Using Blockchains for Censorship-Resistant Bootstrapping in Anonymity Networks

With Tor being a popular anonymity network, many censors and ISPs have blocked access to it. Tor relies on privately and selectively distributing IPs of circumvention proxies (i.e., bridges) to censored clients for censorship evasion. However, existing distributors are still vulnerable to blocking or compromising anonymity. This paper introduces Antiblok, a new and practical channel for bridge distribution leveraging blockchain, a globally decentralized environment. A key insight of Antiblok is that all blockchain transactions are under pseudonymous identities, allowing requesting clients to fetch bridge information while maintaining anonymity, regardless of the trustworthiness of blockchain nodes. To prevent the use of off-chain communication channels, we present an account sharing protocol based on DH key exchange. The unblockability of Antiblok depends on the economic consequences of blocking the Ethereum system. We show that Antiblok effectively thwarts client-side blocking of the distribution channel for Tor bridges, and we describe the security of our design.