Advanced Access Content System

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.

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 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).

Volume IDs

Volume IDs are unique identifiers or serial numbers that are stored on pre-recorded discs with special hardware. They cannot be duplicated on consumers' recordable media. The point of this is to prevent simple bit-by-bit copies, since the Volume ID is required (though not sufficient) for decoding content. On Blu-ray discs, the Volume ID is stored in the BD-ROM Mark.

To read the Volume ID, a cryptographic certificate (the Private Host Key) signed by the AACS LA is required. However, this has been circumvented by modifying the firmware of some HD DVD and Blu-ray drives.

Decryption process

To view the movie, the player must first decrypt the content on the disc. The decryption process is somewhat convoluted. The disc contains 4 items—the Media Key Block (MKB), the Volume ID, the Encrypted Title Keys, and the Encrypted Content. The MKB is encrypted in a subset difference tree approach. Essentially, a set of keys are arranged in a tree such that any given key can be used to find every other key except its parent keys. This way, to revoke a given device key, the MKB needs only be encrypted with that device key's parent key.

Once the MKB is decrypted, it provides the Media Key, or the km. The km is combined with the Volume ID (which the program can only get by presenting a cryptographic certificate to the drive, as described above) in a one-way encryption scheme (AES-G) to produce the Volume Unique Key (Kvu). The Kvu is used to decrypt the encrypted title keys, and that is used to decrypt the encrypted content.

Analog Outputs

AACS-compliant players must follow guidelines pertaining to outputs over analog connections. This is set by a flag called the Image Constraint Token (ICT), which restricts the resolution for analog outputs to 960×540. Full 1920×1080 resolution is restricted to HDMI or DVI outputs that support HDCP. The decision to set the flag to restrict output ("down-convert") is left to the content provider. Warner Pictures is a proponent of ICT, and it is expected that Paramount and Universal will implement down-conversion as well. AACS guidelines require that any title which implements the ICT must clearly state so on the packaging. The German magazine "Der Spiegel" has reported about an unofficial agreement between film studios and electronics manufacturers to not use ICT until 2010 – 2012. However, some titles have already been released that apply ICT.

Audio watermarking

On 5 June 2009, the licensing agreements for AACS were finalized, which were updated to make Cinavia audio watermark detection on commercial Blu-ray disc players a requirement.

Managed Copy

Managed Copy refers to a system by which consumers can make legal copies of films and other digital content protected by AACS. This requires the device to obtain authorization by contacting a remote server on the Internet. The copies will still be protected by DRM, so infinite copying is not possible (unless it is explicitly allowed by the content owner). It is mandatory for content providers to give the consumer this flexibility in both the HD DVD and the Blu-ray standards (commonly called Mandatory Managed Copy). The Blu-ray standards adopted Mandatory Managed Copy later than HD DVD, after HP requested it.

Possible scenarios for Managed Copy include (but are not limited to):

• Create an exact duplicate onto a recordable disc for backup
• Create a full-resolution copy for storage on a media server
• Create a scaled-down version for watching on a portable device

This feature was not included in the interim standard, so the first devices on the market did not have this capability. It was expected to be a part of the final AACS specification.

In June 2009, the final AACS agreements were ratified and posted online, and include information on the Managed Copy aspects of AACS.