Wojciech Mazurczyk, Krysztof Szczypiorski and Maciej Karať have long been interested in steganography - the science of writing and transmitting secret messages within regular messages.
After learning that Skype transmits voice data in 130-byte packets and the silences in 70-byte packets, they came upon the idea of using the latter to conceal the sending and receiving of additional messages, Trusted Third Party reports (via Google Translate).
In order to do that, they created SkypeHide (or SkyDe), a piece of software that hijacks some of the "silence" packets and then embeds the encrypted messages into them. On the receiving computer, the same software extracts it and decrypts it.
The hidden messages can consist of text, audio or video, and can be transmitted at a rate of almost 1 kilobit per second. And, according to Mazurczyk, the secret data is indistinguishable from silence-period traffic.
More details about the software and how it works will be soon shared with the public, as the researchers are set to present the research at the 1st ACM Workshop on Information Hiding and Multimedia Security which is to be held in Montpellier, France, in June.