For your eyes only (Image: Patrick Sinkel/AP/PA)

Got a secret message to send? Say it with silence. A new technique can embed secret data during a phone call on Skype. “There are concerns that Skype calls can be intercepted and analysed,” says Wojciech Mazurczyk at the Institute of Telecommunications in Warsaw, Poland. So his team’s SkypeHide system lets users hide extra, non-chat messages during a call.

Mazurczyk and his colleagues Maciej Karaś and Krzysztof Szczypiorski analysed Skype data traffic during calls and discovered an opportunity in the way Skype “transmits” silence. Rather than send no data between spoken words, Skype sends 70-bit-long data packets instead of the 130-bit ones that carry speech.

The team hijacks these silence packets, injecting encrypted message data into some of them. The Skype receiver simply ignores the secret-message data, but it can nevertheless be decoded at the other end, the team has found. “The secret data is indistinguishable from silence-period traffic, so detection of SkypeHide is very difficult,” says Mazurczyk. They found they could transmit secret text, audio or video during Skype calls at a rate of almost 1 kilobit per second alongside phone calls.

The team aims to present SkypeHide at a steganography conference in Montpellier, France, in June. They have already published their paper online.