TitleSafely Measuring Tor
Publication TypeConference Paper
Year of Publication2016
AuthorsJansen, R., and A. Johnson
Conference Name23rd ACM Conference on Computer and Communications Security
Date Published10/2016

Tor is a popular network for anonymous communication.
The usage and operation of Tor is not well-understood, however,
because its privacy goals make common measurement
approaches ineffective or risky. We present PrivCount, a system
for measuring the Tor network designed with user privacy
as a primary goal. PrivCount securely aggregates measurements
across Tor relays and over time to produce differentially
private outputs. PrivCount improves on prior approaches
by enabling flexible exploration of many diverse
kinds of Tor measurements while maintaining accuracy and
privacy for each. We use PrivCount to perform a measurement
study of Tor of sufficient breadth and depth to inform
accurate models of Tor users and traffic. Our results indicate
that Tor has 710,000 users connected but only 550,000
active at a given time, that Web traffic now constitutes 91%
of data bytes on Tor, and that the strictness of relays’ connection
policies significantly affects the type of application data
they forward.

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