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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."

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benrothke writes "Narrating a compelling and interesting story about cryptography is not an easy endeavor. Many authors have tried and failed miserably; attempting to create better anecdotes about the adventure of Alice and Bob. David Kahn probably did the best job of it when wrote The Codebreakers: The story of secret writing in 1967 and set the gold standard on the information security narrative. Kahn's book was so provocative and groundbreaking that the US Government originally censored many parts of it. While Secret History: The Story of Cryptology is not as groundbreaking, it also has no government censorship. With that, the book is fascinating read that provides a combination of cryptographic history and the underlying mathematics behind it." Keep reading for the rest of Ben's review.

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