It may sound a little tough to believe, but Voice over IP phone protocol has become an effective medium to send hidden messages. According to research done in Poland, there are “many ways” to hide messages in the data stream of VoIP calls.

Sending hidden messages, or steganography, isn’t new; methods include hiding messages in wax tablets in ancient Greece, or more recently, invisible ink and microdots. Modern steganography makes use of digitally transmitted images and sound files to hide messages.

More than “simply scrambling messages,” steganography won’t allow anyone to listen in. Hidden messages may only show as subtle changes to images, the content of spam messages or a little extra noise in voice over IP communications, which makes the messages extremely hard to detect. Software to unbundle and decipher the message is required even if it is detected.

This form of steganography—hidden messages within sound files—isn’t new, either.

Security researchers Wojciech Mazurczyk and Krzysztof Szczypiorski at Warsaw University of Technology, Poland, have worked to explain and demonstrate steganography in VoIP.

They developed two techniques, based on VoIP systems’ redundancy and packet loss. The first takes advantage of unused fields in real-time control protocol and real-time transport protocol to pass the messages. The second hides messages using delayed audio packets. There are other possibilities detailed in the report, as well.

During experimentation, they sent more than 1.3 megabits during a nine-minute VoIP call, and the protocol method proved quicker.

VoIP is already frustrating law enforcement, since they have been unable to decipher messages from suspected terrorists using Skype, purely based on Skype’s own cryptography. Police in Germany may be developing software to intercept Skype calls.