Advanced Access Content System: Difference between revisions

Revision as of 21:53, 22 August 2024

Revision as of 10:30, 5 January 2025

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{{Use dmy dates|date=February 2020}}

[[Image:AACS dataflow.svg|right|thumb|AACS decryption process]]

The '''Advanced Access Content System''' ('''AACS''') is a [[standardization|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 LA|AACS Licensing Administrator, LLC]] (AACS LA), a [[consortium]] that includes [[The Walt Disney Company|Disney]], [[Intel]], [[Microsoft]], [[Panasonic Corporation|Panasonic]], [[Warner Bros.]], [[IBM]], [[Toshiba]] and [[Sony]]. AACS has been operating under an "interim agreement" since the final specification (including provisions for [[#Managed Copy|Managed Copy]]) has not yet been finalized.

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==System overview==

===Encryption===

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 combination of 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 [[Content Scramble System|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 underusing thea 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, makingrendering 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 watermarking|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).<ref>{{cite web