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The Advanced Access Content System (AACS) is a standard for content distribution and digital rights management, which will allow restricting access to and copying of the next generation of optical discs and DVDs.

The group developing it includes Disney, Intel, Microsoft, Matsushita, Warner Brothers, IBM, Toshiba, and Sony. The standard has been adopted as the access restriction scheme for HD-DVD and Blu-ray.

The proposal is based on broadcast encryption using Naor-Naor-Lotspiech subset difference trees. The proposal was voted one of the technologies most likely to fail by IEEE Spectrum magazine. Concerns about the approach include its similarity to past systems that failed, such as Content Scrambling System (CSS), and the inability to preserve security against attacks that compromise large numbers of players. The specifications for the product have been publicly released (as of April 2005).

System overview

AACS utilizes cryptography to control the use of digital media. AACS-protected content is encrypted under one or more title keys using the Advanced Encryption Standard (AES). Title keys are derived from a combination of a media key and several elements, including the volume ID of the media (e.g., a physical serial number embedded on a DVD), and a cryptographic hash of the title usage rules.

The principal difference between AACS and earlier content management systems such as CSS is in the means by which title-specific decryption keys are distributed. Under CSS, all players of a given model are provisioned with the same, shared decryption key. Content is encrypted under the title-specific key, which is itself encrypted under each model's key. In CSS, each volume contains a collection of several hundred encrypted keys, one for each licensed player model. In principle, this approach allows licensors to "revoke" a given player model (prevent it from playing back future content) by omitting to include the encryption corresponding to that model. In practice, however, revoking all players of a particular model is costly, as it causes many users to lose playback capability. Furthermore, the inclusion of a shared key across many players makes key compromise significantly more likely, as was demonstrated by a number of compromises in the mid-1990s.

The approach of AACS provisions each individual player with a unique set of decryption keys which are used in a broadcast encryption scheme. This approach allows licensors to "revoke" individual players, or more specifically, the decryption keys associated with the player. Thus, if a given player's keys are compromised by an attacker, the AACS licensing authority can simply revoke those keys in future content, making the keys/player useless for decrypting new titles.

See also

• Content Scrambling System

External links

• AACS homepage
• Specifications
• Whitepaper on the technology (PDF)
• Reuters report on USA Today
• Independent Security Evaluators (ISE) evaluation of AACS

• Digital rights management