Changing of the Guards: A Framework for Understanding and Improving Entry Guard Selection in Tor Tariq Elahi†, Kevin Bauer†, Mashael AlSabah†, Roger Dingledine‡, Ian Goldberg† †University of Waterloo ‡The Tor Project, Inc. †{mtelahi,k4bauer,malsabah,iang}@cs.uwaterloo.ca ‡
[email protected] ABSTRACT parties with anonymity from their communication partners as well Tor is the most popular low-latency anonymity overlay network as from passive third parties observing the network. This is done for the Internet, protecting the privacy of hundreds of thousands by distributing trust over a series of Tor routers, which the network of people every day. To ensure a high level of security against cer- clients select to build paths to their Internet destinations. tain attacks, Tor currently utilizes special nodes called entry guards If the adversary can anticipate or compel clients to choose com- as each client’s long-term entry point into the anonymity network. promised routers then clients can lose their anonymity. Indeed, While the use of entry guards provides clear and well-studied secu- the client router selection protocol is a key ingredient in main- rity benefits, it is unclear how well the current entry guard design taining the anonymity properties that Tor provides and needs to achieves its security goals in practice. be secure against adversarial manipulation and leak no information We design and implement Changing of the Guards (COGS), a about clients’ selected routers. simulation-based research framework to study Tor’s entry guard de- When the Tor network was first launched in 2003, clients se- sign. Using COGS, we empirically demonstrate that natural, short- lected routers uniformly at random—an ideal scheme that provides term entry guard churn and explicit time-based entry guard rotation the highest amount of path entropy and thus the least amount of contribute to clients using more entry guards than they should, and information to the adversary.