Users can hide extra, non-chat messages during a call through his team's SkypeHide system.
Mazurczyk and his colleagues Maciej Karas and Krysztof 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 team found that the receiver simply ignores the secret-message data, but it can nevertheless be decoded at the other end.
Mazurczyk said that the secret data is indistinguishable from silence-period traffic, so detection of SkypeHide is very difficult.
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.