Enemy At the Gateways: Censorship-Resilient Proxy Distribution Using Game Theory

Abstract

A core technique used by popular proxy-based circumvention systems like Tor is to privately and selectively distribute the IP addresses of circumvention proxies among censored clients to keep them unknown to the censors. In Tor, for instance, such privately shared proxies are known as bridges. A key challenge to this mechanism is the insider attack problem: censoring agents can impersonate benign censored clients in order to learn (and then block) the privately shared circumvention proxies. To minimize the risks of the insider attack threat, in-thewild circumvention systems like Tor use various proxy assignment mechanisms in order to minimize the risk of proxy enumeration by the censors, while providing access to a large fraction of censored clients. Unfortunately, existing proxy assignment mechanisms (like the one used by Tor) are based on ad hoc heuristics that offer no theoretical guarantees and are easily evaded in practice. In this paper, we take a systematic approach to the problem of proxy distribution in circumvention systems by establishing a gametheoretic framework. We model the proxy assignment problem as a game between circumvention system operators and the censors, and use game theory to derive the optimal strategies of each of the parties. Using our framework, we derive the best (optimal) proxy assignment mechanism of a circumvention system like Tor in the presence of the strongest censorship adversary who takes her best censorship actions. We perform extensive simulations to evaluate our optimal proxy assignment algorithm under various adversarial and network settings. We show that the algorithm has superior performance compared to the state of the art, i.e., provides stronger resistance to censorship even against the strongest censorship adversary. Our study establishes a generic framework for optimal proxy assignment that can be applied to various types of circumvention systems and under various threat models. We conclude with lessons and recommendations for the design of proxy-based circumvention systems.

Lox: Protecting the Social Graph in Bridge Distribution

“…The bridge distribution problem, in its simplest construction, focuses on distributing viable network bridges to honest users in the presence of a censor that aspires to prevent bridge use. As explored in previous work [13,27,30,37], designing and implementing such a system for practical use requires an understanding of how censors operate in different regions. This includes the resources at a censor's disposal, the tools they are known to deploy and the known dangers to individuals who are discovered using censorship circumvention tools.…”

“…Nasr et al [30] take a game-theoretic approach to the bridge distribution problem, finding that a bootstrapping period, during which only trusted users are invited to use the system, is essential to counter the optimal censor strategy. In line with this finding, the systems we discussed above already involve restricting access to trusted groups of users or else verifying their accounts prior to registration [13,27,37].…”

“…The ability for censors to create sock-puppet accounts to enumerate and block bridges is a well-known problem noted in prior work [13,27,30,37]. Unlike prior work that limits bridge distribution to trusted users [27,37] or relies on a third party to limit duplicate accounts [13] at the expense of protecting the social graph, we provide an open-entry avenue to join Lox with fewer protections.…”

MassBrowser: Unblocking the Censored Web for the Masses, by the Masses

“…Nonetheless, our Operator can deploy standard Sybil protection mechanisms against such an expensive DoS attack. We have particularly implemented the Sybil protection mechanism of Nasr et al [48], which uses a strategic reputation system to pair clients and proxies.…”

Blind matching algorithm based proxy distribution against internet censorship

“…Existing literature tries to solve the proxy distribution issue in the context of proxy based censorship circumvention systems by leveraging either heuristic-based methods [10][11][12][13][14] or gametheoretical approaches [15]. Additionally, several advanced approaches are provided in recent research to solve such kind of generic resources assignment problems in the wide field of networking and communications, such as clustering [16] and coalition formation [17].…”