Defeating Skype Encryption Without a Key

06-27-2011 • SecurityWeek

Skype allows customers to communicate over Voice over Internet Protocol (VoIP) platforms. And because it is encrypted, Skype, which was recently purchased by Microsoft for $8.5 Billion, is used by many businesses today for their international phone calls. What researchers have found, however, is a novel way to decrypt those conversations without ever knowing the encryption key. Researchers Andrew M. White, Austin R. Matthews, Kevin Z. Snow, and Fabian Monrose from the Department of Computer Science and the Department of Linguistics at the University of North Carolina at Chapel Hill used an attack, dubbed “Phonotactic Reconstruction”, in their research paper, amusingly subtitled “Hookt on Fon-iks,” to predict clear text words from encrypted sequences. What they did was segment sequences of the VoIP packets into sub-sequences mapped into candidate words, then, based on rules of grammar, hypothesized these sub-sequences into whole sentences. In other words, they were able to reconstruc