Internet censorship is used in many parts of the world to prohibit free access to online information. Different techniques such as IP address or URL blocking, DNS hijacking, or deep packet inspection are used to block access to specific content on the Internet. In response, several censorship circumvention systems were proposed that attempt to bypass existing filters. Especially systems that hide the communication in different types of cover protocols attracted a lot of attention. However, recent research results suggest that this kind of covert traffic can be easily detected by censors. In this paper, we present SkypeLine, a censorship circumvention system that leverages Direct-Sequence Spread Spectrum (DSSS) based steganography to hide information in Voice-over-IP (VoIP) communication. SkypeLine introduces two novel modulation techniques that hide data by modulating information bits on the voice carrier signal using pseudo-random, orthogonal noise sequences and repeating the spreading operation several times. Our design goals focus on undetectability in presence of a strong adversary and improved data rates. As a result, the hiding is inconspicuous, does not alter the statistical characteristics of the carrier signal, and is robust against alterations of the transmitted packets. We demonstrate the performance of SkypeLine based on two simulation studies that cover the theoretical performance and robustness. Our measurements demonstrate that the data rates achieved with our techniques substantially exceed existing DSSS approaches. Furthermore, we prove the real-world applicability of the presented system with an exemplary prototype for Skype.
['Katharina Kohls', 'Thorsten Holz', 'Dorothea Kolossa', 'Christina Pöpper']
SkypeLine: Robust Hidden Data Transmission for VoIP