Extensive Analysis and Large-Scale Empirical Evaluation of Tor Bridge Discovery
Tor is a well-known low-latency anonymous communication system that is able to bypass Internet censorship. However, publicly announced Tor routers are being blocked by various parties. To counter this blocking, Tor introduced nonpublic bridges as the first-hop relay into its core network. In this paper, the authors analyzed the effectiveness of two categories of bridge-discovery approaches: enumerating bridges from bridge https and email servers, and inferring bridges by malicious Tor middle routers. Large-scale experiments were conducted and validated their theoretic analysis. They discovered 2365 Tor bridges through the two enumeration approaches and 2369 bridges by only one Tor middle router in 14 days.