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Breaking Vanish: A Story of Security Research in Action

September 29, 2009 by

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.]

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Improving the Government's User Interface

September 21, 2009 by

The White House’s attempts to gather input from citizens have hit some bumps, wrote Anand Giridharadas recently in the New York Times. This administration has done far more than its predecessors to let citizens provide input directly to government via the Internet, but they haven’t always received the input they expected. Giridharadas writes:

During the transition, the administration created an online “Citizen’s Briefing Book” for people to submit ideas to the president…. They received 44,000 proposals and 1.4 million votes for those proposals. The results were quietly published, but they were embarrassing…

In the middle of two wars and an economic meltdown, the highest-ranking idea was to legalize marijuana, an idea nearly twice as popular as repealing the Bush tax cuts on the wealthy. Legalizing online poker topped the technology ideas, twice as popular as nationwide wi-fi. Revoking the Church of Scientology’s tax-exempt status garnered three times more votes than raising funding for childhood cancer.

Once in power, the White House crowdsourced again. In March, its Office of Science and Technology Policy hosted an online “brainstorm” about making government more transparent. Good ideas came; but a stunning number had no connection to transparency, with many calls for marijuana legalization and a raging (and groundless) debate about the authenticity of President Obama’s birth certificate.

It’s obvious what happened: relatively small groups of highly motivated people visited the site, and their input outweighed the discussion of more pressing national issues. This is not a new phenomenon — there’s a long history of organized groups sending letters out of proportion with their numbers.

Now, these groups obviously have the right to speak, and the fact that some groups proved to be better organized and motivated than others is useful information for policymakers to have. But if that is all that policymakers learn, we have lost an important opportunity. Government needs to hear from these groups, but it needs to hear from the rest of the public too.

It’s tempting to decide that this is inevitable, and that online harvesting of public opinion will have little value. But I think that goes too far.

What the administration’s experience teaches, I think, is that measuring public opinion online is difficult, and that the most obvious measurement methods can run into trouble. Instead of giving up, the best response is to think harder about how to gather information and how to analyze the information that is available. What works for a small, organized group, or even a political campaign, won’t necessarily work for the United States as a whole. What we need are new interfaces, new analysis methods, and experiments to reveal what tends to work.

Designing user interfaces is almost always harder than it looks. Designing the user interface of government is an enormous challenge, but getting it right can yield enormous benefits.

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NY Times Should Report on NY Times Ad Malware

September 14, 2009 by

Yesterday morning, while reading the New York Times online, I was confronted with an attempted security attack, apparently delivered through an advertisement. A window popped up, mimicking an antivirus scanner. After “scanning” my computer, it reported finding viruses and invited me to download a free antivirus scanner. The displays implied, without quite saying so, that the messages came from my antivirus vendor and that the download would come from there too. Knowing how these things work, I recognized it right away as an attack, probably carried by an ad. So I didn’t click on anything, and I’m fairly certain my computer wasn’t infected.

I wasn’t the only person who saw this attack. The Times posted a brief note on its site yesterday, and followed up today with a longer blog post.

What is interesting about the Times’s response is that it consists of security warnings, rather than journalism. Security warnings are good as far as they go; the Times owed that much to its users, at least. But it’s also newsworthy that a major, respected news site was facilitating cybercrime, even unintentionally. Somebody should report on this story — and who better than the Times itself?

It’s probably an interesting story, involving the ugly underside of the online ad business. Most likely, ad space in the Times was sold and, presumably, resold to an actual attacker; or a legitimate ad placement service was penetrated. Either way, other people are at risk of the same attack. Even better, the story opens issues such as the difficulties of securing the web, what vendors are doing to improve matters, what the bad buys are trying to achieve, and what happens to the victims.

An enterprising technology reporter might find a fascinating story here — and it’s right under the noses of the Times staff. Let’s hope they jump on it.

UPDATE (Sept. 15): As Barry points out in the comments below, the Times wrote a good article the day after this post appeared. It turns out that the booby-trapped ad was not sold through an ad network, as one might have expected. Instead, the ad space was sold directly by the Times, to a party who was pretending to be Vonage. The perpetrators ran Vonage ads for a while, then switched over to serving the malicious ads.

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Finnish Court Orders Re-Vote After E-Voting Snafu

September 10, 2009 by

The Supreme Administrative Court of Finland has ruled that three municipal elections, the first in Finland to use electronic voting, must be redone because of voting machine problems. (English summary; ruling in Finnish)

The troubles started with a usability problem, which caused 232 voters (about 2% of voters) to leave the voting booth without fully casting their ballots. Electronic Frontiers Finland explains what went wrong:

It seems that the system required the voter to insert a smart card to identify the voter, type in their selected candidate number, then press “ok”, check the candidate details on the screen, and then press “ok” again. Some voters did not press “ok” for the second time, but instead removed their smart card from the voting terminal prematurely, causing their ballots not to be cast.

This usability issue was exacerbated by Ministry of Justice instructions, which specifically said that in order to cancel the voting process, the user should click on “cancel” and after that, remove the smart card. Thus, some voters did not realise that their vote had not been registered.

If you want to see what this looks like for a voter, check out the online demo of the voting process, from the Finnish Ministry of Justice (in English).

Well designed voting systems tend to have a prominent, clearly labeled control or action that the voter uses to officially cast his or her vote. This might be a big red “CAST VOTE” button. The Finnish system mistakenly used the same “OK” button used previously in the process, making voter mistakes more likely. Adding to the problem, the voter’s smart card was protruding from the front of the machine, making it all too easy for a voter to grab the card and walk away.

No voting machine can stop a “fleeing voter” scenario, where a voter simply walks away during the voting process (we conventionally say “fleeing” even if the voter leaves by mistake), but some systems are much better than others in this respect. Diebold’s touchscreen voting machines, for all their faults, got this design element right, pulling the voter’s smart card all of the way into the machine and ejecting it only when the voter was supposed to leave — thus turning the voter’s desire to return the smart card into a countermeasure against premature voter departure, rather than a cause of it. (ATM machines often use this same trick of holding the card inside the machine to stop the user from grabbing the card and walking away at the wrong time.) Some older lever machines use an even simpler method against fleeing voters: the same big red handle that casts the ballot also opens the curtains so the voter can leave.

I’d be curious to know what the rules are about fleeing voters in Finland. I know that New Jersey procedures say that if a voter leaves without performing the final step of pushing the “Cast Vote” button, poll workers are supposed to push the button on the voter’s behalf (without looking at the voter’s choices). Crucially, the design of the New Jersey voting machine (for all its faults) makes it almost certain that such a non-cast ballot will be discovered promptly — the machine makes a noise when the ballot is cast, and the machine will complain if the poll worker tries to enable the next voter’s ballot before the previous voter’s ballot has been cast.

It seems likely that the Finnish machine, in addition to its usability problems that led to fleeing voters, had other design/process problems that made a non-completed ballot less noticeable to poll workers. (I don’t know this for sure; the answer isn’t in any English-language document I have seen.)

Fortunately, the damage was not as bad as it might have been, because the e-voting system was used in only three municipalities, as a pilot program, rather than nationwide. Presumably, nationwide use of the flawed system is now unlikely.

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Consolidation in E-Voting Market: ES&S Buys Premier

September 4, 2009 by

Yesterday Diebold sold its e-voting division, known as Premier Election Systems, to ES&S, one of Premier’s competitors. The price was low: about $5 million.

ES&S is reportedly the largest e-voting company, and Premier was the second-largest, so the deal represents a substantial consolidation in the market. The odds of one major e-voting company breaking from the pack and embracing up-to-date security engineering are now even slimmer than before. Premier had seemed like the company most likely to change its ways.

The sale represents the end of an embarrassing era for Diebold. The company must have had high hopes when it first bought a small e-voting company, but the new Diebold e-voting division never approached the parent companies standards for security and product quality. Over time the small e-voting division became an embarrassment, and the parent company distanced itself by renaming the division from Diebold to Premier and publicizing the division’s independence. Now Diebold is finally rid of its e-voting division and can return to doing what it does relatively well.

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