April 1, 2015

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Bitcoin and game theory: we’re still scratching the surface

In an earlier post I argued why Bitcoin’s stability is fundamentally a game-theoretic proposition, and ended with some questions:

Can we effectively model the system with all its interacting components in the language of strategies and payoff-maximization? Is the resulting model tractable — can we analyze it mathematically or using simulations? And most importantly, do its predictions match what we observe in practice?

Let’s look at those questions in the context of a “block withholding attack” between mining pools.

Recall that mining pools are groups of individual miners who pool their computing power as well as their rewards. Suppose two mining pools — let’s call them blue and red — are both seeking to maximize their mining rewards.  Let’s say the manager of the red pool decides to infiltrate the blue pool and decrease their efficiency using some of the mining power that red (directly or indirectly) controls. This can be done by submitting shares (partial proofs of work) to earn a share of rewards, but withholding any valid blocks which are found and therefore not contributing any productive work to the blue pool. At first sight this seems like cutting off your nose to spite your face — sure, blue’s efficiency will be hurt, but red is wasting hash power as well.

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Be wary of one-time pads and other crypto unicorns

Yesterday, a new messaging app called Zendo got some very favorable coverage from Tech Crunch. At the core of their sales pitch is the fact that they use one-time pads for encryption. With a few strong assumptions, namely that the pads are truly random and are only used once, it’s true that this scheme is “unbreakable” or more precisely that it offers information-theoretic guarantees that no eavesdropper can learn anything about the encrypted message. Zendo’s founder calls it a “crypto unicorn” and claims it is a game-changer in terms of security.

It isn’t. In this post I’ll explain why we don’t need (and shouldn’t want) to use one-time pads for a consumer secure-messaging app and why we should generally be wary of products like Zendo making grandiose claims about solving security problems through magic crypto. [Read more...]

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Why Your Netflix Traffic is Slow, and Why the Open Internet Order Won’t (Necessarily) Make It Faster

The FCC recently released the Open Internet Order, which has much to say about “net neutrality” whether (and in what circumstances) an Internet service provider is permitted to prioritize traffic. I’ll leave more detailed thoughts on the order itself to future posts; in this post, I would like to clarify what seems to be a fairly widespread misconception about the sources of Internet congestion, and why “net neutrality” has very little to do with the performance problems between Netflix and consumer ISPs such as Comcast.

Much of the popular media has led consumers to believe that the reason that certain Internet traffic—specifically, Netflix video streams—were experiencing poor performance because Internet service providers are explicitly slowing down Internet traffic. John Oliver accuses Comcast of intentionally slowing down Netflix traffic (an Oatmeal cartoon reiterates this claim). These caricatures are false, and they demonstrate a fundamental misunderstanding of how Internet connectivity works, what led to the congestion in the first place, and the economics of how the problems were ultimately resolved.
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Security flaw in New South Wales puts thousands of online votes at risk

Update Mar. 23 1:30 PM AEDT: Our response to the NSWEC’s response

New South Wales, Australia, is holding state elections this month, and they’re offering a new Internet voting system developed by e-voting vendor Scytl and the NSW Electoral Commission. The iVote system, which its creators describe as private, secure and verifiable, is predicted to see record turnout for online voting. Voting has been happening for six days, and already iVote has received more than 66,000 votes. Up to a quarter million voters (about 5% of the total) are expected to use the system by the time voting closes next Saturday.

Since we’ve both done extensive research on the design and analysis of Internet voting systems, we decided to perform an independent security review of iVote. We’ll prepare a more extensive technical report after the election, but we’re writing today to share news about critical vulnerabilities we found that have put tens of thousands of votes at risk. We discovered a major security hole allowing a man-in-the middle attacker to read and manipulate votes. We also believe there are ways to circumvent the verification mechanism.

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What should we do about re-identification? A precautionary approach to big data privacy

Computer science research on re-identification has repeatedly demonstrated that sensitive information can be inferred even from de-identified data in a wide variety of domains. This has posed a vexing problem for practitioners and policy makers. If the absence of “personally identifying information” cannot be relied on for privacy protection, what are the alternatives? Joanna Huey, Ed Felten, and I tackle this question in a new paper “A Precautionary Approach to Big Data Privacy”. Joanna presented the paper at the Computers, Privacy & Data Protection conference earlier this year.

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On compromising app developers to go after their users

In a recent article by Scahill and Begley, we learned that the CIA is interested in targeting Apple products. I largely agree with the quote from Steve Bellovin, that “spies gonna spy”, so of course they’re interested in targeting the platform that rides in the pockets of many of their intelligence collection targets. What could be a tastier platform for intelligence collection than a device with a microphone, cellular network connection, GPS, and a battery, which your targets willingly carry around in their pockets? Even better, your targets will spare you the trouble of recharging your spying device for you. Of course you target their iPhones! (And Androids. And Blackberries.)

To my mind, the real eyebrow raising moment was that the CIA is also allegedly targeting app developers through “whacking” Apple’s Xcode tool, presumably allowing all subsequent software shipped from the developer to the app store to contain some sort of malicious implant, which will then be distributed within that developer’s app. Nothing has been disclosed about how widespread these attacks are (if ever used at all), what developers might have been targeted, or how the implants might function.
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Threshold signatures for Bitcoin wallets are finally here

Today we are pleased to release our paper presenting a new ECDSA threshold signature scheme that is particularly well-suited for securing Bitcoin wallets. We teamed up with cryptographer Rosario Gennaro to build this scheme. Threshold signatures can be thought of as “stealth multi-signatures.” [Read more...]

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FREAK Attack: The Chickens of ‘90s Crypto Restriction Come Home to Roost

Today researchers disclosed a new security flaw in TLS/SSL, the protocol used to secure web connections. The flaw is significant in itself, but it is also a good example of what can go wrong when government asks to build weaknesses into security systems.

Back in the early 1990s, it was illegal to export most products from the U.S. if they had strong cryptography. To be exportable, a system had to use small keys that could be defeated by a brute-force search over the (reduced) key space. Because of this, the secure web protocol, SSL, was designed to allow either party to a communication to ask to use a special export mode. [Note for crypto geeks: “export mode” refers to certain cipher suites whose names start with “EXP”.] When it became legal to export strong crypto, the export mode feature was not removed from the protocol because some software still depended on it. Export mode is still an option today.

This creates the possibility that a network “man in the middle” (MITM) can downgrade the security of a connection. If Alice and Bob are setting up a connection, the MITM can tell Alice that Bob is asking for export mode, and vice versa. This kind of “downgrade attack” is well known, and the TLS/SSL protocol has features designed to detect it. In this case, for complicated reasons beyond the scope of this post, the anti-downgrade protections could be evaded by a clever MITM.

Having tricked Alice and Bob into using export mode, an adversary could then crack the 512-bit RSA keys used in this mode. Back in the ‘90s that would have required a heavy-duty computation, but today it takes about 7 hours on Amazon EC2 and costs about $100.

Many web sites are vulnerable to this attack, allowing an adversary in the network to spoof or spy on traffic to vulnerable sites. About 12% of popular sites appear to be vulnerable, including americanexpress.com, groupon.com, bloomberg.com, kohls.com, marriott.com, and usajobs.gov.

Even the National Security Agency’s own site is vulnerable. That’s not a big national security problem in itself because NSA doesn’t distribute state secrets from its public site. But there is an important lesson here about the consequences of crypto policy decisions: the NSA’s actions in the ‘90s to weaken exportable cryptography boomeranged on the agency, undermining the security of its own site twenty years later.

Next time you hear a government official ask to modify a security system to protect their own access to data, ask yourself: What are the side effects? How do we know we won’t regret this later?

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A clear line between offense and defense

The New York Times, in an editorial today entitled “Arms Control for a Cyberage“, writes,

The problem is that unlike conventional weapons, with cyberweapons “there’s no clear line between offense and defense,” as President Obama noted this month in an interview with Re/code, a technology news publication. Defense in cyberwarfare consists of pre-emptively locating the enemy’s weakness, which means getting into its networks.

This is simply wrong.
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We can de-anonymize programmers from coding style. What are the implications?

In a recent post, I talked about our paper showing how to identify anonymous programmers from their coding styles. We used a combination of lexical features (e.g., variable name choices), layout features (e.g., spacing), and syntactic features (i.e., grammatical structure of source code) to represent programmers’ coding styles. The previous post focused on the overall results and techniques we used. Today I’ll talk about applications and explain how source code authorship attribution can be used in software forensics, plagiarism detection, copyright or copyleft investigations, and other domains.

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