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Linguists Use Sounds to Bypass Skype Crypto

Decryption is difficult and computationally expensive. So what if, instead of decrypting the content of a message, you found a correlation between the encrypted data and its meaning – without having to crack the code itself?

Such an approach has been demonstrated by a group of University of North Carolina linguists working with computer scientists on encrypted Skype calls. While their research paper only managed to partially recover conversations, an encryption scheme that leaks even some of the data it’s meant to protect is no longer secure.

It works like this: spoken English has a set of known – and quite settled – rules for its phonetic grammar.

For non-linguists, this means the order in which we can and cannot put different sounds together. The “ds” sound, or phoneme, at the end of sounds is fairly common at the end of English words, but doesn’t occur at the beginning.

Systems like speech-to-text converters use these rules to break strings of sounds into individual words; they match sounds against a dictionary of legal phoneme combinations and map these into words. What the researchers discovered is that encryption leaves a pattern that can be subjected to this kind of analysis – without decrypting the data.

When you encode spoken English for VoIP using (in the case of Skype) CELP (code excited linear projection), you will end up with patterns in the data that match the patterns in the sounds. In particular, those patterns end up being reflected in the size of the data frame: the more complex the sound that’s being encoded, the larger the frame, resulting in a correlation between frame size and the original sounds spoken.

When the data created by CELP is encrypted, it retains the original frame size – and that means that even encrypted Skype data will retain the correlation between the size of the data frame and the original phonemes.

The technique gets another helping hand: at least some of the time, boundaries between sounds correspond to sudden changes in frame size, hinting at the difference between “Han Solo” and “Hans Solo”.

The researchers mapped the size of encrypted data frames in the Skype stream back to likely patterns of phonemes, and used that mapping – which they called “Phonetic Reconstruction” – to reconstruct the call, without decrypting the data.

So how well does it work? Not so well that we should all abandon Skype tomorrow. However, the researchers noted that if an encryption scheme is to be considered secure, “no reconstruction, even a partial one, should be possible; indeed, any cryptographic system that leaked as much information as shown here would immediately be deemed insecure.”

Bigger phoneme-word dictionaries (covering more dialects and languages) and faster processing would improve the accuracy of this kind of analysis.

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Cambridge boffins rebuff banking industry take down request

Computer scientists from Cambridge University have rebuffed attempts by a banking association to persuade them to take down a thesis covering the shortcomings of Chip-and-PIN as a payment verification method.

Omar Choudary’s masters thesis contains too much information about how it might be possible to fool a retailing terminal into thinking a PIN authorising a purchase had been entered, as far as the bankers are concerned. Noted cryptographer and banking security expert Professor Ross Anderson gives short shrift to the argument that publishing the research exceeds the bounds of responsible disclosure, politely but firmly telling the UK Cards Association that the research was already in the public domain and that Choudary’s work would stay online.

Anderson is one of Choudary’s supervisors in the latter’s research.

Choudary’s research on so-called NO-PIN attacks builds on work by Steven Murdoch, Saar Drimer and Anderson that was disclosed to the banking industry last year and published back in February.

Chip-and-PIN is used throughout Europe and in Canada as a method to authorise credit and debit card payments. The attack unearthed by the Cambridge researchers creates a means to trick a card into thinking a chip-and-signature transaction is taking place while the terminal thinks it’s authorised by chip-and-PIN. The flaw creates a means to make transactions that are “Verified by PIN” using a stolen (uncancelled) card without knowing the PIN code. The ruse works by installing a wedge between the card and terminal.

The same approach cannot be applied to make ATM transactions.

In the months since the potential loophole was uncovered only Barclays Bank has responded by modifying its technology to block the potential scam, Anderson reports.

Choudary is one of the authors of an upcoming paper on Chip-and-PIN security, due to be unveiled at the Financial Cryptography 2011 conference in February