HiddenTor: Toward a User-Centric and Private Query System for Tor BridgeDB

Tor bridges are crucial, unlisted relays designed to enhance system accessibility and circumvent censorship in the Tor network. Currently, Tor BridgeDB will randomly distribute 1-3 bridge relays to the user per request. Yet, those randomly selected bridges may not meet users' specific needs, e.g., adequate bandwidth for large-file sharing in a certain region. Also, a user's usage metadata (e.g., bridge choices) collected by Tor BridgeDB would inevitably reveal sensitive information about the user, discouraging the use of this censorship-circumvention service. In light of them, we introduce HiddenTor, a user-centric and privacy-focused bridge distribution system that allows Tor users to retrieve bridges privately and precisely (i.e., based on a range of specific criteria). At its core, HiddenTor designs a condition-based private information retrieval (PIR) protocol by building atop a suite of lightweight cryptographic primitives (i.e., function secret sharing). Besides, HiddenTor also crafts several optimization designs to balance the trade-offs between query efficiency and service reliability. The extensive experimental results have confirmed the feasibility and practicality of HiddenTor. For example, our prototype can efficiently handle private queries over 3000 bridges in approximately 2 seconds, which can further be reduced to 0.21 seconds using parallel computing techniques.