Users of the Tor anonymous communication system are at risk of being tracked by an adversary who can monitor both the traffic entering and leaving the network. This weakness is well known to the designers and currently there is no known practical way to resist such attacks, while maintaining the low-latency demanded by applications such as web browsing. For this reason, it seems intuitively clear that when selecting a path through the Tor network, it would be beneficial to select the nodes to be in different countries. Hopefully government-level adversaries will find it problematic to track cross-border connections as mutual legal assistance is slow, if it even works at all. Non-government adversaries might also find that their influence drops off at national boundaries too.

Implementing secure IP-based geolocation is hard, but even if it were possible, the technique might not help and could perhaps even harm security. The PET Award nominated paper, “Location Diversity in Anonymity Networks“, by Nick Feamster and Roger Dingledine showed that international Internet connections cross a comparatively small number of tier-1 ISPs. Thus, by forcing one or more of these companies to co-operate, a large proportion of connections through an anonymity network could be traced.

The results of Feamster and Dingledine’s paper suggest that it may be better to bounce anonymity traffic around within a country, because it is less likely that there will be a single ISP monitoring incoming and outgoing traffic to several nodes. However, this only appears to be the case because they used BGP data to build a map of Autonomous Systems (ASes), which roughly correspond to ISPs. Actually, inter-ISP traffic (especially in Europe) might travel through an Internet eXchange (IX), a fact not apparent from BGP data. Our paper, “Sampled Traffic Analysis by Internet-Exchange-Level Adversaries“, by Steven J. Murdoch and Piotr Zieliński, examines the consequences of this observation.

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