Skype has caused all sorts of headaches for governments the world over because it enabled a form of voice and text chatting that was hard for traditional listening services to tap into—at least until recently. And now Polish researchers have discovered a new and more tricky way to use the net-based VoIP system to transmit secret messages: Encoded invisibly into the silent gaps in an otherwise typical call.
The system relies on a feature of Skype whereby instead of simply sending no data when there's no sounds to transmit—such as between words or sentences—Skype simply sends shorter data packets (70 bits long instead of 130-bit packets for speech). The Polish team's system hijacks these short snippets of data and replaces the bits with different info that they choose. The Skype software ignores the "silent" packets, but a suitable system can still decode the silent messages. As a bonus the entire setup is hard to detect and is capable of sending up to 1 kilobit per second of secret data of any type, be it text or video.
This is a form of steganography, a secret messaging system that hides information in plain sight alongside a photo, a painting, or some other text. It's long used by spies (and a key plot device in the recent episode of the Sherlock Holmes TV show Elementary, as fans will remember).
Have you ever worried that someone could listen in to your Skype calls for nefarious purposes?