Effective Tor Bridge Distribution
Philipp Winter · 2012
Tor bridges are undocumented relays which are supposed to help censored users access the Tor network when all public relays are blocked. They serve as a stepping stone into the public network. Their effectiveness relies on them not leaking to censoring countries. If this happens, they get blocked and become useless in this particular country. Bridges are distributed over HTTPS, e-mail and (physical) social networks. It is reported that at least China was able to get an exceeding amount of bridge descriptors distributed over HTTPS and e-mail [2]. Distribution over social networks is more robust but less effective. Basically, the problem of bridge distribution boils down to a secret which has to reach censored users but must not reach censors. This is impossible by definition since censors can always act like legitimate users and, worse, can launch sybil attacks.