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).
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.