Understanding Tor Usage with Privacy-Preserving Measurement
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The Tor anonymity network is difficult to measure because, if not done carefully, measurements could risk the privacy (and potentially the safety) of the network's users. Recent work has proposed the use of differential privacy and secure aggregation techniques to safely measure Tor, and preliminary proof-of-concept prototype tools have been developed in order to demonstrate the utility of these techniques. In this work, we significantly enhance two such tools--PrivCount and Private Set-Union Cardinality--in order to support the safe exploration of new types of Tor usage behavior that have never before been measured. Using the enhanced tools, we conduct a detailed measurement study of Tor covering three major aspects of Tor usage: how many users connect to Tor and from where do they connect, with which destinations do users most frequently communicate, and how many onion services exist and how are they used. Our findings include that Tor has 8 million daily users (a factor of four more than previously believed) while Tor user IPs turn over almost twice in a 4 day period. We also find that 40 torproject.org domain name, 10 and 80 million sites list. Finally, we find that 90 are invalid, and more than 90
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