Increasing Trust in Tor Node List Using Blockchain

Tor is a low-latency free anonymization network based on onion routing. In Tor, directory servers maintain a list of all nodes. It is, however, possible for a powerful adversary (e.g., law enforcement agencies) to seize or compromise enough directory servers and thus forge that list. Therefore, clients that obtained such a forged list of nodes can be effectively deanonymized. As a countermeasure, we propose to utilize a permissioned blockchain with a single voting committee that is privately “elected” by a verifiable random function (VRF). Since the blockchain provides us with integrity guarantees by design, we increase trust in the directory servers by decentralizing management of Tor nodes present in the shared list. We apply skiplist as an optimization reducing a validation overhead of newly joined nodes and clients. The proposed approach has only a small performance impact on the existing Tor infrastructure.