How to intercept Skype calls

Researchers have discovered a trick that allows encrypted VoIP (Voice over Internet Protocol) calls like Skype to be deciphered without the need to crack encryption.

The method, dubbed "Phonotactic Reconstruction", exploited the Linear Predictive Filter, a system used by Voice over Internet Protocol platforms to transmit conversations by creating data sets from spoken English.

The fragments of spoken words

Data sets were used to rebuild Skype conversations because they resemble the fragments of spoken words, or phonemes, on which they were modelled.

Linguists were helped furthermore because the scripts and file sizes of the data sets, even afterwards encryption, matched phonemes. Rules of spoken language mean that phonemes have a strict placement within words, which helped linguists reconstruct conversations.

The generalised performance is not as strong as we

"During the generalised performance is not as strong as we would have liked … one would hope that such recovery would not be at all possible since VoIP audio is encrypted precisely to prevent such breaches of privacy."

While Linear Predictive Filtering weakens cryptography because encrypted data bears a statistical relationship to the source data, it is used by VoIP systems because it provides the best type of compression.