rBridge: User Reputation based Tor Bridge Distribution with Privacy Preservation

In simulated event-driven (crisis) blocking where all corrupt users simultaneously block bridges on day 300, available bridges drop from ~500 to ~150 and thirsty users spike to 25%; maintaining 50 reserve bridges (~10% of deployed stock) halves the thirsty-user count, and 100 reserve bridges nearly eliminates thirstiness among users who had accumulated sufficient credits.

§4.3.2 evaluation generic

Knowing a user's bridge assignment narrows the adversary's anonymity set to the small group sharing that bridge, deanonymizing Tor users even when the bridge itself is not compromised; rBridge addresses this using 1-out-of-m Oblivious Transfer, Pedersen commitments, and non-interactive zero-knowledge proofs so the bridge distributor learns nothing about which bridges a user holds.

§5, §5.1 defense generic

China's GFW was able to enumerate all Tor bridges distributed via IP address or Gmail account in under a month, demonstrating that standard small-subset distribution strategies are insufficient against a state-level adversary controlling large numbers of accounts and Sybils.

§2.1 detection ip-blocking cn

rBridge tolerates up to ~30% malicious users with acceptable bridge protection, but fails at f≥50%; with f=5% under aggressive blocking, over 95% of users are never bridge-starved and ~50% of bridges are never blocked, while conservative blocking (corrupt users waiting 225 days before acting) causes ~10% of users to be thirsty 15% of the time because delayed blockers accumulate enough credits to inject additional malicious invitees.

§4.3.2 evaluation generic

rBridge outperforms Proximax by at least one order of magnitude across all robustness metrics under aggressive blocking with 5% malicious users: to support 200 users for 30 days, Proximax requires at least 2400 bridges while rBridge needs only 108, and in Proximax fewer than 5% of bridges produce more than 20 user-hours versus 99% in rBridge.

§4.3.3 evaluation generic