October 2, 2022

Search Results for: aacs

AACS Updated, Broken Again

[Other posts in this series]

We predicted in past posts that AACS, the encryption system intended to protect HD-DVD and Blu-ray movies, would suffer a gradual meltdown from its inability to respond quickly enough to attacks. Like most DRM, AACS depends on the secrecy of encryption keys built into hardware and software players. An attacker who discovers a player’s keys can defeat the protection on any disc that works with that player. AACS was designed with a defense against such attacks: after a player has been compromised, producers can alter new discs so that they no longer work with the compromised player’s keys. Whether this defense (which we call “key blacklisting”) will do much to stop copying depends how much time elapses before each leaked key is blacklisted.

Next week marks three months after the first compromised player key appeared on the Internet (and more than five months after cracks for individual discs began to appear). Discs slated for release on Tuesday will be the first to contain an update to AACS that blacklists the leaked keys.

What took so long? One limitation comes from the licensing agreement signed with player manufacturers, which requires that they receive ninety days’ notice before their keys are blacklisted, so that they have enough time to update their products.

Customers who obtained the new discs a few days early confirmed that the previously leaked keys no longer worked. It seemed as if AACS had recovered from the attacks just as its designers intended.

However, a new twist came yesterday, when SlySoft, an Antigua-based company that sells software to defeat various forms of copy protection, updated its AnyDVD product to allow it to copy the new AACS discs. Apparently, SlySoft had extracted a key from a different player and had kept the attack a secret. They waited until all the other compromised keys were blacklisted before switching to the new one.

The AACS Licensing Authority will be able to figure out which player SlySoft cracked by examining the program, and they will eventually blacklist this new key as well. However, all discs on store shelves will remain copyable for months, since disc producers must wait another ninety days before making the change.

To be successful in the long run, AACS needs to outpace such attacks. Its backers might be able to accelerate the blacklisting cycle somewhat by revising their agreements with player manufacturers, but the logistics of mastering discs and shipping them to market mean the shortest practical turnaround time will be at least several weeks. Attackers don’t even have to wait this long before they start to crack another player. Like Slysoft, they can extract keys from several players and keep some of them secret until all publicly known keys are blacklisted. Then they can release the other keys one at a time to buy additional time.

All of this is yet more bad news for AACS.

Digg Users Revolt Over AACS Key

I wrote yesterday about efforts by AACS LA, the entity that controls the AACS copy protection system used in HD-DVD and Blu-ray discs, to stop people from republishing a sixteen-byte cryptographic key that can unlock most existing discs. Much of the action took place at Digg, a site that aggregates Web page recommendations from many people. (At Digg, you can recommend pages on the Web that you find interesting, and Digg will show you the most-recommended pages in various categories.

Digg had received a demand letter from AACS LA, asking Digg to take down links to sites containing the key. After consulting with lawyers, Digg complied, and Digg’s administrators started canceling entries on the site.

Then Digg’s users revolted. As word got around about what Digg was doing, users launched a deluge of submissions to Digg, all mentioning or linking to the key. Digg’s administrators tried to keep up, but submissions showed up faster than the administrators could cancel them. For a while yesterday, the entire front page of Digg – the “hottest” pages according to Digg’s algorithms – consisted of links to the AACS key.

Last night, Digg capitulated to its users. Digg promised to stop removing links to the key, and Digg founder Kevin Rose even posted the key to the site himself. Rose wrote on Digg’s official blog,

In building and shaping the site I’ve always tried to stay as hands on as possible. We’ve always given site moderation (digging/burying) power to the community. Occasionally we step in to remove stories that violate our terms of use (eg. linking to pornography, illegal downloads, racial hate sites, etc.). So today was a difficult day for us. We had to decide whether to remove stories containing a single code based on a cease and desist declaration. We had to make a call, and in our desire to avoid a scenario where Digg would be interrupted or shut down, we decided to comply and remove the stories with the code.

But now, after seeing hundreds of stories and reading thousands of comments, you’ve made it clear. You’d rather see Digg go down fighting than bow down to a bigger company. We hear you, and effective immediately we won’t delete stories or comments containing the code and will deal with whatever the consequences might be.

If we lose, then what the hell, at least we died trying.

This is a remarkable event. Critics of Web 2.0 technologies like Digg often say that users are being exploited, that the “communities” on these sites are shams and the company running the site is really in control. Here, the Digg company found that it doesn’t entirely control the Digg site – if users want something on the site badly enough, they can put it there. If Digg wasn’t going to shut down entirely (or become clogged with postings of the key), it had no choice but to acquiesce and allow links to the key. But Digg went beyond acquiescence, siding with its users against AACS LA, by posting the key itself and practically inviting a lawsuit from AACS LA.

Digg’s motive here probably has more to do with profit and market share than with truth, justice, and the American way. It’s not a coincidence that Digg’s newly discovered values coincide with the desires of its users. Still, the important fact is that users could bend Digg to their will. It turns out that the “government” of Digg’s community gets its power from the consent of the governed. Users of other Web 2.0 sites will surely take note.

AACS Plays Whack-a-Mole with Extracted Key

The people who control AACS, the copy protection technology used on HD-DVD and Blu-ray discs, are apparently trying to shut down websites that publish a certain 128-bit integer. The number is apparently a “processing key” used in AACS. Together with a suitable computer program, the key allows the decryption of video content on most existing HD-DVD and Blu-ray discs.

I won’t publish the key here but you can spot it all over the Web. It’s a long string starting with “09 F9”.

The key has been published on a few websites for months, but in recent days the AACS “Licensing Authority” (AACS LA) has taken to sending out demand letters to websites that publish the key, claiming that the key is a circumvention technology under the DMCA. News of these demand letters, and the subsequent disappearance of content and whole sites from the Net, has triggered an entirely predictable backlash, with thousands of people reposting the key to their own sites.

The key will inevitably remain available, and AACSLA are just making themselves look silly by trying to suppress it. We’ve seen this script before. The key will show up on T-shirts and in song lyrics. It will be chalked on the sidewalk outside the AACS LA office. And so on.

It’s hard to see the logic in AACS LA’s strategy here. Their end goal is (or should be) to stop unauthorized online distribution of high-def video files ripped from HD-DVD or Blu-ray discs. The files in question are enormous and cumbersome to store and distribute, containing more than a gigabyte of content. If you can’t stop distribution of these huge files, surely there’s no hope of stopping distribution of a little sixteen-byte key, or even of decryption software containing the key. Whatever tactics can stop distribution of the key should be even more effective against distribution of movies.

My guess is that AACS LA miscalculated, thinking that a few demand letters would succeed in suppressing the key. As the key spread, it seemed natural to continue sending letters – to do otherwise would be an admission of defeat. Now the key is spread so widely that there’s no point in sending any more letters.

The next question is whether AACS LA will try to sue somebody who defied a demand letter. There’s no real strategic point to such a suit, but even big organizations act out of spite sometimes.

AACS: Slow Start on Traitor Tracing

[Previous posts in this series: 1, 2, 3, 4, 5, 6, 7, 8.]

Alex wrote on Thursday about the next step in the breakdown of AACS, the encryption scheme used on next-gen DVD discs (HD-DVD and Blu-ray): last week a person named Arnezami discovered and published a processing key that apparently can be used to decrypt all existing discs.

We’ve been discussing AACS encryption, on and off, for several weeks now. To review the state of play: the encryption scheme serves two purposes: key distribution and traitor tracing. Key distribution ensures that every player device, except devices that have been blacklisted, can decrypt a disc. Traitor tracing helps the authorities track down which player has been compromised, if key information is leaked. The AACS authorities encode the header information for each disc in such a way that keys are distributed properly and traitor tracing can occur.

Or that’s the theory, at least. In practice, the authorities are making very little use of the traitor tracing facilities. We’re not sure why this is. They surely have an interest in tracing traitors, and failing to encode discs to facilitate traitor tracing is just a lost opportunity.

The main traitor tracing feature is the so-called sequence key mechanism. This mechanism is not used at all on any of the discs we have seen, nor have we seen any reports of its use.

A secondary traitor tracing feature involves the use of processing keys. Each player device has a unique set of a few hundred device keys, from which it can calculate a few billion different processing keys. Each processing key is computable by only a fraction of the players in the world. Each disc’s headers include a list of the processing keys that can decrypt the disc; any one of the listed processing keys is sufficient to decrypt the disc.

For some reason, all existing discs seem to list the same set of 512 processing keys. Each player will be able to compute exactly one of these processing keys. So when Arnezami leaked a processing key, the authorities could deduce that he must have extracted it from a player that knew that particular processing key. In other words, it narrowed down the identity of his player to about 0.2% of all possible players.

Because all existing discs use the same set of processing keys, the processing key leaked by Arnezami can decrypt any existing disc. Had the authorities used different sets of processing keys on different discs – which was perfectly feasible – then a single processing key would not have unlocked so many discs. Arnezami would have had to extract and publish many processing keys, which would have made his job more difficult, and would have further narrowed down which player he had.

The ability to use different processing key sets on different discs is part of the AACS traitor tracing facility. In failing to do this, the authorities once again failed to use the traitor tracing mechanisms at their disposal.

Why aren’t the authorities working as hard as they can to traitor-trace compromised players? Sure, the sequence key and processing key mechanisms are a bit complex, but if the authorities weren’t going to use these mechanisms, then why would they have gone to the difficulty and expense of designing them and requiring all players to implement them? It’s a mystery to us.

AACS: A Tale of Three Keys

[Previous posts in this series: 1, 2, 3, 4, 5, 6, 7.]

This week brings further developments in the gradual meltdown of AACS (the encryption scheme used for HD-DVD and Blu-Ray discs). Last Sunday, a member of the Doom9 forum, writing under the pseudonym Arnezami, managed to extract a “processing key” from an HD-DVD player application. Arnezami says that this processing key can be used to decrypt all existing HD-DVD and Blu-Ray discs. Though currently this attack is more powerful than previous breaks, which focused on a different kind of key, its usefulness will probably diminish as AACS implementers adapt.

To explain what’s at stake, we need to describe a few more details about the way AACS manages keys. Recall that AACS player applications and devices are assigned secret device keys. Devices can use these keys to calculate a much larger set of keys called processing keys. Each AACS movie is encrypted with a unique title key, and several copies of the title key, encrypted with different processing keys, are stored on the disc. To play a disc, a device figures out which of the encrypted title keys it has the ability to decrypt. Then it uses its device keys to compute the necessary processing key, uses the processing key to decrypt the title key, and uses the title key to extract the content.

These three kinds of keys have different security properties that make them more or less valuable to attackers. Device keys are the most useful. If you know the device keys for a player, you can decrypt any disc that the player can. Title keys are the least useful, because each title key works only for a single movie. (Attacks on any of these keys will be limited by disc producers’ ability to blacklist compromised players. If they can determine which device has been compromised, they can change future discs so that the broken player, or its leaked device keys, won’t be able to decrypt them.)

To date, no device keys have been compromised. All successful breaks, before Arnezami, have involved extracting title keys from player software. These attacks are rather cumbersome–before non-technical users can decrypt a movie, somebody with the means to extract the title key needs to obtain a copy of the disc and publish its title key online. Multiple web sites for sharing title keys have been deployed, but these are susceptible to legal and technical threats.

So is the new attack on the processing key comparable to learning a venerable device key or a lowly title key? The answer is that, due to a strange quirk in the way the processing keys used on existing discs were selected, the key Arnezami published apparently can be used to decrypt every HD-DVD or Blu-Ray disc on the market. For the time being, knowing Arnezami’s processing key is as powerful as knowing a device key. For instance, someone could use the processing key to build a player or ripper that is able to treat all current discs as if they were unencrypted, without relying on online services or waiting for other users to extract title keys.

Yet this power will not last long. For future discs, processing key attacks will probably be no more valuable than title key attacks, working only on a single disc or a few discs at most. We’ll explain why in tomorrow’s post.