Networking

Equal-Sized Cells Mean Equal-Sized Packets in Tor?

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Executive Summary

Tor is a well-known low-latency anonymous communication system. To prevent the traffic analysis attack, Tor packs application data into equal-sized cells. However, the authors found that equal-sized cells at the application layer do not necessarily produce equal-sized packets at the network layer. Therefore, they introduced a packet size based attack that compromises Tor's communication anonymity with no need of controlling Tor routers. An attacker can manipulate size of packets between a web site and an exit onion router and embeds a signal into the target traffic. An accomplice at the user side can sniff the traffic and recognize this signal.

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