Modern Censorship: Blocking Access to the Tor Network

Censorship has become a matter of fact in various places all around the world. Freedom of speech therefor has been replaced by governmentally ordered restrictions referring to the availability of different kinds of information, such as news websites or social media like Facebook or Twitter. This fact became especially obvious during the revolutions in Africa and the Middle East during 2012 where affected residents were not able to communicate with the rest of the world. One method to circumvent censorship is to use The Onion Router (Tor), a low-latency anonymity network that also addresses this problem directly. As techniques used to circumvent censorship develop, the methods used to detect and block those techniques also enhance. Therefore this book describes the methods used to detect (and block) Tor. Additionally, we focus on the remaining problem of timing attacks against Tor and introduce our implementation to harden the problem of disclosing anonymity by adversaries that control wide parts of a network, such as governments. Our implementation can be run on Tor nodes to generate additional (random) latencies when forwarding packets. Combining our implementation with other methods described in this book results in a lower probability of successfully detaining any freedom of information.