MIT Research: Encryption Less Secure Than We Thought

A group of researchers from MIT and the University of Ireland has presented a paper (PDF) showing that one of the most important assumptions behind cryptographic security is wrong. As a result, certain encryption-breaking methods will work better than previously thought. "The problem, Médard explains, is that information-theoretic analyses of secure systems have generally used the wrong notion of entropy. They relied on so-called Shannon entropy, named after the founder of information theory, Claude Shannon, who taught at MIT from 1956 to 1978. Shannon entropy is based on the average probability that a given string of bits will occur in a particular type of digital file. In a general-purpose communications system, that’s the right type of entropy to use, because the characteristics of the data traffic will quickly converge to the statistical averages. ... But in cryptography, the real concern isn't with the average case but with the worst case. A codebreaker needs only one reliable correlation between the encrypted and unencrypted versions of a file in order to begin to deduce further correlations. ... In the years since Shannon’s paper, information theorists have developed other notions of entropy, some of which give greater weight to improbable outcomes. Those, it turns out, offer a more accurate picture of the problem of codebreaking. When Médard, Duffy and their students used these alternate measures of entropy, they found that slight deviations from perfect uniformity in source files, which seemed trivial in the light of Shannon entropy, suddenly loomed much larger. The upshot is that a computer turned loose to simply guess correlations between the encrypted and unencrypted versions of a file would make headway much faster than previously expected. 'It’s still exponentially hard, but it’s exponentially easier than we thought,' Duffy says."

Comments

Re:Huh?

[Trepidity]

As usual, the paper makes more sense than the press release, but is less grandiose in its claims.

It's a fairly technical result that finds some appeals to the asymptotic equipartition property lead to too-strong claims, compared to a more precise analysis.

Re:good news for NSA

[Shaiku]

I read the article. The impression I got was that it will still take the same time today that it would have taken yesterday to break encryption, but it turns out that the metric used to demonstrate an algorithm's effectiveness at hiding information was inadequate for electronic communication. In a nutshell, the latest math explains that most encryption systems are vulnerable to side-channel attacks, even if you might not have realized it. But side-channel attacks have been employed for a long time, so those who do security already knew this anecdotally.