Troll Patrol: Anonymous User Reporting of Bridge Censorship

Active scanning of bridges for censorship verification must be minimized because each scan independently risks exposing the scanning infrastructure to the censor, with discovery probability modeled as a monotonically increasing function of scan volume. Troll Patrol reduces required scans by limiting active verification to only bridges for which user-submitted negative reports have been received, and revokes reporting privileges from users who accumulate too many false reports to deter scan-flooding attacks.

§2.2, §7 defense active-probingip-blocking generic

Bridge usage statistics fail to detect censorship for low-use bridges: in a Belarus 2020–2021 case study, email-distributed obfs4 bridges that were actively blocked regularly received 0 connections from Belarus even on unblocked days, making censorship-vs-inactivity indistinguishable. A censor can also artificially inflate counts by making its own connections from multiple IP addresses, so high connection counts do not imply absence of censorship.

§3, Appendix A evaluation ip-blockingdpi by

Positive user reports (reporting successful connections) cannot reliably indicate whether a bridge is accessible: censors possess all technical capabilities of regular users and can submit fabricated positive reports after blocking a bridge. Restricting positive reports to high-reputation users would exclude most honest users early in a bridge's lifetime, when each Tor Lox bridge is initially distributed to only 10 users globally.

§4 defense ip-blocking generic

Troll Patrol's anonymous negative-reporting protocols, implemented as an extension to the Tor Project's Lox system, impose only a small overhead over existing Lox protocols: Report Submit adds a 1,300-byte request (vs. 877 bytes for Check Blockage) with a 2.72 ms client computation time; existing protocols increase by approximately 100–200 bytes per request due to two new credential attributes. All new protocol timings are sub-5 ms on the client side.

§6, Table 3 defense generic

Troll Patrol uses zero-knowledge proofs over anonymous credential attributes to track per-user false report counts (f) and pending session state (p) without linking reports across sessions or revealing user identity. A user proves in zero knowledge that f plus any new false reports does not exceed threshold m before submitting; users who exceed m lose reporting ability without any identity disclosure or impact on their reputation within the broader bridge distribution system.

§5.3, §5.4 defense generic