Ramblings & ephemera

AACS, next-gen encryption for DVDs

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

AACS relies on the well-established AES (with 128-bit keys) to safeguard the disc data. Just like DVD players, HD DVD and Blu-ray drives will come with a set of Device Keys handed out to the manufacturers by AACS LA. Unlike the CSS encryption used in DVDs, though, AACS has a built-in method for revoking sets of keys that are cracked and made public. AACS-encrypted discs will feature a Media Key Block that all players need to access in order to get the key needed to decrypt the video files on the disc. The MKB can be updated by AACS LA to prevent certain sets of Device Keys from functioning with future titles – a feature that AACS dubs “revocation.” …

AACS also supports a new feature called the Image Constraint Token. When set, the ICT will force video output to be degraded over analog connections. ICT has so far gone unused, though this could change at any time. …

While AACS is used by both HD disc formats, the Blu-ray Disc Association (BDA) has added some features of its own to make the format “more secure” than HD DVD. The additions are BD+ and ROM Mark; though both are designed to thwart pirates, they work quite differently.

While the generic AACS spec includes key revocation, BD+ actually allows the BDA to update the entire encryption system once players have already shipped. Should encryption be cracked, new discs will include information that will alter the players’ decryption code. …

The other new technology, ROM Mark, affects the manufacturing of Blu-ray discs. All Blu-ray mastering equipment must be licensed by the BDA, and they will ensure that all of it carries ROM Mark technology. Whenever a legitimate disc is created, it is given a “unique and undetectable identifier.” It’s not undetectable to the player, though, and players can refuse to play discs without a ROM Mark. The BDA has the optimistic hope that this will keep industrial-scale piracy at bay. We’ll see.

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