Enhancing Censorship Resistance in the Tor Anonymity Network

The Tor network was originally designed as low-latency anonymity network. However, over time Tor earned a reputation as also being a useful tool to circumvent Internet censorship—at times, the network counted 30,000 users only from China. Censors reacted by tightening their grip on the national communication infrastructure. In particular, they developed different techniques to prevent people from being able to access the Tor network. This arms race now counts several iterations and no end is in sight. This thesis contributes to a censorship-resistant Tor network in two ways. First, it analyses how existing censorship systems work. In particular, the Great Firewall of China is probed in order to obtain a detailed understanding of its capabilities as well as unexplored circumvention opportunities. Second, this thesis proposes practical countermeasures to circumvent Internet censorship. It discusses a novel network protocol which is resistant to the Great Firewall’s active probing attacks. Some of the concepts and results of this thesis led to the creation of software prototypes. All the code is available under a free license. By developing and deploying software, we are not just limited to a theoretical understanding of censorship systems. Rather, we can gain valuable practical experience in the rapidly progressing arms race that is Internet censorship.