When Matasano recommends a blog, I read it. And, I just noticed this great post at a Matasano recommended blog.
There are a number of things to try when developing such attacks, depending on the device and countermeasures present. We’ll assume that the attacker has possession of several instances of the device and a moderate budget. This limits an attacker to non-invasive and slightly invasive methods.
Great discussion and something I commented on a while back when misinterpreting a prediction by Ptacek.
Besides all the stories of side channel attacks used by spies, side channel attacks went quite mainstream with research and attack demonstrations on smartcards a few years back. I can remember a couple of RSA conferences ago when there was a demonstration of SPA on the showroom floor, and most (all?) smartcard vendors have now tried to implement some sorts of countermeasures for the various attacks that have surfaced publicly (and not so publicly).[...]
Anyway, what I would really like to see from the creators of AACS is a discussion of this sort of attack.
It turns out that the attacker’s best strategy is to withhold any newly discovered compromise until a “release window” of size R has passed since the last time the authority blacklisted a player. (R depends in a complicated way on L and C.) Once the release window has passed, the attacker will use the compromise aggressively and the authority will then blacklist the compromised player, which essentially starts the game over. The studio collects revenue during the release window, and sometimes beyond the release window when the attacker gets unlucky and takes a long time to find another compromise.
This attack is based on these AACS design goals outlined by Metzger.
[...]In the high def video disk case, you have several quite different goals from the broadcast case. One is to enforce region encoding so that you can charge U.S. customers a large multiple of what you charge, say, a Chinese customer for exactly the same material. The second is to prevent people from retrieving the content in a form they can send to their friends over the internet. A third goal is to keep customers from being able to use content in unplanned ways so you can up-sell them to a newer format later on. (Note that the DRM scheme does *not* prevent actual commercial piracy of disks, since pirates can simply press bit for bit identical
disks.)
Metzger named the attack too.
As I noted, in the direct broadcast satellite case “slow leaks” of keys over extended periods of time are not of much use to the end users and are not of much damage to the system owners, but in the AACS case “slow leaks” are actually exceptionally damaging. Were the bad guys to release just one key every couple of months it would effectively destroy the system.
There are technical aspects to these “slow leak” key attacks, in that the attackers have to find ways to expose keys to leak, but, given that key exposure looks to have been assumed in the AACS design (and is certainly a reality), an attempt was made to mitigate these failures through limiting the damage done by key exposure, such as limiting the usefulness of a particular key and tracking its origins. This leads to a key revocation mechanism, but key revocation does not come for free. And, it seems the attacks on the key revocation process currently cost the attacker far less than the attacked. That makes the overall AACS an economic failure. And, since money was the driver behind AACS, that hits right at the heart of AACS.
So, what can be done here that bring the costs of key compromise back in favor of the attacked? Make key compromise more expensive to achieve? Make the key issuance more device specific? Increase key revocation roll out speed? This would be an interesting blog post from those that build these systems, and I wonder what steps are being taken to move in this direction.
Alternatively, is AACS DRM just dead in the water and media companies better off spending their money elsewhere? Is finding new revenue models not based on forcing consumers to buy the same content multiple times the way to go? Perhaps watermarking is more important here, such that people going “well beyond” beyond fair use can potentially be located but everyone else is let be?
Of note, there was a time when I thought about building an open source DRM system for alternative media. I talked to some sharp people about this, but, over many a beer, we established that the media in question relied on continuously distributing new content and advertising rather than depending on the same content being resold over and over to make money, making the whole point of DRM pretty much moot before we even moved on to pondering the technical challenges.
