Unobservable Covert Streaming for Internet Censorship Circumvention

This work studies the possibility of using the encrypted video channel of widely used video-conferencing applications, such as Skype, as a carrier for an unobservable covert channel that can be used to access arbitrary information on the Internet. Such channel can be used by citizens living in countries ruled by repressive regimes to exert their civil rights without being detected and/or blocked by a censor. Even if the video is encrypted, a covert channel may still be detected if it induces statistically relevant changes in the traffic pattern of the application. We propose and evaluate different alternatives to encode information in the video-stream, in order to maximize the available throughput while preserving the characteristics of the unmodified stream. We have implemented a prototype of our system, named DeltaShaper, that offers a data-link interface and that can support any protocol that runs over TCP/IP. Our results show that it is possible to achieve a throughput of 0.4 kB/s with no significant impact on the stream, which allows to run standard applications such as FTP, SMTP or a simple web client.