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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 next generation of optical discs and DVDs. The specification was publicly released in April 2005 and 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, Matsushita (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 finalised.
Since appearing in devices in 2006, several AACS decryption keys have been extracted from weakly protected software players and published on the Internet
Encryption
AACS uses cryptography to control the use of digital media. It encrypts content under one or more title keys using the Advanced Encryption Standard (AES). Title keys are derived from 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 CSS, the DRM system used on DVDs, lies in how the device decryption keys are organized.
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. 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, making the keys/player useless for decrypting new titles.
AACS also has traitor tracing. If an attacker tries to keep a compromised key secret (by only publishing decrypted title keys), it can still be tracked down. The standard allows different versions of short sections of a movie to be encrypted with different keys. A certain player will only be able to decrypt one version of each section. By embedding a digital watermark in the different versions and analyzing what sections of the movie the attacker publishes, the compromised keys can eventually 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, hackers claim to have circumvented that particular protection by modifying the firmware of an HD DVD reader.
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.
Audio watermarking
It is possible to use Verance's audio watermarking technology in conjunction with AACS. Studios may insert a mark in the soundtracks of theatrical motion pictures. If an AACS-compliant player detects this mark, it means the disc is playing back a copy made from a theatrical print (probably from illegal camcording), and will cause the player to refuse to play the disc. The mark is made by varying the waveform of speech and music in a regular pattern to convey a digital code. These variations are too subtle to be noticed by most people. Another variation of this system can be used to discourage the playback of discs created through unauthorized duplication of audio watermarked discs purchased or rented by consumers. This variation for home entertainment utilizes a watermark that differs from the cinema mark in that it is permitted in media distributed to consumers.
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.
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