Today, seven colleagues and I released a new paper, “Defeating Vanish with Low-Cost Sybil Attacks Against Large DHTs“. The paper’s authors are Scott Wolchok (Michigan), Owen Hofmann (Texas), Nadia Heninger (Princeton), me, Alex Halderman (Michigan), Christopher Rossbach (Texas), Brent Waters (Texas), and Emmett Witchel (Texas).
Our paper is the next chapter in an interesting story about the making, breaking, and possible fixing of security systems.
The story started with a system called Vanish, designed by a team at the University of Washington (Roxana Geambasu, Yoshi Kohno, Amit Levy, and Hank Levy). Vanish tries to provide “vanishing data objects” (VDOs) that can be created at any time but will only be usable within a short time window (typically eight hours) after their creation. This is an unusual kind of security guarantee: the VDO can be read by anybody who sees it in the first eight hours, but after that period expires the VDO is supposed to be unrecoverable.
Vanish uses a clever design to do this. It takes your data and encrypts it, using a fresh random encryption key. It then splits the key into shares, so that a quorum of shares (say, seven out of ten shares) is required to reconstruct the key. It takes the shares and stores them at random locations in a giant worldwide system called the Vuze DHT. The Vuze DHT throws away items after eight hours. After that the shares are gone, so the key cannot be reconstructed, so the VDO cannot be decrypted — at least in theory.
What is this Vuze DHT? It’s a worldwide peer-to-peer network, containing a million or so computers, that was set up by Vuze, a company that uses the BitTorrent protocol to distribute (licensed) video content. Vuze needs a giant data store for its own purposes, to help peers find the videos they want, and this data store happens to be open so that Vanish can use it. The million-computer extent of the Vuze data store was important, because it gave the Vanish designers a big haystack in which to hide their needles.
Vanish debuted on July 20 with a splashy New York Times article. Reading the article, Alex Halderman and I realized that some of our past thinking about how to extract information from large distributed data structures might be applied to attack Vanish. Alex’s student Scott Wolchok grabbed the project and started doing experiments to see how much information could be extracted from the Vuze DHT. If we could monitor Vuze and continuously record almost all of its contents, then we could build a Wayback Machine for Vuze that would let us decrypt VDOs that were supposedly expired, thereby defeating Vanish’s security guarantees.
Scott’s experiments progressed rapidly, and by early August we were pretty sure that we were close to demonstrating a break of Vanish. The Vanish authors were due to present their work in a few days, at the Usenix Security conference in Montreal, and we hoped to demonstrate a break by then. The question was whether Scott’s already heroic sleep-deprived experimental odyssey would reach its destination in time.
We didn’t want to ambush the Vanish authors with our break, so we took them aside at the conference and told them about our preliminary results. This led to some interesting technical discussions with the Vanish team about technical details of Vuze and Vanish, and about some alternative designs for Vuze and Vanish that might better resist attacks. We agreed to keep them up to date on any new results, so they could address the issue in their talk.
As it turned out, we didn’t establish a break before the Vanish team’s conference presentation, so they did not have to modify their presentation much, and Scott finally got to catch up on his sleep. Later, we realized that evidence to establish a break had actually been in our experimental logs before the Vanish talk, but we hadn’t been clever enough to spot it at the time. Science is hard.
Some time later, I ran into my ex-student Brent Waters, who is now on the faculty at the University of Texas. I mentioned to Brent that Scott, Alex and I had been studying attacks on Vanish and we thought we were pretty close to making an attack work. Amazingly, Brent and some Texas colleagues (Owen Hoffman, Christopher Rossbach, and Emmett Witchel) had also been studying Vanish and had independently devised attacks that were pretty similar to what Scott, Alex, and I had.
We decided that it made sense to join up with the Texas team, work together on finishing and testing the attacks, and then write a joint paper. Nadia Heninger at Princeton did some valuable modeling to help us understand our experimental results, so we added her to the team.
Today we are releasing our joint paper. It describes our attacks and demonstrates that the attacks do indeed defeat Vanish. We have a working system that can decrypt Vanishing data objects (made with the original version of Vanish) after they are supposedly unrecoverable.
Our paper also discusses what went wrong in the original Vanish design. The people who designed Vanish are smart and experienced, but they obviously made some kind of mistake in their original work that led them to believe that Vanish was secure — a belief that we now know is incorrect. Our paper talks about where we think the Vanish authors went wrong, and what security practitioners can learn from the Vanish experience so far.
Meanwhile, the Vanish authors went back to the drawing board and came up with a bunch of improvements to Vanish and Vuze that make our attacks much more expensive. They wrote their own paper about their experience with Vanish and their new modifications to it.
Where does this leave us?
For now, Vanish should be considered too risky to rely on. The standard for security is not “no currently demonstrated attacks”, it is “strong evidence that the system resists all reasonable attacks”. By updating Vanish to resist our attacks, the Vanish authors showed that their system is not a dead letter. But in my view they are still some distance from showing that Vanish is secure . Given the complexity of underlying technologies such as Vuze, I wouldn’t be surprised if more attacks turn out to be possible. The latest version of Vanish might turn out to be sound, or to be unsound, or the whole approach might turn out to be flawed. It’s too early to tell.
Vanish is an interesting approach to a real problem. Whether this approach will turn out to work is still an open question. It’s good to explore this question — and I’m glad that the Vanish authors and others are doing so. At this point, Vanish is of real scientific interest, but I wouldn’t rely on it to secure my data.
[Update (Sept. 30, 2009): I rewrote the paragraphs describing our discussions with the Vanish team at the conference. The original version may have given the wrong impression about our intentions.]