Over the past few years, the Internet has become a powerful means for the masses to interact, coordinate activities, and gather and disseminate information. As such, it is increasingly relevant for many governments worldwide to surveil and censor it, and many censorship programs have been put in place in the last years. Due to lack of publicly available information, as well as the inherent risk...

A core technique used by popular proxy-based circumvention systems like Tor, Psiphon, and Lantern is to secretly share the IP addresses of circumvention proxies with the censored clients for them to be able to use such systems. For instance, such secretly shared proxies are known as bridges in Tor. However, a key challenge to this mechanism is the insider attack problem: censoring agents can im...

The Tor Network is a low-latency anonymity, privacy, and censorship resistance network whose servers are run by volunteers around the Internet. This distribution of trust creates resilience in the face of compromise and censorship; but it also creates performance, security, and usability issues. The TorFlow suite attempts to address this by providing a library and associated tools for measuring...

“Censorship resistant” systems attempt to prevent censors from imposing a particular distribution of content across a system. In this paper, we introduce a variation of censorship resistance (CR) that is resistant to selective filtering even by a censor who is able to inspect (but not alter) the internal contents and computations of each data server, excluding only the server’s private signatur...

Tor has grown beyond its original purpose as an anonymity tool and has become a widely-used cen­ sorship circumvention tool. This is the first study to ex­ amine Tor’s usability in this role. We evaluate, design, and test the Tor configuration interface by placing users in simulated censorship environments, instructing them to use Tor to circumvent censorship, and measuring their interactions w...

Motivated by recent interest in black holes whose asymptotic geometry approaches that of anti-de Sitter spacetime, we give a proof of topological censorship applicable to spacetimes with such asymptotic behavior. Employing a useful rephrasing of topological censorship as a property of homotopies of arbitrary loops, we then explore the consequences of topological censorship for horizon topology ...

Due to the complexity of Einstein's equations, it is often natural to study a question of interest in the framework of a restricted class of solutions. One way to impose a restriction is to consider solutions satisfying a given symmetry condition. There are many possible choices, but the present article is concerned with one particular choice, which we shall refer to as Gowdy symmetry. We begin...

One challenge in detecting online censorship is the need for vantage points within the censoring domains. We focus on the specific subproblem of DNS blacklisting, where servers in a particular administrative domain are instructed not to resolve requests for specific sites. We find that for this problem internal vantage points are not needed, since public DNS servers in a given domain can be dir...