We introduce Torsk, a structured peer-to-peer low-latency anonymity protocol. Torsk is designed as an interoperable replacement for the relay selection and directory service of the popular Tor anonymity network, that decreases the bandwidth cost of relay selection and maintenance from quadratic to quasilinear while introducing no new attacks on the anonymity provided by Tor, and no additional delay to connections made via Tor. The resulting bandwidth savings make a modest-sized Torsk network significantly cheaper to operate, and allows low-bandwidth clients to join the network.Unlike previous proposals for P2P anonymity schemes, Torsk does not require all users to relay traffic for others. Torsk utilizes a combination of two P2P lookup mechanisms with complementary strengths in order to avoid attacks on the confidentiality and integrity of lookups. We show by analysis that previously known attacks on P2P anonymity schemes do not apply to Torsk, and report on experiments conducted with a 336-node wide-area deployment of Torsk, demonstrating its efficiency and feasibility.

Abstract. At Oakland 2005, Murdoch and Danezis described an attack on the Tor anonymity service that recovers the nodes in a Tor circuit, but not the client. We observe that in a peer-to-peer anonymity scheme, the client is part of the circuit and thus the technique can be of greater significance in this setting. We experimentally validate this conclusion by showing that "circuit clogging" can identify client nodes using the MorphMix peer-to-peer anonymity protocol. We also propose and empirically validate the use of the Stochastic Fair Queueing discipline on outgoing connections as an efficient and low-cost mitigation technique.

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