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.

A bit of explanation may be in order. Digital audio files, such as those used by Skype, use tens of thousands of strings of characters called to represent every second of sound. When Skype detects silence on either end of a conversation, it uses a distinctly different, shorter set of characters, which are discarded on the receiving end of the conversation, and the listener hears nothing.

The Institute of Telecommunications researchers developed an app called SkypeHide which sends bits of encrypted information–text, images, even audio–in these otherwise useless character strings. “The secret data is indistinguishable from silence-period traffic, so detection of SkypeHide is very difficult,” said Wojciech Mazurczyk, one of the developers.

SkypeHide isn’t yet commercially available, but will be presented at a steganography conference in Paris in June.