There’s been some concern lately of Skype communications being monitored without users’ consent , after the Microsoft-owned service began allowing law enforcement agencies to surveil text chats. Fueled by these privacy concerns, a group of hackers at Warsaw, Poland’s Institute of Telecommunications have come up with an ingenious sub rosa method of communication on the service: hiding messages inside bits of digital silence.
Mazurczyk and his colleagues Maciej Karaś and Krzysztof Szczypiorski analyzed 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.