Measuring Tor Relay Popularity
Author Posting © ICST Institute for Computer Sciences, Social Informatics and Telecommunications Engineering, 2019. This is the author's version of the work. It is posted here by permission of the ICST Institute for Computer Sciences, Social Informatics and Telecommunications Engineering for personal use, not for redistribution. The definitive version was published in SecureComm 2019: Security and Privacy in Communication Networks, Volume 304, December 2019.
Tor is one of the most popular anonymity networks. It has been reported that over 2 million unique users utilize the Tor network daily. The Tor network is run by over 6, 000 volunteer relays. Each Tor client telescopically builds a circuit by choosing three Tor relays and then uses that circuit to connect to a server. The Tor relay selection algorithm makes sure that no two relays with the same /16 IP address are chosen. Our objective is to determine the popularity of Tor relays when building circuits. With over 44 vantage points (machines running Tor clients) and over 145,000 circuits built, we found that some Tor relays are chosen more often than others. Although a completely balanced selection algorithm is not possible, analysis of our dataset shows that some Tor relays are over 3 times more likely to be chosen than others. An adversary could potentially eavesdrop or correlate more Tor traffic.