Security of Advanced Access Content System

The security of Advanced Access Content System has a subject of discussion amongst security researchers, high definition video enthusiasts, and consumers at large since its inception. A successor to Content Scramble System, the digital rights management mechanism used by commercial DVDs, AACS was intended to improve upon the design of CSS by addressing flaws which had led to the total circumvention of CSS in 1999. The AACS system relies on a subset difference tree combined with a certificate revocation mechanism to ensure the security of high definition video content in the event of a compromise.

Even before AACS was put into use, security researchers expressed doubts about the system's ability to withstand attacks.

Comparison of CSS and AACS

History of attacks

The AACS proposal was voted one of the technologies most likely to fail by "IEEE Spectrum" magazine's readers in the January 2005 issue. [cite web
url = http://www.spectrum.ieee.org/jan05/2703
title = Loser: DVD Copy Protection, Take 2
accessdate = 2007-05-04
author = Tekla S. Perry
year = 2007
month = January
work = Spectrum Online] Concerns about the approach included its similarity to past systems that failed, such as CSS, and the inability to preserve security against attacks that compromise large numbers of players. Jon Lech Johansen, who was part of the team that circumvented CSS, said he expected AACS to be cracked by the end of 2006 or the beginning of 2007. [cite web
url = http://nanocrew.net/2006/01/08/deaacscom/
title = DeAACS.com
accessdate = 2007-05-04
last = Johansen
first = Jon Lech
date = 2006-01-08
work=So sue me] In late 2006, security expert Peter Gutmann released "A Cost Analysis of Windows Vista Content Protection", a technical paper criticizing the implementation of various content protection technologies in Windows Vista.cite paper
author = Peter Gutmann
date = 2006-12-26
title = A Cost Analysis of Windows Vista Content Protection
url = http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.html
accessdate = 2007-01-28 ] cquote
Providing this protection incurs considerable costs in terms of system performance, system stability, technical support overhead, and hardware and software cost. These issues affect not only users of Vista but the entire PC industry, since the effects of the protection measures extend to cover all hardware and software that will ever come into contact with Vista, even if it's not used directly with Vista (for example hardware in a Macintosh computer or on a Linux server).

This paper was later claimed by Microsoft to contain various factual errors. [cite web|url=http://windowsvistablog.com/blogs/windowsvista/archive/2007/01/20/windows-vista-content-protection-twenty-questions-and-answers.aspx|title=Windows Vista Content Protection - Twenty Questions (and Answers)] [cite web|url=http://en.wikipedia.org/wiki/Peter_Gutmann_%28computer_scientist%29#Criticism_of_Peter_Gutmann.27s_analysis_of_Vista_DRM|title=Criticisms of Vista Analysis]

While great care had been taken with AACS to ensure that content was encrypted along the entire path from the disc to the display device, it was discovered in July 2006 that a perfect copy of any still frame from a film could be captured from certain Blu-ray and HD DVD software players by using the Print Screen function of the Windows operating system. [cite web
url = http://hardware.slashdot.org/article.pl?sid=06/07/07/1255224
title = Work Around for New DVD Format Protections
date = 2006-06-07
accessdate = 2007-05-02
work = Slashdot] [cite web
url = http://www.heise-online.co.uk/security/Copy-protection-hole-in-Blu-ray-and-HD-DVD-movies--/news/75103
title = Copy protection hole in Blu-ray and HD DVD movies
accessdate = 2007-05-02
date = 2006-07-07
author = Edward Henning ("ehe")
work = heise Security] It was suggested 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 CSS was cracked, but to date no such copy has been discovered. This exploit has been closed in subsequent software versions.

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. This is an example of the analog hole.

Both title keys and one of the keys used to decrypt them (known as "Processing Keys" in the AACS specifications) have been found by using debuggers to inspect the memory space of running HD-DVD and Blu-ray player programs. [cite web
url = http://www.techamok.com/?pid=1849
title = HD-DVD Content Protection already hacked?
work = TechAmok
date = 2006-12-28
accessdate = 2007-01-02 ] [cite web
url = http://news.bbc.co.uk/2/hi/technology/6301301.stm
title = Hi-def DVD security is bypassed
date = 2007-01-26
accessdate=2007-05-02
work=BBC news] [cite web
last = Block
first = Ryan
url = http://www.engadget.com/2007/01/20/blu-ray-cracked-too/
title = Blu-ray cracked too?
date = 2007-01-20
accessdate = 2007-01-22
work=Engadget ] [cite web
url = http://www.theregister.co.uk/2007/01/23/blu-ray_drm_cracked/
first = John
last = Leyden
work = The Register
title = Blu-ray DRM defeated
date = 2007-01-23
accessdate = 2007-01-22] Hackers also found Device Keys, [cite web
url=http://forum.doom9.org/showthread.php?t=122664
title=WinDVD 8 Device Key Found!
accessdate=2007-05-04
author="ATARI Vampire"
date=2007-02-24
work=Doom9.net forums] which are used to calculate the Processing Key, and a Host Private Key [cite web
url=http://forum.doom9.org/showthread.php?t=122969
title=PowerDVD private key
accessdate=2007-05-04
author="jx6bpm"
date=2007-03-03
work=Doom9.net forums] (a key signed by the AACS LA used for hand-shaking between host and HD drive; required for reading the Volume ID). The first unprotected HD DVD movies appeared on BitTorrent trackers soon afterwards. [cite web
url=http://www.dailytech.com/article.aspx?newsid=5747
title=First Pirated HD DVDs Released
accessdate=2007-05-03
first=Marcus
last=Yam
date=2007-01-17
work=DailyTech] The Processing Key for the first Media Key Block version, which could be used to decrypt any AACS protected content released up to that point, was found and published on the Internet at the Doom9 forums. AACS Licensing Authority sent multiple DMCA takedown notices to web sites hosting the key. [cite web
url=http://www.chillingeffects.org/notice.cgi?sID=3218
title=AACS licensor complains of posted key
accessdate=2007-05-02] Some administrators of sites which consist of user-submitted content, such as Digg and Wikipedia, tried to remove mentions of the key fearing reprisals from AACS LA. [cite web
url=http://blog.wired.com/business/2007/05/wikipedia_locks.html
title=Wikipedia Locks Out "The Number"
first=Paul
last=Boutin
accessdate=2007-05-02
date=2007-05-01 ] [cite web
title=Digg's DRM Revolt
first=Andy
last=Greenberg
date=2007-05-02
work=Forbes
url=http://www.forbes.com/technology/2007/05/02/digital-rights-management-tech-cx_ag_0502digg.html
accessdate=2007-05-04] Both sites' administrators eventually decided to allow publication of the key. [cite web
url=http://news.bbc.co.uk/2/hi/technology/6615047.stm
title=DVD DRM row sparks user rebellion
accessdate = 2007-05-02
date=2007-05-02
work=BBC news]

Cyberlink, the company which sells the PowerDVD player, stated that their software could not have been used as part of these exploits. [cite web
date=2007-01-02
first=Chris
last=Lanier
url=http://msmvps.com/blogs/chrisl/archive/2007/01/02/463980.aspx
title=Cyberlink Responds to Alleged AACS Crack
accessdate=2007-05-02]

On April 16, 2007, the AACS consortium announced that it had revoked the Device Keys used by both Cyberlink PowerDVD and InterVideo WinDVD, and patches were made available for users which provided uncompromised encryption keys and better security for the keys. [cite web
url=http://www.aacsla.com/press/
title=Press Messages: AACS - Advanced Access Content System
accessdate=2007-05-02] [cite web
url=http://dailytech.com/AACS+Responds+to+Cracked+HD+DVD+and+Bluray+Disc+Protections/article5879.htm
title=AACS Responds to Cracked HD DVD and Blu-ray Disc Protections
accessdate=2007-05-03
first=Marcus
last=Yam
date=2007-01-26
work=DailyTech ] To continue having the ability to view new content users were forced to apply the patches, which also hardened the security of player applications.

On 23rd May 2007 the Processing Key for the next version of the Media Key Block was posted to the comments page of a Freedom to Tinker blog post. [cite web
url=http://www.freedom-to-tinker.com/?p=1155#comment-367359
title=You Can Own an Integer Too
accessdate=2007-06-09 [http://web.archive.org/web/20070824092116/http://www.freedom-to-tinker.com/?p=1155#comment-367359 Archived copy of page] (original link dead)]

The use of encryption does not offer any true protection against memory snooping, since the software player must have the encryption key available somewhere in memory and there is no way to protect against a determined PC owner extracting the encryption key (if everything else fails the user could run the program in a virtual machine making it possible to freeze the program and inspect all memory addresses without the program knowing).

The only way to wholly prevent attacks like this would require changes to the PC platform (see Trusted Computing) or that the content distributors do not permit their content to be played on PCs at all (by not providing the companies making software players with the needed encryption keys).

On January 15 2007 a website launched at HDKeys.com containing a database of HD DVD title keys. It also featured a modified copy of the BackupHDDVD software allowing for online key retrieval (the latter was later removed after a DMCA complaint).

SlySoft has released AnyDVD HD which allows users to watch HD DVD and Blu-ray movies on non-HDCP-compliant PC hardware. The movies can be decrypted on the fly directly from the disc, or can be copied to another medium. AnyDVD HD is also capable of automatically removing any unwanted logos and trailers. Slysoft has stated that AnyDVD HD uses several different mechanisms to disable the encryption, and is not dependent on the use of compromised encryption keys. They have also stated that AACS has even more flaws in its implementation than CSS; this renders it highly vulnerable, [cite web
url = http://forum.slysoft.com/showpost.php?s=dd10095bbb7307dcbb16168166d2087e&p=9286&postcount=2
title = Device key revokation
accessdate = 2007-04-09
date = 2007-02-13
author = "peer"
work=Slysoft forums] but they will release no details on their implementation. Users at Doom9 claim that the program makes use of the host certificate of PowerDVD version 6.5, [ cite web
url = http://forum.doom9.org/showthread.php?t=122272
title = AnyDVD method of operation
accessdate = 2007-04-09
date = 2007-02-15
author = "evdberg"
work = Doom9.net Forums ] but SlySoft has claimed that the program would be unaffected by the AACS revocation system. [cite web
url = http://forum.slysoft.com/showpost.php?p=15263&postcount=10
title = And after the process. key is revoked?
accessdate = 2007-04-09
date = 2007-03-02
author = "James"
work=Slysoft forums ]

Media key block renewals

References