Guard Sets for Onion Routing

“Entry” guards protect the Tor onion routing system from variants of the “predecessor” attack, that would allow an adversary with control of a fraction of routers to eventually de-anonymize some users. Research has however shown the three guard scheme has drawbacks and Dingledine et al. proposed in 2014 for each user to have a single long-term guard. We first show that such a guard selection strategy would be optimal if the Tor network was failure-free and static. However under realistic failure conditions the one guard proposal still suffers from the classic fingerprinting attacks, uniquely identifying users. Furthermore, under dynamic network conditions using single guards offer smaller anonymity sets to users of fresh guards. We propose and analyze an alternative guard selection scheme by way of grouping guards together to form shared guard sets. We compare the security and performance of guard sets with the three guard scheme and the one guard proposal. We show guard sets do provide increased resistance to a number of attacks, while foreseeing no significant degradation in performance or bandwidth utilization. DOI 10.1515/popets-2015-0017 Received 2015-02-15; revised 2015-05-13; accepted 2015-05-15.