Advanced Access Content System

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The Advanced Access Content System (AACS) is a standard for content distribution and digital rights management, which will allow restricting access to and copying of the next generation of optical discs and DVDs.

The group developing it includes Disney, Intel, Microsoft, Matsushita (Panasonic), Warner Brothers, IBM, Toshiba, and Sony. The standard has been adopted as the access restriction scheme for HD DVD and Blu-ray Disc.

The proposal is based on broadcast encryption using Naor-Naor-Lotspiech subset difference trees. The proposal was voted one of the technologies most likely to fail by IEEE Spectrum magazine [1]. Concerns about the approach include its similarity to past systems that failed, such as Content Scrambling System (CSS), and the inability to preserve security against attacks that compromise large numbers of players. Indeed, Jon Lech Johansen (known colloquially as "DVD Jon") who defeated the original CSS encryption expects AACS to be cracked by Winter 2006/2007[2]. The specifications for the product have been publicly released (as of April 2005).

1 System overview
2 Attempts at defeating AACS copy protection
- 2.1 Initial steps
- 2.2 The future
3 External links

[edit] System overview

AACS utilizes cryptography to control the use of digital media. AACS-protected content is encrypted under one or more title keys using the Advanced Encryption Standard (AES). Title keys are derived from a combination of a media key and several elements, including the volume ID of the media (e.g., a physical serial number embedded on a DVD), and a cryptographic hash of the title usage rules.

The principal difference between AACS and earlier content management systems such as CSS is in the means by which title-specific decryption keys are distributed. 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. In CSS, each volume 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 the encryption key corresponding to that model. 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 by an attacker, the AACS licensing authority can simply revoke those keys in future content, making the keys/player useless for decrypting new titles.

[edit] Attempts at defeating AACS copy protection

[edit] Initial steps

In July 2006, the first steps towards enabling full AACS-encrypted films to be copied were taken[3][4]. While great care has been taken with AACS to ensure that contents are encrypted right up to the display device, it was discovered that a perfect copy of any still frame from a film could be made simply by utilising the Print Screen function of the Windows operating system. It is expected that this approach could be automated to allow a perfect copy of an entire film to be made, in much the same way that DVD films were copied before the advent of DeCSS. It should be noted that such approaches do not constitute compromises of the AACS encryption itself, relying instead on an officially licensed software player to perform the decryption. As such, the output data will not be in the form of the compressed video from the disc, but rather decompressed video. The large size of this will necessitate that the video be recompressed, for example using x264.

[edit] The future

Jon Lech Johansen, the Norwegian hacker famous for developing the DeCSS tool which removes CSS encoding from DVDs, announced on January 8, 2006 that he would be developing a piece of software called DeAACS which would remove AACS encryption from Blu-ray and HD-DVD discs. He expects the software to be released by March 2007.