An increasing number of countries implement Internet censorship at different levels and for a variety of reasons. Consequently, there is an ongoing arms race where censorship resistance schemes (CRS) seek to enable unfettered user access to Internet resources while censors come up with new ways to restrict access. In particular, the link between the censored client and entry point to the CRS has been a censorship flash point, and consequently the focus of circumvention tools. To foster interoperability and speed up development, Tor introduced Pluggable Transports — a framework to flexibly implement schemes that transform traffic flows between Tor client and the bridge such that a censor fails to block them. Dozens of tools and proposals for pluggable transports have emerged over the last few years, each addressing specific censorship scenarios. As a result, the area has become too complex to discern a big picture.
Our recent report takes away some of this complexity by presenting a model of censor capabilities and an evaluation stack that presents a layered approach to evaluate pluggable transports. We survey 34 existing pluggable transports and highlight their inflexibility to lend themselves to feature sharability for broader defense coverage. This evaluation has led to a new design for Pluggable Transports – the Tweakable Transport: a tool for efficiently building and evaluating a wide range of Pluggable Transports so as to increase the difficulty and cost of reliably censoring the communication channel.
Continue reading Systemization of Pluggable Transports for Censorship Resistance