Censorship Circumvention: Staying Ahead in a Cat-and-Mouse Game

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• Nikita Borisov, University of Illinois at Urbana-Champaign
• Thursday 29 August 2013, 16:00-17:00
• Auditorium, Microsoft Research Ltd, 21 Station Road, Cambridge, CB1 2FB.

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The Internet enables access to a wide variety of information sources; many countries and organizations, however, try to restrict such access for political and social reasons. People whose access has been censored make use of a variety of circumvention technologies to find the information they need; in turn, the censors use increasingly sophisticated tools to render these technologies ineffective. One of the most powerful techniques available to the censors has been the insider attack, wherein the censor pretends to be a user of a system in order to learn secret information about its functions. For example, censors continually update a blacklist of IP addresses belonging to circumvention proxies. I will discuss some new techniques designed specifically resist this insider threat.

rBridge focuses on the distribution of proxy addresses to users. It tracks the reputation score of each user, representing the likelihood of this user revealing a proxy address to the censors, and uses an introduction mechanism to resist Sybil attacks. A particular challenge of rBridge is to preserve the privacy of its users by keeping the knowledge about which users know which proxies secret.

Cirripede is an alternate approach that seeks to eliminate the insider threat entirely. It uses redirection proxies that are activated by a special cryptographic signal, which can be generated using only public information but can only be recognized by the proxies. Instead of hiding the location of the proxies, Cirripede places them in highly connected ISPs, such that blocking Cirripede would result in high collateral damage.

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