. Like Slysoft, they can extract keys from several players and keep some of them secret until all publicly known keys are blacklisted. Then they can release the other keys one at a time to buy additional time.

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