Detection and analysis of eavesdropping in anonymous communication networks

Abstract

Anonymous communication networks, like Tor, partially protect the confidentiality of user traffic by encrypting all communications within the overlay network. However, when the relayed traffic reaches the boundaries of the network, toward its destination, the original user traffic is inevitably exposed to the final node on the path. As a result, users transmitting sensitive data, like authentication credentials, over such networks, risk having their data intercepted and exposed, unless end-to-end encryption is used. Eavesdropping can be performed by malicious or compromised relay nodes, as well as any rogue network entity on the path toward the actual destination. Furthermore, end-to-end encryption does not assure defense against man-in-the-middle attacks. In this work, we explore the use of decoys at multiple levels for the detection of traffic interception by malicious nodes of proxy-based anonymous communication systems. Our approach relies on the injection of traffic that exposes bait credentials for decoy services requiring user authentication, and URLs to seemingly sensitive decoy documents which, when opened, invoke scripts alerting about being accessed. Our aim was to entice prospective eavesdroppers to access our decoy servers and decoy documents, using the snooped credentials and URLs. We have deployed our prototype implementation in the Tor network using decoy IMAP, SMTP, and HTTP servers. During the course of over 30 months, our system has detected 18 cases of traffic eavesdropping that involved 14 different Tor exit nodes.

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Notes

  1. 1.

    A TCP-based service can keep its IP address hidden (and thus its identity) by replacing the IP address with a hidden service URL. These URLs end in a virtual top-level domain called “.onion” and are resolved by a Tor clients while initiating connection to the hidden service.

  2. 2.

    In contrast to SMTP relay (port 25), SMTP through port 587 is dedicated to message submission for delivery only for users that have registered accounts on the server.

  3. 3.

    In other words, for each exit node that allows access to IMAP, we created a unique username and password. This unique association of the exit node and the exposed user credential helps identify the eavesdropping exit nodes that snoop on these exposed credentials and connect back to our decoy server.

  4. 4.

    Mail clients generally execute a set of commands on the server to fetch the various user directories associated with an account. The absence of such commands and zero payload length could be a strong indication that the adversary does not use any known mail client. We have studied the various protocol messages exchanged by various popular mail client programs.

  5. 5.

    This difference is primarily due to the different lengths of IMAP and SMTP messages. The overhead due to Tor protocol messages, involving circuit setup, key exchanges, accounting, and circuit termination, does not vary significantly between IMAP and SMTP.

  6. 6.

    By default, a fixed set of entry nodes used by Tor clients to defend against traffic analysis attacks that can be launched by malicious entry and exit nodes.

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Acknowledgments

This work was supported by DARPA and ONR through Contracts DARPA-W011NF-11-1-0140 and ONR-MURI-N00014-07-1-090, respectively. Any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect those of the US Government, DARPA, or ONR.

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Correspondence to Sambuddho Chakravarty.

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Chakravarty, S., Portokalidis, G., Polychronakis, M. et al. Detection and analysis of eavesdropping in anonymous communication networks. Int. J. Inf. Secur. 14, 205–220 (2015). https://doi.org/10.1007/s10207-014-0256-7

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Keywords

  • Tor
  • Anonymity networks
  • Proxies
  • Eavesdropping
  • Decoys