Advanced Access Content System

The Advanced Access Content System (AACS) is a standard for content distribution and digital rights management, intended to restrict access to and copying of the post-DVD generation of optical discs. The specification was publicly released in April 2005. The standard has been adopted as the access restriction scheme for HD DVD and Blu-ray Disc (BD). It is developed by AACS Licensing Administrator, LLC (AACS LA), a consortium that includes Disney, Intel, Microsoft, Panasonic, Warner Bros., IBM, Toshiba and Sony. AACS has been operating under an "interim agreement" since the final specification (including provisions for Managed Copy) has not yet been finalized.

Since appearing in devices in 2006, several AACS decryption keys have been extracted from software players and published on the Internet, allowing decryption by unlicensed software.

AACS uses cryptography to control and restrict the use of digital media. It encrypts content under one or more title keys using the Advanced Encryption Standard (AES). Title keys are decrypted using a media key (encoded in a Media Key Block) and the Volume ID of the media (e.g., a physical serial number embedded on a pre-recorded disc).

The principal difference between AACS and CSS (the DRM system used on DVDs) lies in how the device decryption keys and codes are organized.

Under CSS, all players of a given model group are provisioned with the same shared activated decryption key. Content is encrypted using a title-specific key, which is itself encrypted under each model's key. Thus, each disc 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 encrypt future title keys with the player model's key. 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 and published, the AACS LA can simply revoke those keys in future content, rendering the keys and the player useless for decrypting new titles.

AACS also incorporates traitor tracing techniques. The standard allows for multiple versions of short sections of a movie to be encrypted with different keys, while a given player will only be able to decrypt one version of each section. The manufacturer embeds varying digital watermarks (such as Cinavia) in these sections, and upon subsequent analysis of the pirated release the compromised keys can be identified and revoked (this feature is called Sequence keys in the AACS specifications).