Detecting Censor Detection

Abstract: Our goal is to empirically discover how censors react to the introduction of new proxy servers that can be used to circumvent their information controls. We examine a specific case, that of obfuscated Tor bridges, and conduct experiments designed to discover how long it takes censors to block them (if they do block at all). Through a year's worth of active measurements from China, Iran, Kazakhstan, and other countries, we learn when bridges become blocked. In China we found the most interesting behavior, including long and varying delays before blocking, frequent failures during which blocked bridges became reachable, and an advancement in blocking technique midway through the experiment. Throughout, we observed surprising behavior by censors, not in accordance with what we would have predicted, calling into question our assumptions and suggesting potential untapped avenues for circumvention.