CORDON—A Taxonomy of Internet Censorship Resistance Strategies

We present a taxonomy of Internet censorship resistance strategies and techniques extracted from analyzing proposed and implemented systems. We categorize the strategies into the following six types to form the CORDON taxonomy: Collateral damage, where the damage caused by censorship would outweigh its benefits; Outside scope of influence, where the censor is powerless to act due to it having no control over entities or traffic; Rate limiting, where the censor’s monitoring abilities are curtailed; Decoupled communication, where bidirectional communications are asynchronous and asymmetric to take advantage of weaknesses in the censor’s defences or hide the fact that the communications are related; Overwhelm, where the censor is deluged with large amounts of network traffic, paths and vectors to increase the cost and difficulty of effective censorship; and No target, where the censor is unable to accurately detect and identify the people, infrastructure and network traffic to target. For each strategy, we identify the common supporting techniques used or proposed by resistance systems. We provide a detailed discussion of censors and their limitations, and outline the censor’s decision-making process based on its utility, resources, and capabilities. We identify the censor’s attack surfaces to provide context tying the censor capabilities and the resistance techniques together. We further identify future work needed to solve the fundamental problems facing all systems today: rendezvous protocols, bootstrapping without the need for client software, transports that stay ahead of the censor capabilities, and systems that scale better than the current batch. Applying CORDON to censorship resistance research will provide researchers a better understanding of the techniques and interactions that will help produce more effective and reliable censorship resistance solutions. Keywords-taxonomy; censorship resistance; censorship circumvention; systematization of knowledge