Abstract
Tor is the most popular anonymous communication system. In Tor, each user chooses onion routers (ORs) to construct a circuit to relay the traffic. The final OR of the circuit, called exit node, forwards regular traffic for the Tor user to the destination. As a result, the exit nodes are often accused of the anonymous users’ illegal activities. In this paper, we propose an extension for Tor, called A-Tor, to provide accountable anonymity. A-Tor protects the exit nodes with verifiable evidences that the illegal or malicious packets are originated from the certain users but not the exit nodes. An A-Tor user firstly constructs a Tor circuit to apply for an anonymous certificate. Then, a second Tor circuit is constructed to access the destination server as in Tor, and the anonymous certificate is presented as a credential to the exit node; otherwise, the exit node refuses to forward his/her packets. A-Tor provides anonymity with the same level of assurance as Tor, and cooperative ORs are able to trace the anonymous A-Tor user (when illegal or malicious packets are detected in the future). Moreover, non-repudiation is achieved in the revocation of anonymity; that is, during the application of anonymous certificates and the subsequent anonymous communications through Tor circuits, a chain of evidences are generated by the A-Tor user and the ORs, and these evidences cannot be forged by collusive ORs. The performance overhead introduced by the A-Tor extension is also evaluated.
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Acknowledgments
Q. Cai, and J. Lin were partially supported by National 973 Program of China under Award No. 2014CB 340603. B. Luo was partially supported in part by US National Science Foundation under NSF CNS-1422206, NSF DGE-1565570l.
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Cai, Q., Lutes, J., Lin, J., Luo, B. (2018). A-Tor: Accountable Anonymity in Tor. In: Lin, X., Ghorbani, A., Ren, K., Zhu, S., Zhang, A. (eds) Security and Privacy in Communication Networks. SecureComm 2017. Lecture Notes of the Institute for Computer Sciences, Social Informatics and Telecommunications Engineering, vol 238. Springer, Cham. https://doi.org/10.1007/978-3-319-78813-5_46
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