We present Tor, a circuit-based low-latency anonymous communication service. This second-generation Onion Routing system addresses limitations in the original design by adding perfect forward secrecy, congestion control, directory servers, integrity checking, configurable exit policies, and a practical design for location-hidden services via rendezvous points. Tor works on the real-world Internet, requires no special privileges or kernel modifications, requires little synchronization or coordination between nodes, and provides a reasonable tradeoff between anonymity, usability, and efficiency. We briefly describe our experiences with an international network of more than 30 nodes. We close with a list of open problems in anonymous communication.

/pdf/tor-the-second-generation-onion-router-2qfrg0ppqb.pdf

Tor: the second-generation onion router

We present Mixminion, a message-based anonymous remailer protocol with secure single-use reply blocks. Mix nodes cannot distinguish Mixminion forward messages from reply messages, so forward and reply messages share the same anonymity set. We add directory servers that allow users to learn public keys and performance statistics of participating remailers, and we describe nymservers that provide long-term pseudonyms using single-use reply blocks as a primitive. Our design integrates link encryption between remailers to provide forward anonymity. Mixminion works in a real-world Internet environment, requires little synchronization or coordination between nodes, and protects against known anonymity-breaking attacks as well as or better than other systems with similar design parameters.

/pdf/mixminion-design-of-a-type-iii-anonymous-remailer-protocol-4tx21i8clc.pdf

Mixminion: design of a type III anonymous remailer protocol

Tor is the second generation onion router supporting the anonymous transport of TCP streams over the Internet. Its low latency makes it very suitable for common tasks, such as Web browsing, but insecure against traffic-analysis attacks by a global passive adversary. We present new traffic-analysis techniques that allow adversaries with only a partial view of the network to infer which nodes are being used to relay the anonymous streams and therefore greatly reduce the anonymity provided by Tor. Furthermore, we show that otherwise unrelated streams can be linked back to the same initiator Our attack is feasible for the adversary anticipated by the Tor designers. Our theoretical attacks are backed up by experiments performed on the deployed, albeit experimental, Tor network. Our techniques should also be applicable to any low latency anonymous network. These attacks highlight the relationship between the field of traffic-analysis and more traditional computer security issues, such as covert channel analysis. Our research also highlights that the inability to directly observe network links does not prevent an attacker from performing traffic-analysis: the adversary can use the anonymising network as an oracle to infer the traffic load on remote nodes in order to perform traffic-analysis.

/pdf/low-cost-traffic-analysis-of-tor-1j5rxn82ze.pdf

Low-cost traffic analysis of Tor

Hidden services were deployed on the Tor anonymous communication network in 2004. Announced properties include server resistance to distributed DoS. Both the EFF and Reporters Without Borders have issued guides that describe using hidden services via Tor to protect the safety of dissidents as well as to resist censorship. We present fast and cheap attacks that reveal the location of a hidden server. Using a single hostile Tor node we have located deployed hidden servers in a matter of minutes. Although we examine hidden services over Tor, our results apply to any client using a variety of anonymity networks. In fact, these are the first actual intersection attacks on any deployed public network: thus confirming general expectations from prior theory and simulation. We recommend changes to route selection design and implementation for Tor. These changes require no operational increase in network overhead and are simple to make; but they prevent the attacks we have demonstrated. They have been implemented.

/pdf/locating-hidden-servers-awfudane97.pdf

Locating hidden servers

Mix networks are a popular mechanism for anonymous Internet communications. By routing IP traffic through an overlay chain of mixes, they aim to hide the relationship between its origin and destination. Using a realistic model of interactive Internet traffic, we study the problem of defending low-latency mix networks against attacks based on correlating inter-packet intervals on two or more links of the mix chain. We investigate several attack models, including an active attack which involves adversarial modification of packet flows in order to “fingerprint” them, and analyze the tradeoffs between the amount of cover traffic, extra latency, and anonymity properties of the mix network. We demonstrate that previously proposed defenses are either ineffective, or impose a prohibitively large latency and/or bandwidth overhead on communicating applications. We propose a new defense based on adaptive padding.

/pdf/timing-analysis-in-low-latency-mix-networks-attacks-and-mh9tm8blv2.pdf

Timing analysis in low-latency mix networks: attacks and defenses

We discuss problems and trade-offs with systems providing anonymity for web browsing (or more generally any communication system that requires low latency interaction). We focus on two main systems: the Freedom network [12] and PipeNet [8]. Although Freedom is efficient and reasonably secure against denial of service attacks, it is vulnerable to some generic traffic analysis attacks, which we describe. On the other hand, we look at PipeNet, a simple theoretical model which protects against the traffic analysis attacks we point out, but is vulnerable to denial of services attacks and has efficiency problems. In light of these observations, we discuss the trade-offs that one faces when trying to construct an efficient low latency communication system that protects users anonymity.

/pdf/traffic-analysis-attacks-and-trade-offs-in-anonymity-9apo1o3i93.pdf

Traffic Analysis Attacks and Trade-Offs in Anonymity Providing Systems

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Traffic Analysis Attacks and Trade-Offs in Anonymity Providing Systems