A team of researchers at Warsaw University of Technology released a trio of white papers that explain how it's possible to use steganography to hide communication within VoIP and SIP protocols and to establish hidden routes for such communication to travel using common network topology.
In their recently released white papers, Krzysztof Szczypiorski, Igor Margasinski, and Wojciech Mazurczyk explain how their techniques hide communications in various layers of VoIP traffic by taking advantage of unused or rarely used data fields. The team also describe how to take advantage of agent-based network environments to develop steganographic routers. Typical agent-based environments include various peer-to-peer technologies as well as network services, such as IDS systems, installed on numerous systems that communicate with one another. Steganographic routing involves building vector-based routing tables on a series of agents that randomly discover other agents that know how to handle the covert communications.
While the overall idea of using steganography to implement covert communications isn't new, the team's overall approach represents new techniques that haven't been discussed extensively in public before.
The white papers are available at Cornell University Library in PDF format.