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2487 lines
116 KiB
HTML
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<title> Tor: The Second-Generation Onion Router </title>
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<h1 align="center">Tor: The Second-Generation Onion Router </h1>
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<div class="p"><!----></div>
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<h3 align="center">
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Roger Dingledine, The Free Haven Project, <tt>arma@freehaven.net</tt><br>
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Nick Mathewson, The Free Haven Project, <tt>nickm@freehaven.net</tt><br>
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Paul Syverson, Naval Research Lab, <tt>syverson@itd.nrl.navy.mil</tt> </h3>
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<div class="p"><!----></div>
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<div class="p"><!----></div>
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<h2> Abstract</h2>
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We present Tor, a circuit-based low-latency anonymous communication
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service. This second-generation Onion Routing system addresses limitations
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in the original design by adding perfect forward secrecy, congestion
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control, directory servers, integrity checking, configurable exit policies,
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and a practical design for location-hidden services via rendezvous
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points. Tor works on the real-world
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Internet, requires no special privileges or kernel modifications, requires
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little synchronization or coordination between nodes, and provides a
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reasonable tradeoff between anonymity, usability, and efficiency.
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We briefly describe our experiences with an international network of
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more than 30 nodes. We close with a list of open problems in anonymous communication.
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<div class="p"><!----></div>
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<div class="p"><!----></div>
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<div class="p"><!----></div>
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<h2><a name="tth_sEc1">
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1</a> Overview</h2>
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<a name="sec:intro">
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</a>
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<div class="p"><!----></div>
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Onion Routing is a distributed overlay network designed to anonymize
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TCP-based applications like web browsing, secure shell,
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and instant messaging. Clients choose a path through the network and
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build a <em>circuit</em>, in which each node (or "onion router" or "OR")
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in the path knows its predecessor and successor, but no other nodes in
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the circuit. Traffic flows down the circuit in fixed-size
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<em>cells</em>, which are unwrapped by a symmetric key at each node
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(like the layers of an onion) and relayed downstream. The
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Onion Routing project published several design and analysis
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papers [<a href="#or-ih96" name="CITEor-ih96">27</a>,<a href="#or-jsac98" name="CITEor-jsac98">41</a>,<a href="#or-discex00" name="CITEor-discex00">48</a>,<a href="#or-pet00" name="CITEor-pet00">49</a>]. While a wide area Onion
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Routing network was deployed briefly, the only long-running
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public implementation was a fragile
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proof-of-concept that ran on a single machine. Even this simple deployment
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processed connections from over sixty thousand distinct IP addresses from
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all over the world at a rate of about fifty thousand per day.
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But many critical design and deployment issues were never
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resolved, and the design has not been updated in years. Here
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we describe Tor, a protocol for asynchronous, loosely federated onion
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routers that provides the following improvements over the old Onion
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Routing design:
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<div class="p"><!----></div>
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<b>Perfect forward secrecy:</b> In the original Onion Routing design,
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a single hostile node could record traffic and
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later compromise successive nodes in the circuit and force them
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to decrypt it. Rather than using a single multiply encrypted data
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structure (an <em>onion</em>) to lay each circuit,
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Tor now uses an incremental or <em>telescoping</em> path-building design,
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where the initiator negotiates session keys with each successive hop in
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the circuit. Once these keys are deleted, subsequently compromised nodes
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cannot decrypt old traffic. As a side benefit, onion replay detection
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is no longer necessary, and the process of building circuits is more
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reliable, since the initiator knows when a hop fails and can then try
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extending to a new node.
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<div class="p"><!----></div>
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<b>Separation of "protocol cleaning" from anonymity:</b>
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Onion Routing originally required a separate "application
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proxy" for each supported application protocol-most of which were
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never written, so many applications were never supported. Tor uses the
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standard and near-ubiquitous SOCKS [<a href="#socks4" name="CITEsocks4">32</a>] proxy interface, allowing
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us to support most TCP-based programs without modification. Tor now
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relies on the filtering features of privacy-enhancing
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application-level proxies such as Privoxy [<a href="#privoxy" name="CITEprivoxy">39</a>], without trying
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to duplicate those features itself.
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<div class="p"><!----></div>
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<b>No mixing, padding, or traffic shaping (yet):</b> Onion
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Routing originally called for batching and reordering cells as they arrived,
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assumed padding between ORs, and in
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later designs added padding between onion proxies (users) and
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ORs [<a href="#or-ih96" name="CITEor-ih96">27</a>,<a href="#or-jsac98" name="CITEor-jsac98">41</a>]. Tradeoffs between padding protection
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and cost were discussed, and <em>traffic shaping</em> algorithms were
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theorized [<a href="#or-pet00" name="CITEor-pet00">49</a>] to provide good security without expensive
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padding, but no concrete padding scheme was suggested.
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Recent research [<a href="#econymics" name="CITEeconymics">1</a>]
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and deployment experience [<a href="#freedom21-security" name="CITEfreedom21-security">4</a>] suggest that this
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level of resource use is not practical or economical; and even full
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link padding is still vulnerable [<a href="#defensive-dropping" name="CITEdefensive-dropping">33</a>]. Thus,
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until we have a proven and convenient design for traffic shaping or
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low-latency mixing that improves anonymity against a realistic
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adversary, we leave these strategies out.
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<div class="p"><!----></div>
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<b>Many TCP streams can share one circuit:</b> Onion Routing originally
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built a separate circuit for each
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application-level request, but this required
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multiple public key operations for every request, and also presented
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a threat to anonymity from building so many circuits; see
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Section <a href="#sec:maintaining-anonymity">9</a>. Tor multiplexes multiple TCP
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streams along each circuit to improve efficiency and anonymity.
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<div class="p"><!----></div>
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<b>Leaky-pipe circuit topology:</b> Through in-band signaling
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within the circuit, Tor initiators can direct traffic to nodes partway
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down the circuit. This novel approach
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allows traffic to exit the circuit from the middle-possibly
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frustrating traffic shape and volume attacks based on observing the end
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of the circuit. (It also allows for long-range padding if
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future research shows this to be worthwhile.)
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<div class="p"><!----></div>
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<b>Congestion control:</b> Earlier anonymity designs do not
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address traffic bottlenecks. Unfortunately, typical approaches to
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load balancing and flow control in overlay networks involve inter-node
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control communication and global views of traffic. Tor's decentralized
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congestion control uses end-to-end acks to maintain anonymity
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while allowing nodes at the edges of the network to detect congestion
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or flooding and send less data until the congestion subsides.
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<div class="p"><!----></div>
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<b>Directory servers:</b> The earlier Onion Routing design
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planned to flood state information through the network-an approach
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that can be unreliable and complex. Tor takes a simplified view toward distributing this
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information. Certain more trusted nodes act as <em>directory
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servers</em>: they provide signed directories describing known
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routers and their current state. Users periodically download them
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via HTTP.
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<div class="p"><!----></div>
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<b>Variable exit policies:</b> Tor provides a consistent mechanism
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for each node to advertise a policy describing the hosts
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and ports to which it will connect. These exit policies are critical
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in a volunteer-based distributed infrastructure, because each operator
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is comfortable with allowing different types of traffic to exit
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from his node.
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<div class="p"><!----></div>
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<b>End-to-end integrity checking:</b> The original Onion Routing
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design did no integrity checking on data. Any node on the
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circuit could change the contents of data cells as they passed by-for
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example, to alter a connection request so it would connect
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to a different webserver, or to `tag' encrypted traffic and look for
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corresponding corrupted traffic at the network edges [<a href="#minion-design" name="CITEminion-design">15</a>].
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Tor hampers these attacks by verifying data integrity before it leaves
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the network.
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<div class="p"><!----></div>
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<div class="p"><!----></div>
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<b>Rendezvous points and hidden services:</b>
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Tor provides an integrated mechanism for responder anonymity via
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location-protected servers. Previous Onion Routing designs included
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long-lived "reply onions" that could be used to build circuits
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to a hidden server, but these reply onions did not provide forward
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security, and became useless if any node in the path went down
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or rotated its keys. In Tor, clients negotiate <i>rendezvous points</i>
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to connect with hidden servers; reply onions are no longer required.
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<div class="p"><!----></div>
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Unlike Freedom [<a href="#freedom2-arch" name="CITEfreedom2-arch">8</a>], Tor does not require OS kernel
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patches or network stack support. This prevents us from anonymizing
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non-TCP protocols, but has greatly helped our portability and
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deployability.
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<div class="p"><!----></div>
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<div class="p"><!----></div>
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We have implemented all of the above features, including rendezvous
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points. Our source code is
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available under a free license, and Tor
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is not covered by the patent that affected distribution and use of
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earlier versions of Onion Routing.
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We have deployed a wide-area alpha network
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to test the design, to get more experience with usability
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and users, and to provide a research platform for experimentation.
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As of this writing, the network stands at 32 nodes spread over two continents.
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<div class="p"><!----></div>
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We review previous work in Section <a href="#sec:related-work">2</a>, describe
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our goals and assumptions in Section <a href="#sec:assumptions">3</a>,
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and then address the above list of improvements in
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Sections <a href="#sec:design">4</a>, <a href="#sec:rendezvous">5</a>, and <a href="#sec:other-design">6</a>.
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We summarize
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in Section <a href="#sec:attacks">7</a> how our design stands up to
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known attacks, and talk about our early deployment experiences in
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Section <a href="#sec:in-the-wild">8</a>. We conclude with a list of open problems in
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Section <a href="#sec:maintaining-anonymity">9</a> and future work for the Onion
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Routing project in Section <a href="#sec:conclusion">10</a>.
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<div class="p"><!----></div>
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<div class="p"><!----></div>
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<h2><a name="tth_sEc2">
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2</a> Related work</h2>
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<a name="sec:related-work">
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</a>
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<div class="p"><!----></div>
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Modern anonymity systems date to Chaum's <b>Mix-Net</b>
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design [<a href="#chaum-mix" name="CITEchaum-mix">10</a>]. Chaum
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proposed hiding the correspondence between sender and recipient by
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wrapping messages in layers of public-key cryptography, and relaying them
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through a path composed of "mixes." Each mix in turn
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decrypts, delays, and re-orders messages before relaying them
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onward.
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<div class="p"><!----></div>
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Subsequent relay-based anonymity designs have diverged in two
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main directions. Systems like <b>Babel</b> [<a href="#babel" name="CITEbabel">28</a>],
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<b>Mixmaster</b> [<a href="#mixmaster-spec" name="CITEmixmaster-spec">36</a>],
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and <b>Mixminion</b> [<a href="#minion-design" name="CITEminion-design">15</a>] have tried
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to maximize anonymity at the cost of introducing comparatively large and
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variable latencies. Because of this decision, these <em>high-latency</em>
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networks resist strong global adversaries,
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but introduce too much lag for interactive tasks like web browsing,
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Internet chat, or SSH connections.
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<div class="p"><!----></div>
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Tor belongs to the second category: <em>low-latency</em> designs that
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try to anonymize interactive network traffic. These systems handle
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a variety of bidirectional protocols. They also provide more convenient
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mail delivery than the high-latency anonymous email
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networks, because the remote mail server provides explicit and timely
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delivery confirmation. But because these designs typically
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involve many packets that must be delivered quickly, it is
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difficult for them to prevent an attacker who can eavesdrop both ends of the
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communication from correlating the timing and volume
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of traffic entering the anonymity network with traffic leaving it [<a href="#SS03" name="CITESS03">45</a>].
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These
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protocols are similarly vulnerable to an active adversary who introduces
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timing patterns into traffic entering the network and looks
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for correlated patterns among exiting traffic.
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Although some work has been done to frustrate these attacks, most designs
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protect primarily against traffic analysis rather than traffic
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confirmation (see Section <a href="#subsec:threat-model">3.1</a>).
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<div class="p"><!----></div>
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The simplest low-latency designs are single-hop proxies such as the
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<b>Anonymizer</b> [<a href="#anonymizer" name="CITEanonymizer">3</a>]: a single trusted server strips the
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data's origin before relaying it. These designs are easy to
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analyze, but users must trust the anonymizing proxy.
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Concentrating the traffic to this single point increases the anonymity set
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(the people a given user is hiding among), but it is vulnerable if the
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adversary can observe all traffic entering and leaving the proxy.
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<div class="p"><!----></div>
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More complex are distributed-trust, circuit-based anonymizing systems.
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In these designs, a user establishes one or more medium-term bidirectional
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end-to-end circuits, and tunnels data in fixed-size cells.
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Establishing circuits is computationally expensive and typically
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requires public-key
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cryptography, whereas relaying cells is comparatively inexpensive and
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typically requires only symmetric encryption.
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Because a circuit crosses several servers, and each server only knows
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the adjacent servers in the circuit, no single server can link a
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user to her communication partners.
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<div class="p"><!----></div>
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The <b>Java Anon Proxy</b> (also known as JAP or Web MIXes) uses fixed shared
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routes known as <em>cascades</em>. As with a single-hop proxy, this
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approach aggregates users into larger anonymity sets, but again an
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attacker only needs to observe both ends of the cascade to bridge all
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the system's traffic. The Java Anon Proxy's design
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calls for padding between end users and the head of the
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cascade [<a href="#web-mix" name="CITEweb-mix">7</a>]. However, it is not demonstrated whether the current
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implementation's padding policy improves anonymity.
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<div class="p"><!----></div>
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<b>PipeNet</b> [<a href="#back01" name="CITEback01">5</a>,<a href="#pipenet" name="CITEpipenet">12</a>], another low-latency design proposed
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around the same time as Onion Routing, gave
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stronger anonymity but allowed a single user to shut
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down the network by not sending. Systems like <b>ISDN
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mixes</b> [<a href="#isdn-mixes" name="CITEisdn-mixes">38</a>] were designed for other environments with
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different assumptions.
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<div class="p"><!----></div>
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In P2P designs like <b>Tarzan</b> [<a href="#tarzan:ccs02" name="CITEtarzan:ccs02">24</a>] and
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<b>MorphMix</b> [<a href="#morphmix:fc04" name="CITEmorphmix:fc04">43</a>], all participants both generate
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traffic and relay traffic for others. These systems aim to conceal
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whether a given peer originated a request
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or just relayed it from another peer. While Tarzan and MorphMix use
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layered encryption as above, <b>Crowds</b> [<a href="#crowds-tissec" name="CITEcrowds-tissec">42</a>] simply assumes
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an adversary who cannot observe the initiator: it uses no public-key
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encryption, so any node on a circuit can read users' traffic.
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<div class="p"><!----></div>
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<b>Hordes</b> [<a href="#hordes-jcs" name="CITEhordes-jcs">34</a>] is based on Crowds but also uses multicast
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responses to hide the initiator. <b>Herbivore</b> [<a href="#herbivore" name="CITEherbivore">25</a>] and
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<b>P</b><sup><b>5</b></sup> [<a href="#p5" name="CITEp5">46</a>] go even further, requiring broadcast.
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These systems are designed primarily for communication among peers,
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although Herbivore users can make external connections by
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requesting a peer to serve as a proxy.
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<div class="p"><!----></div>
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Systems like <b>Freedom</b> and the original Onion Routing build circuits
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all at once, using a layered "onion" of public-key encrypted messages,
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each layer of which provides session keys and the address of the
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next server in the circuit. Tor as described herein, Tarzan, MorphMix,
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<b>Cebolla</b> [<a href="#cebolla" name="CITEcebolla">9</a>], and Rennhard's <b>Anonymity Network</b> [<a href="#anonnet" name="CITEanonnet">44</a>]
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build circuits
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in stages, extending them one hop at a time.
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Section <a href="#subsubsec:constructing-a-circuit">4.2</a> describes how this
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approach enables perfect forward secrecy.
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<div class="p"><!----></div>
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Circuit-based designs must choose which protocol layer
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to anonymize. They may intercept IP packets directly, and
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relay them whole (stripping the source address) along the
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circuit [<a href="#freedom2-arch" name="CITEfreedom2-arch">8</a>,<a href="#tarzan:ccs02" name="CITEtarzan:ccs02">24</a>]. Like
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Tor, they may accept TCP streams and relay the data in those streams,
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ignoring the breakdown of that data into TCP
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segments [<a href="#morphmix:fc04" name="CITEmorphmix:fc04">43</a>,<a href="#anonnet" name="CITEanonnet">44</a>]. Finally, like Crowds, they may accept
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application-level protocols such as HTTP and relay the application
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requests themselves.
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Making this protocol-layer decision requires a compromise between flexibility
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and anonymity. For example, a system that understands HTTP
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can strip
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identifying information from requests, can take advantage of caching
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to limit the number of requests that leave the network, and can batch
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or encode requests to minimize the number of connections.
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On the other hand, an IP-level anonymizer can handle nearly any protocol,
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even ones unforeseen by its designers (though these systems require
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kernel-level modifications to some operating systems, and so are more
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complex and less portable). TCP-level anonymity networks like Tor present
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a middle approach: they are application neutral (so long as the
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application supports, or can be tunneled across, TCP), but by treating
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application connections as data streams rather than raw TCP packets,
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they avoid the inefficiencies of tunneling TCP over
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TCP.
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<div class="p"><!----></div>
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Distributed-trust anonymizing systems need to prevent attackers from
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adding too many servers and thus compromising user paths.
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Tor relies on a small set of well-known directory servers, run by
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independent parties, to decide which nodes can
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join. Tarzan and MorphMix allow unknown users to run servers, and use
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a limited resource (like IP addresses) to prevent an attacker from
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controlling too much of the network. Crowds suggests requiring
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written, notarized requests from potential crowd members.
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<div class="p"><!----></div>
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Anonymous communication is essential for censorship-resistant
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systems like Eternity [<a href="#eternity" name="CITEeternity">2</a>], Free Haven [<a href="#freehaven-berk" name="CITEfreehaven-berk">19</a>],
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Publius [<a href="#publius" name="CITEpublius">53</a>], and Tangler [<a href="#tangler" name="CITEtangler">52</a>]. Tor's rendezvous
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points enable connections between mutually anonymous entities; they
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are a building block for location-hidden servers, which are needed by
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Eternity and Free Haven.
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<div class="p"><!----></div>
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<div class="p"><!----></div>
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<h2><a name="tth_sEc3">
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3</a> Design goals and assumptions</h2>
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<a name="sec:assumptions">
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</a>
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<div class="p"><!----></div>
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<font size="+1"><b>Goals</b></font><br />
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Like other low-latency anonymity designs, Tor seeks to frustrate
|
|
attackers from linking communication partners, or from linking
|
|
multiple communications to or from a single user. Within this
|
|
main goal, however, several considerations have directed
|
|
Tor's evolution.
|
|
|
|
<div class="p"><!----></div>
|
|
<b>Deployability:</b> The design must be deployed and used in the
|
|
real world. Thus it
|
|
must not be expensive to run (for example, by requiring more bandwidth
|
|
than volunteers are willing to provide); must not place a heavy
|
|
liability burden on operators (for example, by allowing attackers to
|
|
implicate onion routers in illegal activities); and must not be
|
|
difficult or expensive to implement (for example, by requiring kernel
|
|
patches, or separate proxies for every protocol). We also cannot
|
|
require non-anonymous parties (such as websites)
|
|
to run our software. (Our rendezvous point design does not meet
|
|
this goal for non-anonymous users talking to hidden servers,
|
|
however; see Section <a href="#sec:rendezvous">5</a>.)
|
|
|
|
<div class="p"><!----></div>
|
|
<b>Usability:</b> A hard-to-use system has fewer users-and because
|
|
anonymity systems hide users among users, a system with fewer users
|
|
provides less anonymity. Usability is thus not only a convenience:
|
|
it is a security requirement [<a href="#econymics" name="CITEeconymics">1</a>,<a href="#back01" name="CITEback01">5</a>]. Tor should
|
|
therefore not
|
|
require modifying familiar applications; should not introduce prohibitive
|
|
delays;
|
|
and should require as few configuration decisions
|
|
as possible. Finally, Tor should be easily implementable on all common
|
|
platforms; we cannot require users to change their operating system
|
|
to be anonymous. (Tor currently runs on Win32, Linux,
|
|
Solaris, BSD-style Unix, MacOS X, and probably others.)
|
|
|
|
<div class="p"><!----></div>
|
|
<b>Flexibility:</b> The protocol must be flexible and well-specified,
|
|
so Tor can serve as a test-bed for future research.
|
|
Many of the open problems in low-latency anonymity
|
|
networks, such as generating dummy traffic or preventing Sybil
|
|
attacks [<a href="#sybil" name="CITEsybil">22</a>], may be solvable independently from the issues
|
|
solved by
|
|
Tor. Hopefully future systems will not need to reinvent Tor's design.
|
|
|
|
<div class="p"><!----></div>
|
|
<b>Simple design:</b> The protocol's design and security
|
|
parameters must be well-understood. Additional features impose implementation
|
|
and complexity costs; adding unproven techniques to the design threatens
|
|
deployability, readability, and ease of security analysis. Tor aims to
|
|
deploy a simple and stable system that integrates the best accepted
|
|
approaches to protecting anonymity.<br />
|
|
|
|
<div class="p"><!----></div>
|
|
<font size="+1"><b>Non-goals</b></font><a name="subsec:non-goals">
|
|
</a><br />
|
|
In favoring simple, deployable designs, we have explicitly deferred
|
|
several possible goals, either because they are solved elsewhere, or because
|
|
they are not yet solved.
|
|
|
|
<div class="p"><!----></div>
|
|
<b>Not peer-to-peer:</b> Tarzan and MorphMix aim to scale to completely
|
|
decentralized peer-to-peer environments with thousands of short-lived
|
|
servers, many of which may be controlled by an adversary. This approach
|
|
is appealing, but still has many open
|
|
problems [<a href="#tarzan:ccs02" name="CITEtarzan:ccs02">24</a>,<a href="#morphmix:fc04" name="CITEmorphmix:fc04">43</a>].
|
|
|
|
<div class="p"><!----></div>
|
|
<b>Not secure against end-to-end attacks:</b> Tor does not claim
|
|
to completely solve end-to-end timing or intersection
|
|
attacks. Some approaches, such as having users run their own onion routers,
|
|
may help;
|
|
see Section <a href="#sec:maintaining-anonymity">9</a> for more discussion.
|
|
|
|
<div class="p"><!----></div>
|
|
<b>No protocol normalization:</b> Tor does not provide <em>protocol
|
|
normalization</em> like Privoxy or the Anonymizer. If senders want anonymity from
|
|
responders while using complex and variable
|
|
protocols like HTTP, Tor must be layered with a filtering proxy such
|
|
as Privoxy to hide differences between clients, and expunge protocol
|
|
features that leak identity.
|
|
Note that by this separation Tor can also provide services that
|
|
are anonymous to the network yet authenticated to the responder, like
|
|
SSH. Similarly, Tor does not integrate
|
|
tunneling for non-stream-based protocols like UDP; this must be
|
|
provided by an external service if appropriate.
|
|
|
|
<div class="p"><!----></div>
|
|
<b>Not steganographic:</b> Tor does not try to conceal who is connected
|
|
to the network.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc3.1">
|
|
3.1</a> Threat Model</h3>
|
|
<a name="subsec:threat-model">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
A global passive adversary is the most commonly assumed threat when
|
|
analyzing theoretical anonymity designs. But like all practical
|
|
low-latency systems, Tor does not protect against such a strong
|
|
adversary. Instead, we assume an adversary who can observe some fraction
|
|
of network traffic; who can generate, modify, delete, or delay
|
|
traffic; who can operate onion routers of his own; and who can
|
|
compromise some fraction of the onion routers.
|
|
|
|
<div class="p"><!----></div>
|
|
In low-latency anonymity systems that use layered encryption, the
|
|
adversary's typical goal is to observe both the initiator and the
|
|
responder. By observing both ends, passive attackers can confirm a
|
|
suspicion that Alice is
|
|
talking to Bob if the timing and volume patterns of the traffic on the
|
|
connection are distinct enough; active attackers can induce timing
|
|
signatures on the traffic to force distinct patterns. Rather
|
|
than focusing on these <em>traffic confirmation</em> attacks,
|
|
we aim to prevent <em>traffic
|
|
analysis</em> attacks, where the adversary uses traffic patterns to learn
|
|
which points in the network he should attack.
|
|
|
|
<div class="p"><!----></div>
|
|
Our adversary might try to link an initiator Alice with her
|
|
communication partners, or try to build a profile of Alice's
|
|
behavior. He might mount passive attacks by observing the network edges
|
|
and correlating traffic entering and leaving the network-by
|
|
relationships in packet timing, volume, or externally visible
|
|
user-selected
|
|
options. The adversary can also mount active attacks by compromising
|
|
routers or keys; by replaying traffic; by selectively denying service
|
|
to trustworthy routers to move users to
|
|
compromised routers, or denying service to users to see if traffic
|
|
elsewhere in the
|
|
network stops; or by introducing patterns into traffic that can later be
|
|
detected. The adversary might subvert the directory servers to give users
|
|
differing views of network state. Additionally, he can try to decrease
|
|
the network's reliability by attacking nodes or by performing antisocial
|
|
activities from reliable nodes and trying to get them taken down-making
|
|
the network unreliable flushes users to other less anonymous
|
|
systems, where they may be easier to attack. We summarize
|
|
in Section <a href="#sec:attacks">7</a> how well the Tor design defends against
|
|
each of these attacks.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc4">
|
|
4</a> The Tor Design</h2>
|
|
<a name="sec:design">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
The Tor network is an overlay network; each onion router (OR)
|
|
runs as a normal
|
|
user-level process without any special privileges.
|
|
Each onion router maintains a TLS [<a href="#TLS" name="CITETLS">17</a>]
|
|
connection to every other onion router.
|
|
Each user
|
|
runs local software called an onion proxy (OP) to fetch directories,
|
|
establish circuits across the network,
|
|
and handle connections from user applications. These onion proxies accept
|
|
TCP streams and multiplex them across the circuits. The onion
|
|
router on the other side
|
|
of the circuit connects to the requested destinations
|
|
and relays data.
|
|
|
|
<div class="p"><!----></div>
|
|
Each onion router maintains a long-term identity key and a short-term
|
|
onion key. The identity
|
|
key is used to sign TLS certificates, to sign the OR's <em>router
|
|
descriptor</em> (a summary of its keys, address, bandwidth, exit policy,
|
|
and so on), and (by directory servers) to sign directories. The onion key is used to decrypt requests
|
|
from users to set up a circuit and negotiate ephemeral keys.
|
|
The TLS protocol also establishes a short-term link key when communicating
|
|
between ORs. Short-term keys are rotated periodically and
|
|
independently, to limit the impact of key compromise.
|
|
|
|
<div class="p"><!----></div>
|
|
Section <a href="#subsec:cells">4.1</a> presents the fixed-size
|
|
<em>cells</em> that are the unit of communication in Tor. We describe
|
|
in Section <a href="#subsec:circuits">4.2</a> how circuits are
|
|
built, extended, truncated, and destroyed. Section <a href="#subsec:tcp">4.3</a>
|
|
describes how TCP streams are routed through the network. We address
|
|
integrity checking in Section <a href="#subsec:integrity-checking">4.4</a>,
|
|
and resource limiting in Section <a href="#subsec:rate-limit">4.5</a>.
|
|
Finally,
|
|
Section <a href="#subsec:congestion">4.6</a> talks about congestion control and
|
|
fairness issues.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.1">
|
|
4.1</a> Cells</h3>
|
|
<a name="subsec:cells">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Onion routers communicate with one another, and with users' OPs, via
|
|
TLS connections with ephemeral keys. Using TLS conceals the data on
|
|
the connection with perfect forward secrecy, and prevents an attacker
|
|
from modifying data on the wire or impersonating an OR.
|
|
|
|
<div class="p"><!----></div>
|
|
Traffic passes along these connections in fixed-size cells. Each cell
|
|
is 512 bytes, and consists of a header and a payload. The header includes a circuit
|
|
identifier (circID) that specifies which circuit the cell refers to
|
|
(many circuits can be multiplexed over the single TLS connection), and
|
|
a command to describe what to do with the cell's payload. (Circuit
|
|
identifiers are connection-specific: each circuit has a different
|
|
circID on each OP/OR or OR/OR connection it traverses.)
|
|
Based on their command, cells are either <em>control</em> cells, which are
|
|
always interpreted by the node that receives them, or <em>relay</em> cells,
|
|
which carry end-to-end stream data. The control cell commands are:
|
|
<em>padding</em> (currently used for keepalive, but also usable for link
|
|
padding); <em>create</em> or <em>created</em> (used to set up a new circuit);
|
|
and <em>destroy</em> (to tear down a circuit).
|
|
|
|
<div class="p"><!----></div>
|
|
Relay cells have an additional header (the relay header) at the front
|
|
of the payload, containing a streamID (stream identifier: many streams can
|
|
be multiplexed over a circuit); an end-to-end checksum for integrity
|
|
checking; the length of the relay payload; and a relay command.
|
|
The entire contents of the relay header and the relay cell payload
|
|
are encrypted or decrypted together as the relay cell moves along the
|
|
circuit, using the 128-bit AES cipher in counter mode to generate a
|
|
cipher stream. The relay commands are: <em>relay
|
|
data</em> (for data flowing down the stream), <em>relay begin</em> (to open a
|
|
stream), <em>relay end</em> (to close a stream cleanly), <em>relay
|
|
teardown</em> (to close a broken stream), <em>relay connected</em>
|
|
(to notify the OP that a relay begin has succeeded), <em>relay
|
|
extend</em> and <em>relay extended</em> (to extend the circuit by a hop,
|
|
and to acknowledge), <em>relay truncate</em> and <em>relay truncated</em>
|
|
(to tear down only part of the circuit, and to acknowledge), <em>relay
|
|
sendme</em> (used for congestion control), and <em>relay drop</em> (used to
|
|
implement long-range dummies).
|
|
We give a visual overview of cell structure plus the details of relay
|
|
cell structure, and then describe each of these cell types and commands
|
|
in more detail below.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<a name="tth_fIg1">
|
|
</a> <center><img src="cell-struct.png" alt="cell-struct.png" />
|
|
</center>
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.2">
|
|
4.2</a> Circuits and streams</h3>
|
|
<a name="subsec:circuits">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Onion Routing originally built one circuit for each
|
|
TCP stream. Because building a circuit can take several tenths of a
|
|
second (due to public-key cryptography and network latency),
|
|
this design imposed high costs on applications like web browsing that
|
|
open many TCP streams.
|
|
|
|
<div class="p"><!----></div>
|
|
In Tor, each circuit can be shared by many TCP streams. To avoid
|
|
delays, users construct circuits preemptively. To limit linkability
|
|
among their streams, users' OPs build a new circuit
|
|
periodically if the previous ones have been used,
|
|
and expire old used circuits that no longer have any open streams.
|
|
OPs consider rotating to a new circuit once a minute: thus
|
|
even heavy users spend negligible time
|
|
building circuits, but a limited number of requests can be linked
|
|
to each other through a given exit node. Also, because circuits are built
|
|
in the background, OPs can recover from failed circuit creation
|
|
without harming user experience.<br />
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<a name="tth_fIg1">
|
|
</a> <center><img src="interaction.png" alt="interaction.png" />
|
|
|
|
<center>Figure 1: Alice builds a two-hop circuit and begins fetching a web page.</center>
|
|
<a name="fig:interaction">
|
|
</a>
|
|
</center>
|
|
<div class="p"><!----></div>
|
|
<font size="+1"><b>Constructing a circuit</b></font><a name="subsubsec:constructing-a-circuit">
|
|
</a><br />
|
|
A user's OP constructs circuits incrementally, negotiating a
|
|
symmetric key with each OR on the circuit, one hop at a time. To begin
|
|
creating a new circuit, the OP (call her Alice) sends a
|
|
<em>create</em> cell to the first node in her chosen path (call him Bob).
|
|
(She chooses a new
|
|
circID C<sub>AB</sub> not currently used on the connection from her to Bob.)
|
|
The <em>create</em> cell's
|
|
payload contains the first half of the Diffie-Hellman handshake
|
|
(g<sup>x</sup>), encrypted to the onion key of the OR (call him Bob). Bob
|
|
responds with a <em>created</em> cell containing g<sup>y</sup>
|
|
along with a hash of the negotiated key K=g<sup>xy</sup>.
|
|
|
|
<div class="p"><!----></div>
|
|
Once the circuit has been established, Alice and Bob can send one
|
|
another relay cells encrypted with the negotiated
|
|
key.<a href="#tthFtNtAAB" name="tthFrefAAB"><sup>1</sup></a> More detail is given in
|
|
the next section.
|
|
|
|
<div class="p"><!----></div>
|
|
To extend the circuit further, Alice sends a <em>relay extend</em> cell
|
|
to Bob, specifying the address of the next OR (call her Carol), and
|
|
an encrypted g<sup>x<sub>2</sub></sup> for her. Bob copies the half-handshake into a
|
|
<em>create</em> cell, and passes it to Carol to extend the circuit.
|
|
(Bob chooses a new circID C<sub>BC</sub> not currently used on the connection
|
|
between him and Carol. Alice never needs to know this circID; only Bob
|
|
associates C<sub>AB</sub> on his connection with Alice to C<sub>BC</sub> on
|
|
his connection with Carol.)
|
|
When Carol responds with a <em>created</em> cell, Bob wraps the payload
|
|
into a <em>relay extended</em> cell and passes it back to Alice. Now
|
|
the circuit is extended to Carol, and Alice and Carol share a common key
|
|
K<sub>2</sub> = g<sup>x<sub>2</sub> y<sub>2</sub></sup>.
|
|
|
|
<div class="p"><!----></div>
|
|
To extend the circuit to a third node or beyond, Alice
|
|
proceeds as above, always telling the last node in the circuit to
|
|
extend one hop further.
|
|
|
|
<div class="p"><!----></div>
|
|
This circuit-level handshake protocol achieves unilateral entity
|
|
authentication (Alice knows she's handshaking with the OR, but
|
|
the OR doesn't care who is opening the circuit-Alice uses no public key
|
|
and remains anonymous) and unilateral key authentication
|
|
(Alice and the OR agree on a key, and Alice knows only the OR learns
|
|
it). It also achieves forward
|
|
secrecy and key freshness. More formally, the protocol is as follows
|
|
(where E<sub>PK<sub>Bob</sub></sub>(·) is encryption with Bob's public key,
|
|
H is a secure hash function, and <font face="symbol">|</font
|
|
> is concatenation):
|
|
|
|
<div class="p"><!----></div>
|
|
<a name="tth_tAb1">
|
|
</a>
|
|
<table>
|
|
<tr><td align="right">Alice </td><td align="center">-> </td><td align="center">Bob </td><td>: E<sub>PK<sub>Bob</sub></sub>(g<sup>x</sup>) </td></tr>
|
|
<tr><td align="right">Bob </td><td align="center">-> </td><td align="center">Alice </td><td>: g<sup>y</sup>, H(K <font face="symbol">|</font
|
|
> "<span class="roman">handshake</span>")
|
|
</td></tr></table>
|
|
|
|
|
|
<div class="p"><!----></div>
|
|
In the second step, Bob proves that it was he who received g<sup>x</sup>,
|
|
and who chose y. We use PK encryption in the first step
|
|
(rather than, say, using the first two steps of STS, which has a
|
|
signature in the second step) because a single cell is too small to
|
|
hold both a public key and a signature. Preliminary analysis with the
|
|
NRL protocol analyzer [<a href="#meadows96" name="CITEmeadows96">35</a>] shows this protocol to be
|
|
secure (including perfect forward secrecy) under the
|
|
traditional Dolev-Yao model.<br />
|
|
|
|
<div class="p"><!----></div>
|
|
<font size="+1"><b>Relay cells</b></font><br />
|
|
Once Alice has established the circuit (so she shares keys with each
|
|
OR on the circuit), she can send relay cells.
|
|
Upon receiving a relay
|
|
cell, an OR looks up the corresponding circuit, and decrypts the relay
|
|
header and payload with the session key for that circuit.
|
|
If the cell is headed away from Alice the OR then checks whether the
|
|
decrypted cell has a valid digest (as an optimization, the first
|
|
two bytes of the integrity check are zero, so in most cases we can avoid
|
|
computing the hash).
|
|
If valid, it accepts the relay cell and processes it as described
|
|
below. Otherwise,
|
|
the OR looks up the circID and OR for the
|
|
next step in the circuit, replaces the circID as appropriate, and
|
|
sends the decrypted relay cell to the next OR. (If the OR at the end
|
|
of the circuit receives an unrecognized relay cell, an error has
|
|
occurred, and the circuit is torn down.)
|
|
|
|
<div class="p"><!----></div>
|
|
OPs treat incoming relay cells similarly: they iteratively unwrap the
|
|
relay header and payload with the session keys shared with each
|
|
OR on the circuit, from the closest to farthest.
|
|
If at any stage the digest is valid, the cell must have
|
|
originated at the OR whose encryption has just been removed.
|
|
|
|
<div class="p"><!----></div>
|
|
To construct a relay cell addressed to a given OR, Alice assigns the
|
|
digest, and then iteratively
|
|
encrypts the cell payload (that is, the relay header and payload) with
|
|
the symmetric key of each hop up to that OR. Because the digest is
|
|
encrypted to a different value at each step, only at the targeted OR
|
|
will it have a meaningful value.<a href="#tthFtNtAAC" name="tthFrefAAC"><sup>2</sup></a>
|
|
This <em>leaky pipe</em> circuit topology
|
|
allows Alice's streams to exit at different ORs on a single circuit.
|
|
Alice may choose different exit points because of their exit policies,
|
|
or to keep the ORs from knowing that two streams
|
|
originate from the same person.
|
|
|
|
<div class="p"><!----></div>
|
|
When an OR later replies to Alice with a relay cell, it
|
|
encrypts the cell's relay header and payload with the single key it
|
|
shares with Alice, and sends the cell back toward Alice along the
|
|
circuit. Subsequent ORs add further layers of encryption as they
|
|
relay the cell back to Alice.
|
|
|
|
<div class="p"><!----></div>
|
|
To tear down a circuit, Alice sends a <em>destroy</em> control
|
|
cell. Each OR in the circuit receives the <em>destroy</em> cell, closes
|
|
all streams on that circuit, and passes a new <em>destroy</em> cell
|
|
forward. But just as circuits are built incrementally, they can also
|
|
be torn down incrementally: Alice can send a <em>relay
|
|
truncate</em> cell to a single OR on a circuit. That OR then sends a
|
|
<em>destroy</em> cell forward, and acknowledges with a
|
|
<em>relay truncated</em> cell. Alice can then extend the circuit to
|
|
different nodes, without signaling to the intermediate nodes (or
|
|
a limited observer) that she has changed her circuit.
|
|
Similarly, if a node on the circuit goes down, the adjacent
|
|
node can send a <em>relay truncated</em> cell back to Alice. Thus the
|
|
"break a node and see which circuits go down"
|
|
attack [<a href="#freedom21-security" name="CITEfreedom21-security">4</a>] is weakened.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.3">
|
|
4.3</a> Opening and closing streams</h3>
|
|
<a name="subsec:tcp">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
When Alice's application wants a TCP connection to a given
|
|
address and port, it asks the OP (via SOCKS) to make the
|
|
connection. The OP chooses the newest open circuit (or creates one if
|
|
needed), and chooses a suitable OR on that circuit to be the
|
|
exit node (usually the last node, but maybe others due to exit policy
|
|
conflicts; see Section <a href="#subsec:exitpolicies">6.2</a>.) The OP then opens
|
|
the stream by sending a <em>relay begin</em> cell to the exit node,
|
|
using a new random streamID. Once the
|
|
exit node connects to the remote host, it responds
|
|
with a <em>relay connected</em> cell. Upon receipt, the OP sends a
|
|
SOCKS reply to notify the application of its success. The OP
|
|
now accepts data from the application's TCP stream, packaging it into
|
|
<em>relay data</em> cells and sending those cells along the circuit to
|
|
the chosen OR.
|
|
|
|
<div class="p"><!----></div>
|
|
There's a catch to using SOCKS, however-some applications pass the
|
|
alphanumeric hostname to the Tor client, while others resolve it into
|
|
an IP address first and then pass the IP address to the Tor client. If
|
|
the application does DNS resolution first, Alice thereby reveals her
|
|
destination to the remote DNS server, rather than sending the hostname
|
|
through the Tor network to be resolved at the far end. Common applications
|
|
like Mozilla and SSH have this flaw.
|
|
|
|
<div class="p"><!----></div>
|
|
With Mozilla, the flaw is easy to address: the filtering HTTP
|
|
proxy called Privoxy gives a hostname to the Tor client, so Alice's
|
|
computer never does DNS resolution.
|
|
But a portable general solution, such as is needed for
|
|
SSH, is
|
|
an open problem. Modifying or replacing the local nameserver
|
|
can be invasive, brittle, and unportable. Forcing the resolver
|
|
library to prefer TCP rather than UDP is hard, and also has
|
|
portability problems. Dynamically intercepting system calls to the
|
|
resolver library seems a promising direction. We could also provide
|
|
a tool similar to <em>dig</em> to perform a private lookup through the
|
|
Tor network. Currently, we encourage the use of privacy-aware proxies
|
|
like Privoxy wherever possible.
|
|
|
|
<div class="p"><!----></div>
|
|
Closing a Tor stream is analogous to closing a TCP stream: it uses a
|
|
two-step handshake for normal operation, or a one-step handshake for
|
|
errors. If the stream closes abnormally, the adjacent node simply sends a
|
|
<em>relay teardown</em> cell. If the stream closes normally, the node sends
|
|
a <em>relay end</em> cell down the circuit, and the other side responds with
|
|
its own <em>relay end</em> cell. Because
|
|
all relay cells use layered encryption, only the destination OR knows
|
|
that a given relay cell is a request to close a stream. This two-step
|
|
handshake allows Tor to support TCP-based applications that use half-closed
|
|
connections.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.4">
|
|
4.4</a> Integrity checking on streams</h3>
|
|
<a name="subsec:integrity-checking">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Because the old Onion Routing design used a stream cipher without integrity
|
|
checking, traffic was
|
|
vulnerable to a malleability attack: though the attacker could not
|
|
decrypt cells, any changes to encrypted data
|
|
would create corresponding changes to the data leaving the network.
|
|
This weakness allowed an adversary who could guess the encrypted content
|
|
to change a padding cell to a destroy
|
|
cell; change the destination address in a <em>relay begin</em> cell to the
|
|
adversary's webserver; or change an FTP command from
|
|
<tt>dir</tt> to <tt>rm *</tt>. (Even an external
|
|
adversary could do this, because the link encryption similarly used a
|
|
stream cipher.)
|
|
|
|
<div class="p"><!----></div>
|
|
Because Tor uses TLS on its links, external adversaries cannot modify
|
|
data. Addressing the insider malleability attack, however, is
|
|
more complex.
|
|
|
|
<div class="p"><!----></div>
|
|
We could do integrity checking of the relay cells at each hop, either
|
|
by including hashes or by using an authenticating cipher mode like
|
|
EAX [<a href="#eax" name="CITEeax">6</a>], but there are some problems. First, these approaches
|
|
impose a message-expansion overhead at each hop, and so we would have to
|
|
either leak the path length or waste bytes by padding to a maximum
|
|
path length. Second, these solutions can only verify traffic coming
|
|
from Alice: ORs would not be able to produce suitable hashes for
|
|
the intermediate hops, since the ORs on a circuit do not know the
|
|
other ORs' session keys. Third, we have already accepted that our design
|
|
is vulnerable to end-to-end timing attacks; so tagging attacks performed
|
|
within the circuit provide no additional information to the attacker.
|
|
|
|
<div class="p"><!----></div>
|
|
Thus, we check integrity only at the edges of each stream. (Remember that
|
|
in our leaky-pipe circuit topology, a stream's edge could be any hop
|
|
in the circuit.) When Alice
|
|
negotiates a key with a new hop, they each initialize a SHA-1
|
|
digest with a derivative of that key,
|
|
thus beginning with randomness that only the two of them know.
|
|
Then they each incrementally add to the SHA-1 digest the contents of
|
|
all relay cells they create, and include with each relay cell the
|
|
first four bytes of the current digest. Each also keeps a SHA-1
|
|
digest of data received, to verify that the received hashes are correct.
|
|
|
|
<div class="p"><!----></div>
|
|
To be sure of removing or modifying a cell, the attacker must be able
|
|
to deduce the current digest state (which depends on all
|
|
traffic between Alice and Bob, starting with their negotiated key).
|
|
Attacks on SHA-1 where the adversary can incrementally add to a hash
|
|
to produce a new valid hash don't work, because all hashes are
|
|
end-to-end encrypted across the circuit. The computational overhead
|
|
of computing the digests is minimal compared to doing the AES
|
|
encryption performed at each hop of the circuit. We use only four
|
|
bytes per cell to minimize overhead; the chance that an adversary will
|
|
correctly guess a valid hash
|
|
is
|
|
acceptably low, given that the OP or OR tear down the circuit if they
|
|
receive a bad hash.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.5">
|
|
4.5</a> Rate limiting and fairness</h3>
|
|
<a name="subsec:rate-limit">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Volunteers are more willing to run services that can limit
|
|
their bandwidth usage. To accommodate them, Tor servers use a
|
|
token bucket approach [<a href="#tannenbaum96" name="CITEtannenbaum96">50</a>] to
|
|
enforce a long-term average rate of incoming bytes, while still
|
|
permitting short-term bursts above the allowed bandwidth.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
Because the Tor protocol outputs about the same number of bytes as it
|
|
takes in, it is sufficient in practice to limit only incoming bytes.
|
|
With TCP streams, however, the correspondence is not one-to-one:
|
|
relaying a single incoming byte can require an entire 512-byte cell.
|
|
(We can't just wait for more bytes, because the local application may
|
|
be awaiting a reply.) Therefore, we treat this case as if the entire
|
|
cell size had been read, regardless of the cell's fullness.
|
|
|
|
<div class="p"><!----></div>
|
|
Further, inspired by Rennhard et al's design in [<a href="#anonnet" name="CITEanonnet">44</a>], a
|
|
circuit's edges can heuristically distinguish interactive streams from bulk
|
|
streams by comparing the frequency with which they supply cells. We can
|
|
provide good latency for interactive streams by giving them preferential
|
|
service, while still giving good overall throughput to the bulk
|
|
streams. Such preferential treatment presents a possible end-to-end
|
|
attack, but an adversary observing both
|
|
ends of the stream can already learn this information through timing
|
|
attacks.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.6">
|
|
4.6</a> Congestion control</h3>
|
|
<a name="subsec:congestion">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Even with bandwidth rate limiting, we still need to worry about
|
|
congestion, either accidental or intentional. If enough users choose the
|
|
same OR-to-OR connection for their circuits, that connection can become
|
|
saturated. For example, an attacker could send a large file
|
|
through the Tor network to a webserver he runs, and then
|
|
refuse to read any of the bytes at the webserver end of the
|
|
circuit. Without some congestion control mechanism, these bottlenecks
|
|
can propagate back through the entire network. We don't need to
|
|
reimplement full TCP windows (with sequence numbers,
|
|
the ability to drop cells when we're full and retransmit later, and so
|
|
on),
|
|
because TCP already guarantees in-order delivery of each
|
|
cell.
|
|
We describe our response below.
|
|
|
|
<div class="p"><!----></div>
|
|
<b>Circuit-level throttling:</b>
|
|
To control a circuit's bandwidth usage, each OR keeps track of two
|
|
windows. The <em>packaging window</em> tracks how many relay data cells the OR is
|
|
allowed to package (from incoming TCP streams) for transmission back to the OP,
|
|
and the <em>delivery window</em> tracks how many relay data cells it is willing
|
|
to deliver to TCP streams outside the network. Each window is initialized
|
|
(say, to 1000 data cells). When a data cell is packaged or delivered,
|
|
the appropriate window is decremented. When an OR has received enough
|
|
data cells (currently 100), it sends a <em>relay sendme</em> cell towards the OP,
|
|
with streamID zero. When an OR receives a <em>relay sendme</em> cell with
|
|
streamID zero, it increments its packaging window. Either of these cells
|
|
increments the corresponding window by 100. If the packaging window
|
|
reaches 0, the OR stops reading from TCP connections for all streams
|
|
on the corresponding circuit, and sends no more relay data cells until
|
|
receiving a <em>relay sendme</em> cell.
|
|
|
|
<div class="p"><!----></div>
|
|
The OP behaves identically, except that it must track a packaging window
|
|
and a delivery window for every OR in the circuit. If a packaging window
|
|
reaches 0, it stops reading from streams destined for that OR.
|
|
|
|
<div class="p"><!----></div>
|
|
<b>Stream-level throttling</b>:
|
|
The stream-level congestion control mechanism is similar to the
|
|
circuit-level mechanism. ORs and OPs use <em>relay sendme</em> cells
|
|
to implement end-to-end flow control for individual streams across
|
|
circuits. Each stream begins with a packaging window (currently 500 cells),
|
|
and increments the window by a fixed value (50) upon receiving a <em>relay
|
|
sendme</em> cell. Rather than always returning a <em>relay sendme</em> cell as soon
|
|
as enough cells have arrived, the stream-level congestion control also
|
|
has to check whether data has been successfully flushed onto the TCP
|
|
stream; it sends the <em>relay sendme</em> cell only when the number of bytes pending
|
|
to be flushed is under some threshold (currently 10 cells' worth).
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
These arbitrarily chosen parameters seem to give tolerable throughput
|
|
and delay; see Section <a href="#sec:in-the-wild">8</a>.
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc5">
|
|
5</a> Rendezvous Points and hidden services</h2>
|
|
<a name="sec:rendezvous">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Rendezvous points are a building block for <em>location-hidden
|
|
services</em> (also known as <em>responder anonymity</em>) in the Tor
|
|
network. Location-hidden services allow Bob to offer a TCP
|
|
service, such as a webserver, without revealing his IP address.
|
|
This type of anonymity protects against distributed DoS attacks:
|
|
attackers are forced to attack the onion routing network
|
|
because they do not know Bob's IP address.
|
|
|
|
<div class="p"><!----></div>
|
|
Our design for location-hidden servers has the following goals.
|
|
<b>Access-control:</b> Bob needs a way to filter incoming requests,
|
|
so an attacker cannot flood Bob simply by making many connections to him.
|
|
<b>Robustness:</b> Bob should be able to maintain a long-term pseudonymous
|
|
identity even in the presence of router failure. Bob's service must
|
|
not be tied to a single OR, and Bob must be able to migrate his service
|
|
across ORs. <b>Smear-resistance:</b>
|
|
A social attacker
|
|
should not be able to "frame" a rendezvous router by
|
|
offering an illegal or disreputable location-hidden service and
|
|
making observers believe the router created that service.
|
|
<b>Application-transparency:</b> Although we require users
|
|
to run special software to access location-hidden servers, we must not
|
|
require them to modify their applications.
|
|
|
|
<div class="p"><!----></div>
|
|
We provide location-hiding for Bob by allowing him to advertise
|
|
several onion routers (his <em>introduction points</em>) as contact
|
|
points. He may do this on any robust efficient
|
|
key-value lookup system with authenticated updates, such as a
|
|
distributed hash table (DHT) like CFS [<a href="#cfs:sosp01" name="CITEcfs:sosp01">11</a>].<a href="#tthFtNtAAD" name="tthFrefAAD"><sup>3</sup></a> Alice, the client, chooses an OR as her
|
|
<em>rendezvous point</em>. She connects to one of Bob's introduction
|
|
points, informs him of her rendezvous point, and then waits for him
|
|
to connect to the rendezvous point. This extra level of indirection
|
|
helps Bob's introduction points avoid problems associated with serving
|
|
unpopular files directly (for example, if Bob serves
|
|
material that the introduction point's community finds objectionable,
|
|
or if Bob's service tends to get attacked by network vandals).
|
|
The extra level of indirection also allows Bob to respond to some requests
|
|
and ignore others.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc5.1">
|
|
5.1</a> Rendezvous points in Tor</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
The following steps are
|
|
performed on behalf of Alice and Bob by their local OPs;
|
|
application integration is described more fully below.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<dl compact="compact">
|
|
|
|
<dt><b></b></dt>
|
|
<dd><li>Bob generates a long-term public key pair to identify his service.</dd>
|
|
<dt><b></b></dt>
|
|
<dd><li>Bob chooses some introduction points, and advertises them on
|
|
the lookup service, signing the advertisement with his public key. He
|
|
can add more later.</dd>
|
|
<dt><b></b></dt>
|
|
<dd><li>Bob builds a circuit to each of his introduction points, and tells
|
|
them to wait for requests.</dd>
|
|
<dt><b></b></dt>
|
|
<dd><li>Alice learns about Bob's service out of band (perhaps Bob told her,
|
|
or she found it on a website). She retrieves the details of Bob's
|
|
service from the lookup service. If Alice wants to access Bob's
|
|
service anonymously, she must connect to the lookup service via Tor.</dd>
|
|
<dt><b></b></dt>
|
|
<dd><li>Alice chooses an OR as the rendezvous point (RP) for her connection to
|
|
Bob's service. She builds a circuit to the RP, and gives it a
|
|
randomly chosen "rendezvous cookie" to recognize Bob.</dd>
|
|
<dt><b></b></dt>
|
|
<dd><li>Alice opens an anonymous stream to one of Bob's introduction
|
|
points, and gives it a message (encrypted with Bob's public key)
|
|
telling it about herself,
|
|
her RP and rendezvous cookie, and the
|
|
start of a DH
|
|
handshake. The introduction point sends the message to Bob.</dd>
|
|
<dt><b></b></dt>
|
|
<dd><li>If Bob wants to talk to Alice, he builds a circuit to Alice's
|
|
RP and sends the rendezvous cookie, the second half of the DH
|
|
handshake, and a hash of the session
|
|
key they now share. By the same argument as in
|
|
Section <a href="#subsubsec:constructing-a-circuit">4.2</a>, Alice knows she
|
|
shares the key only with Bob.</dd>
|
|
<dt><b></b></dt>
|
|
<dd><li>The RP connects Alice's circuit to Bob's. Note that RP can't
|
|
recognize Alice, Bob, or the data they transmit.</dd>
|
|
<dt><b></b></dt>
|
|
<dd><li>Alice sends a <em>relay begin</em> cell along the circuit. It
|
|
arrives at Bob's OP, which connects to Bob's
|
|
webserver.</dd>
|
|
<dt><b></b></dt>
|
|
<dd><li>An anonymous stream has been established, and Alice and Bob
|
|
communicate as normal.
|
|
</dd>
|
|
</dl>
|
|
|
|
<div class="p"><!----></div>
|
|
When establishing an introduction point, Bob provides the onion router
|
|
with the public key identifying his service. Bob signs his
|
|
messages, so others cannot usurp his introduction point
|
|
in the future. He uses the same public key to establish the other
|
|
introduction points for his service, and periodically refreshes his
|
|
entry in the lookup service.
|
|
|
|
<div class="p"><!----></div>
|
|
The message that Alice gives
|
|
the introduction point includes a hash of Bob's public key and an optional initial authorization token (the
|
|
introduction point can do prescreening, for example to block replays). Her
|
|
message to Bob may include an end-to-end authorization token so Bob
|
|
can choose whether to respond.
|
|
The authorization tokens can be used to provide selective access:
|
|
important users can get uninterrupted access.
|
|
During normal situations, Bob's service might simply be offered
|
|
directly from mirrors, while Bob gives out tokens to high-priority users. If
|
|
the mirrors are knocked down,
|
|
those users can switch to accessing Bob's service via
|
|
the Tor rendezvous system.
|
|
|
|
<div class="p"><!----></div>
|
|
Bob's introduction points are themselves subject to DoS-he must
|
|
open many introduction points or risk such an attack.
|
|
He can provide selected users with a current list or future schedule of
|
|
unadvertised introduction points;
|
|
this is most practical
|
|
if there is a stable and large group of introduction points
|
|
available. Bob could also give secret public keys
|
|
for consulting the lookup service. All of these approaches
|
|
limit exposure even when
|
|
some selected users collude in the DoS.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc5.2">
|
|
5.2</a> Integration with user applications</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Bob configures his onion proxy to know the local IP address and port of his
|
|
service, a strategy for authorizing clients, and his public key. The onion
|
|
proxy anonymously publishes a signed statement of Bob's
|
|
public key, an expiration time, and
|
|
the current introduction points for his service onto the lookup service,
|
|
indexed
|
|
by the hash of his public key. Bob's webserver is unmodified,
|
|
and doesn't even know that it's hidden behind the Tor network.
|
|
|
|
<div class="p"><!----></div>
|
|
Alice's applications also work unchanged-her client interface
|
|
remains a SOCKS proxy. We encode all of the necessary information
|
|
into the fully qualified domain name (FQDN) Alice uses when establishing her
|
|
connection. Location-hidden services use a virtual top level domain
|
|
called <tt>.onion</tt>: thus hostnames take the form <tt>x.y.onion</tt> where
|
|
<tt>x</tt> is the authorization cookie and <tt>y</tt> encodes the hash of
|
|
the public key. Alice's onion proxy
|
|
examines addresses; if they're destined for a hidden server, it decodes
|
|
the key and starts the rendezvous as described above.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc5.3">
|
|
5.3</a> Previous rendezvous work</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Rendezvous points in low-latency anonymity systems were first
|
|
described for use in ISDN telephony [<a href="#jerichow-jsac98" name="CITEjerichow-jsac98">30</a>,<a href="#isdn-mixes" name="CITEisdn-mixes">38</a>].
|
|
Later low-latency designs used rendezvous points for hiding location
|
|
of mobile phones and low-power location
|
|
trackers [<a href="#federrath-ih96" name="CITEfederrath-ih96">23</a>,<a href="#reed-protocols97" name="CITEreed-protocols97">40</a>]. Rendezvous for
|
|
anonymizing low-latency
|
|
Internet connections was suggested in early Onion Routing
|
|
work [<a href="#or-ih96" name="CITEor-ih96">27</a>], but the first published design was by Ian
|
|
Goldberg [<a href="#ian-thesis" name="CITEian-thesis">26</a>]. His design differs from
|
|
ours in three ways. First, Goldberg suggests that Alice should manually
|
|
hunt down a current location of the service via Gnutella; our approach
|
|
makes lookup transparent to the user, as well as faster and more robust.
|
|
Second, in Tor the client and server negotiate session keys
|
|
with Diffie-Hellman, so plaintext is not exposed even at the rendezvous
|
|
point. Third,
|
|
our design minimizes the exposure from running the
|
|
service, to encourage volunteers to offer introduction and rendezvous
|
|
services. Tor's introduction points do not output any bytes to the
|
|
clients; the rendezvous points don't know the client or the server,
|
|
and can't read the data being transmitted. The indirection scheme is
|
|
also designed to include authentication/authorization-if Alice doesn't
|
|
include the right cookie with her request for service, Bob need not even
|
|
acknowledge his existence.
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc6">
|
|
6</a> Other design decisions</h2>
|
|
<a name="sec:other-design">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc6.1">
|
|
6.1</a> Denial of service</h3>
|
|
<a name="subsec:dos">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Providing Tor as a public service creates many opportunities for
|
|
denial-of-service attacks against the network. While
|
|
flow control and rate limiting (discussed in
|
|
Section <a href="#subsec:congestion">4.6</a>) prevent users from consuming more
|
|
bandwidth than routers are willing to provide, opportunities remain for
|
|
users to
|
|
consume more network resources than their fair share, or to render the
|
|
network unusable for others.
|
|
|
|
<div class="p"><!----></div>
|
|
First of all, there are several CPU-consuming denial-of-service
|
|
attacks wherein an attacker can force an OR to perform expensive
|
|
cryptographic operations. For example, an attacker can
|
|
fake the start of a TLS handshake, forcing the OR to carry out its
|
|
(comparatively expensive) half of the handshake at no real computational
|
|
cost to the attacker.
|
|
|
|
<div class="p"><!----></div>
|
|
We have not yet implemented any defenses for these attacks, but several
|
|
approaches are possible. First, ORs can
|
|
require clients to solve a puzzle [<a href="#puzzles-tls" name="CITEpuzzles-tls">16</a>] while beginning new
|
|
TLS handshakes or accepting <em>create</em> cells. So long as these
|
|
tokens are easy to verify and computationally expensive to produce, this
|
|
approach limits the attack multiplier. Additionally, ORs can limit
|
|
the rate at which they accept <em>create</em> cells and TLS connections,
|
|
so that
|
|
the computational work of processing them does not drown out the
|
|
symmetric cryptography operations that keep cells
|
|
flowing. This rate limiting could, however, allow an attacker
|
|
to slow down other users when they build new circuits.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
Adversaries can also attack the Tor network's hosts and network
|
|
links. Disrupting a single circuit or link breaks all streams passing
|
|
along that part of the circuit. Users similarly lose service
|
|
when a router crashes or its operator restarts it. The current
|
|
Tor design treats such attacks as intermittent network failures, and
|
|
depends on users and applications to respond or recover as appropriate. A
|
|
future design could use an end-to-end TCP-like acknowledgment protocol,
|
|
so no streams are lost unless the entry or exit point is
|
|
disrupted. This solution would require more buffering at the network
|
|
edges, however, and the performance and anonymity implications from this
|
|
extra complexity still require investigation.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc6.2">
|
|
6.2</a> Exit policies and abuse</h3>
|
|
<a name="subsec:exitpolicies">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
Exit abuse is a serious barrier to wide-scale Tor deployment. Anonymity
|
|
presents would-be vandals and abusers with an opportunity to hide
|
|
the origins of their activities. Attackers can harm the Tor network by
|
|
implicating exit servers for their abuse. Also, applications that commonly
|
|
use IP-based authentication (such as institutional mail or webservers)
|
|
can be fooled by the fact that anonymous connections appear to originate
|
|
at the exit OR.
|
|
|
|
<div class="p"><!----></div>
|
|
We stress that Tor does not enable any new class of abuse. Spammers
|
|
and other attackers already have access to thousands of misconfigured
|
|
systems worldwide, and the Tor network is far from the easiest way
|
|
to launch attacks.
|
|
But because the
|
|
onion routers can be mistaken for the originators of the abuse,
|
|
and the volunteers who run them may not want to deal with the hassle of
|
|
explaining anonymity networks to irate administrators, we must block or limit
|
|
abuse through the Tor network.
|
|
|
|
<div class="p"><!----></div>
|
|
To mitigate abuse issues, each onion router's <em>exit policy</em>
|
|
describes to which external addresses and ports the router will
|
|
connect. On one end of the spectrum are <em>open exit</em>
|
|
nodes that will connect anywhere. On the other end are <em>middleman</em>
|
|
nodes that only relay traffic to other Tor nodes, and <em>private exit</em>
|
|
nodes that only connect to a local host or network. A private
|
|
exit can allow a client to connect to a given host or
|
|
network more securely-an external adversary cannot eavesdrop traffic
|
|
between the private exit and the final destination, and so is less sure of
|
|
Alice's destination and activities. Most onion routers in the current
|
|
network function as
|
|
<em>restricted exits</em> that permit connections to the world at large,
|
|
but prevent access to certain abuse-prone addresses and services such
|
|
as SMTP.
|
|
The OR might also be able to authenticate clients to
|
|
prevent exit abuse without harming anonymity [<a href="#or-discex00" name="CITEor-discex00">48</a>].
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
Many administrators use port restrictions to support only a
|
|
limited set of services, such as HTTP, SSH, or AIM.
|
|
This is not a complete solution, of course, since abuse opportunities for these
|
|
protocols are still well known.
|
|
|
|
<div class="p"><!----></div>
|
|
We have not yet encountered any abuse in the deployed network, but if
|
|
we do we should consider using proxies to clean traffic for certain
|
|
protocols as it leaves the network. For example, much abusive HTTP
|
|
behavior (such as exploiting buffer overflows or well-known script
|
|
vulnerabilities) can be detected in a straightforward manner.
|
|
Similarly, one could run automatic spam filtering software (such as
|
|
SpamAssassin) on email exiting the OR network.
|
|
|
|
<div class="p"><!----></div>
|
|
ORs may also rewrite exiting traffic to append
|
|
headers or other information indicating that the traffic has passed
|
|
through an anonymity service. This approach is commonly used
|
|
by email-only anonymity systems. ORs can also
|
|
run on servers with hostnames like <tt>anonymous</tt> to further
|
|
alert abuse targets to the nature of the anonymous traffic.
|
|
|
|
<div class="p"><!----></div>
|
|
A mixture of open and restricted exit nodes allows the most
|
|
flexibility for volunteers running servers. But while having many
|
|
middleman nodes provides a large and robust network,
|
|
having only a few exit nodes reduces the number of points
|
|
an adversary needs to monitor for traffic analysis, and places a
|
|
greater burden on the exit nodes. This tension can be seen in the
|
|
Java Anon Proxy
|
|
cascade model, wherein only one node in each cascade needs to handle
|
|
abuse complaints-but an adversary only needs to observe the entry
|
|
and exit of a cascade to perform traffic analysis on all that
|
|
cascade's users. The hydra model (many entries, few exits) presents a
|
|
different compromise: only a few exit nodes are needed, but an
|
|
adversary needs to work harder to watch all the clients; see
|
|
Section <a href="#sec:conclusion">10</a>.
|
|
|
|
<div class="p"><!----></div>
|
|
Finally, we note that exit abuse must not be dismissed as a peripheral
|
|
issue: when a system's public image suffers, it can reduce the number
|
|
and diversity of that system's users, and thereby reduce the anonymity
|
|
of the system itself. Like usability, public perception is a
|
|
security parameter. Sadly, preventing abuse of open exit nodes is an
|
|
unsolved problem, and will probably remain an arms race for the
|
|
foreseeable future. The abuse problems faced by Princeton's CoDeeN
|
|
project [<a href="#darkside" name="CITEdarkside">37</a>] give us a glimpse of likely issues.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc6.3">
|
|
6.3</a> Directory Servers</h3>
|
|
<a name="subsec:dirservers">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
First-generation Onion Routing designs [<a href="#freedom2-arch" name="CITEfreedom2-arch">8</a>,<a href="#or-jsac98" name="CITEor-jsac98">41</a>] used
|
|
in-band network status updates: each router flooded a signed statement
|
|
to its neighbors, which propagated it onward. But anonymizing networks
|
|
have different security goals than typical link-state routing protocols.
|
|
For example, delays (accidental or intentional)
|
|
that can cause different parts of the network to have different views
|
|
of link-state and topology are not only inconvenient: they give
|
|
attackers an opportunity to exploit differences in client knowledge.
|
|
We also worry about attacks to deceive a
|
|
client about the router membership list, topology, or current network
|
|
state. Such <em>partitioning attacks</em> on client knowledge help an
|
|
adversary to efficiently deploy resources
|
|
against a target [<a href="#minion-design" name="CITEminion-design">15</a>].
|
|
|
|
<div class="p"><!----></div>
|
|
Tor uses a small group of redundant, well-known onion routers to
|
|
track changes in network topology and node state, including keys and
|
|
exit policies. Each such <em>directory server</em> acts as an HTTP
|
|
server, so clients can fetch current network state
|
|
and router lists, and so other ORs can upload
|
|
state information. Onion routers periodically publish signed
|
|
statements of their state to each directory server. The directory servers
|
|
combine this information with their own views of network liveness,
|
|
and generate a signed description (a <em>directory</em>) of the entire
|
|
network state. Client software is
|
|
pre-loaded with a list of the directory servers and their keys,
|
|
to bootstrap each client's view of the network.
|
|
|
|
<div class="p"><!----></div>
|
|
When a directory server receives a signed statement for an OR, it
|
|
checks whether the OR's identity key is recognized. Directory
|
|
servers do not advertise unrecognized ORs-if they did,
|
|
an adversary could take over the network by creating many
|
|
servers [<a href="#sybil" name="CITEsybil">22</a>]. Instead, new nodes must be approved by the
|
|
directory
|
|
server administrator before they are included. Mechanisms for automated
|
|
node approval are an area of active research, and are discussed more
|
|
in Section <a href="#sec:maintaining-anonymity">9</a>.
|
|
|
|
<div class="p"><!----></div>
|
|
Of course, a variety of attacks remain. An adversary who controls
|
|
a directory server can track clients by providing them different
|
|
information-perhaps by listing only nodes under its control, or by
|
|
informing only certain clients about a given node. Even an external
|
|
adversary can exploit differences in client knowledge: clients who use
|
|
a node listed on one directory server but not the others are vulnerable.
|
|
|
|
<div class="p"><!----></div>
|
|
Thus these directory servers must be synchronized and redundant, so
|
|
that they can agree on a common directory. Clients should only trust
|
|
this directory if it is signed by a threshold of the directory
|
|
servers.
|
|
|
|
<div class="p"><!----></div>
|
|
The directory servers in Tor are modeled after those in
|
|
Mixminion [<a href="#minion-design" name="CITEminion-design">15</a>], but our situation is easier. First,
|
|
we make the
|
|
simplifying assumption that all participants agree on the set of
|
|
directory servers. Second, while Mixminion needs to predict node
|
|
behavior, Tor only needs a threshold consensus of the current
|
|
state of the network. Third, we assume that we can fall back to the
|
|
human administrators to discover and resolve problems when a consensus
|
|
directory cannot be reached. Since there are relatively few directory
|
|
servers (currently 3, but we expect as many as 9 as the network scales),
|
|
we can afford operations like broadcast to simplify the consensus-building
|
|
protocol.
|
|
|
|
<div class="p"><!----></div>
|
|
To avoid attacks where a router connects to all the directory servers
|
|
but refuses to relay traffic from other routers, the directory servers
|
|
must also build circuits and use them to anonymously test router
|
|
reliability [<a href="#mix-acc" name="CITEmix-acc">18</a>]. Unfortunately, this defense is not yet
|
|
designed or
|
|
implemented.
|
|
|
|
<div class="p"><!----></div>
|
|
Using directory servers is simpler and more flexible than flooding.
|
|
Flooding is expensive, and complicates the analysis when we
|
|
start experimenting with non-clique network topologies. Signed
|
|
directories can be cached by other
|
|
onion routers,
|
|
so directory servers are not a performance
|
|
bottleneck when we have many users, and do not aid traffic analysis by
|
|
forcing clients to announce their existence to any
|
|
central point.
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc7">
|
|
7</a> Attacks and Defenses</h2>
|
|
<a name="sec:attacks">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Below we summarize a variety of attacks, and discuss how well our
|
|
design withstands them.<br />
|
|
|
|
<div class="p"><!----></div>
|
|
<font size="+1"><b>Passive attacks</b></font><br />
|
|
<em>Observing user traffic patterns.</em> Observing a user's connection
|
|
will not reveal her destination or data, but it will
|
|
reveal traffic patterns (both sent and received). Profiling via user
|
|
connection patterns requires further processing, because multiple
|
|
application streams may be operating simultaneously or in series over
|
|
a single circuit.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Observing user content.</em> While content at the user end is encrypted,
|
|
connections to responders may not be (indeed, the responding website
|
|
itself may be hostile). While filtering content is not a primary goal
|
|
of Onion Routing, Tor can directly use Privoxy and related
|
|
filtering services to anonymize application data streams.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Option distinguishability.</em> We allow clients to choose
|
|
configuration options. For example, clients concerned about request
|
|
linkability should rotate circuits more often than those concerned
|
|
about traceability. Allowing choice may attract users with different
|
|
needs; but clients who are
|
|
in the minority may lose more anonymity by appearing distinct than they
|
|
gain by optimizing their behavior [<a href="#econymics" name="CITEeconymics">1</a>].
|
|
|
|
<div class="p"><!----></div>
|
|
<em>End-to-end timing correlation.</em> Tor only minimally hides
|
|
such correlations. An attacker watching patterns of
|
|
traffic at the initiator and the responder will be
|
|
able to confirm the correspondence with high probability. The
|
|
greatest protection currently available against such confirmation is to hide
|
|
the connection between the onion proxy and the first Tor node,
|
|
by running the OP on the Tor node or behind a firewall. This approach
|
|
requires an observer to separate traffic originating at the onion
|
|
router from traffic passing through it: a global observer can do this,
|
|
but it might be beyond a limited observer's capabilities.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>End-to-end size correlation.</em> Simple packet counting
|
|
will also be effective in confirming
|
|
endpoints of a stream. However, even without padding, we may have some
|
|
limited protection: the leaky pipe topology means different numbers
|
|
of packets may enter one end of a circuit than exit at the other.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Website fingerprinting.</em> All the effective passive
|
|
attacks above are traffic confirmation attacks,
|
|
which puts them outside our design goals. There is also
|
|
a passive traffic analysis attack that is potentially effective.
|
|
Rather than searching exit connections for timing and volume
|
|
correlations, the adversary may build up a database of
|
|
"fingerprints" containing file sizes and access patterns for
|
|
targeted websites. He can later confirm a user's connection to a given
|
|
site simply by consulting the database. This attack has
|
|
been shown to be effective against SafeWeb [<a href="#hintz-pet02" name="CITEhintz-pet02">29</a>].
|
|
It may be less effective against Tor, since
|
|
streams are multiplexed within the same circuit, and
|
|
fingerprinting will be limited to
|
|
the granularity of cells (currently 512 bytes). Additional
|
|
defenses could include
|
|
larger cell sizes, padding schemes to group websites
|
|
into large sets, and link
|
|
padding or long-range dummies.<a href="#tthFtNtAAE" name="tthFrefAAE"><sup>4</sup></a><br />
|
|
|
|
<div class="p"><!----></div>
|
|
<font size="+1"><b>Active attacks</b></font><br />
|
|
<em>Compromise keys.</em> An attacker who learns the TLS session key can
|
|
see control cells and encrypted relay cells on every circuit on that
|
|
connection; learning a circuit
|
|
session key lets him unwrap one layer of the encryption. An attacker
|
|
who learns an OR's TLS private key can impersonate that OR for the TLS
|
|
key's lifetime, but he must
|
|
also learn the onion key to decrypt <em>create</em> cells (and because of
|
|
perfect forward secrecy, he cannot hijack already established circuits
|
|
without also compromising their session keys). Periodic key rotation
|
|
limits the window of opportunity for these attacks. On the other hand,
|
|
an attacker who learns a node's identity key can replace that node
|
|
indefinitely by sending new forged descriptors to the directory servers.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Iterated compromise.</em> A roving adversary who can
|
|
compromise ORs (by system intrusion, legal coercion, or extralegal
|
|
coercion) could march down the circuit compromising the
|
|
nodes until he reaches the end. Unless the adversary can complete
|
|
this attack within the lifetime of the circuit, however, the ORs
|
|
will have discarded the necessary information before the attack can
|
|
be completed. (Thanks to the perfect forward secrecy of session
|
|
keys, the attacker cannot force nodes to decrypt recorded
|
|
traffic once the circuits have been closed.) Additionally, building
|
|
circuits that cross jurisdictions can make legal coercion
|
|
harder-this phenomenon is commonly called "jurisdictional
|
|
arbitrage." The Java Anon Proxy project recently experienced the
|
|
need for this approach, when
|
|
a German court forced them to add a backdoor to
|
|
their nodes [<a href="#jap-backdoor" name="CITEjap-backdoor">51</a>].
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Run a recipient.</em> An adversary running a webserver
|
|
trivially learns the timing patterns of users connecting to it, and
|
|
can introduce arbitrary patterns in its responses.
|
|
End-to-end attacks become easier: if the adversary can induce
|
|
users to connect to his webserver (perhaps by advertising
|
|
content targeted to those users), he now holds one end of their
|
|
connection. There is also a danger that application
|
|
protocols and associated programs can be induced to reveal information
|
|
about the initiator. Tor depends on Privoxy and similar protocol cleaners
|
|
to solve this latter problem.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Run an onion proxy.</em> It is expected that end users will
|
|
nearly always run their own local onion proxy. However, in some
|
|
settings, it may be necessary for the proxy to run
|
|
remotely-typically, in institutions that want
|
|
to monitor the activity of those connecting to the proxy.
|
|
Compromising an onion proxy compromises all future connections
|
|
through it.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>DoS non-observed nodes.</em> An observer who can only watch some
|
|
of the Tor network can increase the value of this traffic
|
|
by attacking non-observed nodes to shut them down, reduce
|
|
their reliability, or persuade users that they are not trustworthy.
|
|
The best defense here is robustness.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Run a hostile OR.</em> In addition to being a local observer,
|
|
an isolated hostile node can create circuits through itself, or alter
|
|
traffic patterns to affect traffic at other nodes. Nonetheless, a hostile
|
|
node must be immediately adjacent to both endpoints to compromise the
|
|
anonymity of a circuit. If an adversary can
|
|
run multiple ORs, and can persuade the directory servers
|
|
that those ORs are trustworthy and independent, then occasionally
|
|
some user will choose one of those ORs for the start and another
|
|
as the end of a circuit. If an adversary
|
|
controls m > 1 of N nodes, he can correlate at most
|
|
([m/N])<sup>2</sup> of the traffic-although an
|
|
adversary
|
|
could still attract a disproportionately large amount of traffic
|
|
by running an OR with a permissive exit policy, or by
|
|
degrading the reliability of other routers.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Introduce timing into messages.</em> This is simply a stronger
|
|
version of passive timing attacks already discussed earlier.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Tagging attacks.</em> A hostile node could "tag" a
|
|
cell by altering it. If the
|
|
stream were, for example, an unencrypted request to a Web site,
|
|
the garbled content coming out at the appropriate time would confirm
|
|
the association. However, integrity checks on cells prevent
|
|
this attack.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Replace contents of unauthenticated protocols.</em> When
|
|
relaying an unauthenticated protocol like HTTP, a hostile exit node
|
|
can impersonate the target server. Clients
|
|
should prefer protocols with end-to-end authentication.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Replay attacks.</em> Some anonymity protocols are vulnerable
|
|
to replay attacks. Tor is not; replaying one side of a handshake
|
|
will result in a different negotiated session key, and so the rest
|
|
of the recorded session can't be used.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Smear attacks.</em> An attacker could use the Tor network for
|
|
socially disapproved acts, to bring the
|
|
network into disrepute and get its operators to shut it down.
|
|
Exit policies reduce the possibilities for abuse, but
|
|
ultimately the network requires volunteers who can tolerate
|
|
some political heat.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Distribute hostile code.</em> An attacker could trick users
|
|
into running subverted Tor software that did not, in fact, anonymize
|
|
their connections-or worse, could trick ORs into running weakened
|
|
software that provided users with less anonymity. We address this
|
|
problem (but do not solve it completely) by signing all Tor releases
|
|
with an official public key, and including an entry in the directory
|
|
that lists which versions are currently believed to be secure. To
|
|
prevent an attacker from subverting the official release itself
|
|
(through threats, bribery, or insider attacks), we provide all
|
|
releases in source code form, encourage source audits, and
|
|
frequently warn our users never to trust any software (even from
|
|
us) that comes without source.<br />
|
|
|
|
<div class="p"><!----></div>
|
|
<font size="+1"><b>Directory attacks</b></font><br />
|
|
<em>Destroy directory servers.</em> If a few directory
|
|
servers disappear, the others still decide on a valid
|
|
directory. So long as any directory servers remain in operation,
|
|
they will still broadcast their views of the network and generate a
|
|
consensus directory. (If more than half are destroyed, this
|
|
directory will not, however, have enough signatures for clients to
|
|
use it automatically; human intervention will be necessary for
|
|
clients to decide whether to trust the resulting directory.)
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Subvert a directory server.</em> By taking over a directory server,
|
|
an attacker can partially influence the final directory. Since ORs
|
|
are included or excluded by majority vote, the corrupt directory can
|
|
at worst cast a tie-breaking vote to decide whether to include
|
|
marginal ORs. It remains to be seen how often such marginal cases
|
|
occur in practice.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Subvert a majority of directory servers.</em> An adversary who controls
|
|
more than half the directory servers can include as many compromised
|
|
ORs in the final directory as he wishes. We must ensure that directory
|
|
server operators are independent and attack-resistant.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Encourage directory server dissent.</em> The directory
|
|
agreement protocol assumes that directory server operators agree on
|
|
the set of directory servers. An adversary who can persuade some
|
|
of the directory server operators to distrust one another could
|
|
split the quorum into mutually hostile camps, thus partitioning
|
|
users based on which directory they use. Tor does not address
|
|
this attack.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Trick the directory servers into listing a hostile OR.</em>
|
|
Our threat model explicitly assumes directory server operators will
|
|
be able to filter out most hostile ORs.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Convince the directories that a malfunctioning OR is
|
|
working.</em> In the current Tor implementation, directory servers
|
|
assume that an OR is running correctly if they can start a TLS
|
|
connection to it. A hostile OR could easily subvert this test by
|
|
accepting TLS connections from ORs but ignoring all cells. Directory
|
|
servers must actively test ORs by building circuits and streams as
|
|
appropriate. The tradeoffs of a similar approach are discussed
|
|
in [<a href="#mix-acc" name="CITEmix-acc">18</a>].<br />
|
|
|
|
<div class="p"><!----></div>
|
|
<font size="+1"><b>Attacks against rendezvous points</b></font><br />
|
|
<em>Make many introduction requests.</em> An attacker could
|
|
try to deny Bob service by flooding his introduction points with
|
|
requests. Because the introduction points can block requests that
|
|
lack authorization tokens, however, Bob can restrict the volume of
|
|
requests he receives, or require a certain amount of computation for
|
|
every request he receives.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Attack an introduction point.</em> An attacker could
|
|
disrupt a location-hidden service by disabling its introduction
|
|
points. But because a service's identity is attached to its public
|
|
key, the service can simply re-advertise
|
|
itself at a different introduction point. Advertisements can also be
|
|
done secretly so that only high-priority clients know the address of
|
|
Bob's introduction points or so that different clients know of different
|
|
introduction points. This forces the attacker to disable all possible
|
|
introduction points.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Compromise an introduction point.</em> An attacker who controls
|
|
Bob's introduction point can flood Bob with
|
|
introduction requests, or prevent valid introduction requests from
|
|
reaching him. Bob can notice a flood, and close the circuit. To notice
|
|
blocking of valid requests, however, he should periodically test the
|
|
introduction point by sending rendezvous requests and making
|
|
sure he receives them.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Compromise a rendezvous point.</em> A rendezvous
|
|
point is no more sensitive than any other OR on
|
|
a circuit, since all data passing through the rendezvous is encrypted
|
|
with a session key shared by Alice and Bob.
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc8">
|
|
8</a> Early experiences: Tor in the Wild</h2>
|
|
<a name="sec:in-the-wild">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
As of mid-May 2004, the Tor network consists of 32 nodes
|
|
(24 in the US, 8 in Europe), and more are joining each week as the code
|
|
matures. (For comparison, the current remailer network
|
|
has about 40 nodes.) Each node has at least a 768Kb/768Kb connection, and
|
|
many have 10Mb. The number of users varies (and of course, it's hard to
|
|
tell for sure), but we sometimes have several hundred users-administrators at
|
|
several companies have begun sending their entire departments' web
|
|
traffic through Tor, to block other divisions of
|
|
their company from reading their traffic. Tor users have reported using
|
|
the network for web browsing, FTP, IRC, AIM, Kazaa, SSH, and
|
|
recipient-anonymous email via rendezvous points. One user has anonymously
|
|
set up a Wiki as a hidden service, where other users anonymously publish
|
|
the addresses of their hidden services.
|
|
|
|
<div class="p"><!----></div>
|
|
Each Tor node currently processes roughly 800,000 relay
|
|
cells (a bit under half a gigabyte) per week. On average, about 80%
|
|
of each 498-byte payload is full for cells going back to the client,
|
|
whereas about 40% is full for cells coming from the client. (The difference
|
|
arises because most of the network's traffic is web browsing.) Interactive
|
|
traffic like SSH brings down the average a lot-once we have more
|
|
experience, and assuming we can resolve the anonymity issues, we may
|
|
partition traffic into two relay cell sizes: one to handle
|
|
bulk traffic and one for interactive traffic.
|
|
|
|
<div class="p"><!----></div>
|
|
Based in part on our restrictive default exit policy (we
|
|
reject SMTP requests) and our low profile, we have had no abuse
|
|
issues since the network was deployed in October
|
|
2003. Our slow growth rate gives us time to add features,
|
|
resolve bugs, and get a feel for what users actually want from an
|
|
anonymity system. Even though having more users would bolster our
|
|
anonymity sets, we are not eager to attract the Kazaa or warez
|
|
communities-we feel that we must build a reputation for privacy, human
|
|
rights, research, and other socially laudable activities.
|
|
|
|
<div class="p"><!----></div>
|
|
As for performance, profiling shows that Tor spends almost
|
|
all its CPU time in AES, which is fast. Current latency is attributable
|
|
to two factors. First, network latency is critical: we are
|
|
intentionally bouncing traffic around the world several times. Second,
|
|
our end-to-end congestion control algorithm focuses on protecting
|
|
volunteer servers from accidental DoS rather than on optimizing
|
|
performance. To quantify these effects, we did some informal tests using a network of 4
|
|
nodes on the same machine (a heavily loaded 1GHz Athlon). We downloaded a 60
|
|
megabyte file from <tt>debian.org</tt> every 30 minutes for 54 hours (108 sample
|
|
points). It arrived in about 300 seconds on average, compared to 210s for a
|
|
direct download. We ran a similar test on the production Tor network,
|
|
fetching the front page of <tt>cnn.com</tt> (55 kilobytes):
|
|
while a direct
|
|
download consistently took about 0.3s, the performance through Tor varied.
|
|
Some downloads were as fast as 0.4s, with a median at 2.8s, and
|
|
90% finishing within 5.3s. It seems that as the network expands, the chance
|
|
of building a slow circuit (one that includes a slow or heavily loaded node
|
|
or link) is increasing. On the other hand, as our users remain satisfied
|
|
with this increased latency, we can address our performance incrementally as we
|
|
proceed with development.
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
Although Tor's clique topology and full-visibility directories present
|
|
scaling problems, we still expect the network to support a few hundred
|
|
nodes and maybe 10,000 users before we're forced to become
|
|
more distributed. With luck, the experience we gain running the current
|
|
topology will help us choose among alternatives when the time comes.
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc9">
|
|
9</a> Open Questions in Low-latency Anonymity</h2>
|
|
<a name="sec:maintaining-anonymity">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
In addition to the non-goals in
|
|
Section <a href="#subsec:non-goals">3</a>, many questions must be solved
|
|
before we can be confident of Tor's security.
|
|
|
|
<div class="p"><!----></div>
|
|
Many of these open issues are questions of balance. For example,
|
|
how often should users rotate to fresh circuits? Frequent rotation
|
|
is inefficient, expensive, and may lead to intersection attacks and
|
|
predecessor attacks [<a href="#wright03" name="CITEwright03">54</a>], but infrequent rotation makes the
|
|
user's traffic linkable. Besides opening fresh circuits, clients can
|
|
also exit from the middle of the circuit,
|
|
or truncate and re-extend the circuit. More analysis is
|
|
needed to determine the proper tradeoff.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
How should we choose path lengths? If Alice always uses two hops,
|
|
then both ORs can be certain that by colluding they will learn about
|
|
Alice and Bob. In our current approach, Alice always chooses at least
|
|
three nodes unrelated to herself and her destination.
|
|
Should Alice choose a random path length (e.g. from a geometric
|
|
distribution) to foil an attacker who
|
|
uses timing to learn that he is the fifth hop and thus concludes that
|
|
both Alice and the responder are running ORs?
|
|
|
|
<div class="p"><!----></div>
|
|
Throughout this paper, we have assumed that end-to-end traffic
|
|
confirmation will immediately and automatically defeat a low-latency
|
|
anonymity system. Even high-latency anonymity systems can be
|
|
vulnerable to end-to-end traffic confirmation, if the traffic volumes
|
|
are high enough, and if users' habits are sufficiently
|
|
distinct [<a href="#statistical-disclosure" name="CITEstatistical-disclosure">14</a>,<a href="#limits-open" name="CITElimits-open">31</a>]. Can anything be
|
|
done to
|
|
make low-latency systems resist these attacks as well as high-latency
|
|
systems? Tor already makes some effort to conceal the starts and ends of
|
|
streams by wrapping long-range control commands in identical-looking
|
|
relay cells. Link padding could frustrate passive observers who count
|
|
packets; long-range padding could work against observers who own the
|
|
first hop in a circuit. But more research remains to find an efficient
|
|
and practical approach. Volunteers prefer not to run constant-bandwidth
|
|
padding; but no convincing traffic shaping approach has been
|
|
specified. Recent work on long-range padding [<a href="#defensive-dropping" name="CITEdefensive-dropping">33</a>]
|
|
shows promise. One could also try to reduce correlation in packet timing
|
|
by batching and re-ordering packets, but it is unclear whether this could
|
|
improve anonymity without introducing so much latency as to render the
|
|
network unusable.
|
|
|
|
<div class="p"><!----></div>
|
|
A cascade topology may better defend against traffic confirmation by
|
|
aggregating users, and making padding and
|
|
mixing more affordable. Does the hydra topology (many input nodes,
|
|
few output nodes) work better against some adversaries? Are we going
|
|
to get a hydra anyway because most nodes will be middleman nodes?
|
|
|
|
<div class="p"><!----></div>
|
|
Common wisdom suggests that Alice should run her own OR for best
|
|
anonymity, because traffic coming from her node could plausibly have
|
|
come from elsewhere. How much mixing does this approach need? Is it
|
|
immediately beneficial because of real-world adversaries that can't
|
|
observe Alice's router, but can run routers of their own?
|
|
|
|
<div class="p"><!----></div>
|
|
To scale to many users, and to prevent an attacker from observing the
|
|
whole network, it may be necessary
|
|
to support far more servers than Tor currently anticipates.
|
|
This introduces several issues. First, if approval by a central set
|
|
of directory servers is no longer feasible, what mechanism should be used
|
|
to prevent adversaries from signing up many colluding servers? Second,
|
|
if clients can no longer have a complete picture of the network,
|
|
how can they perform discovery while preventing attackers from
|
|
manipulating or exploiting gaps in their knowledge? Third, if there
|
|
are too many servers for every server to constantly communicate with
|
|
every other, which non-clique topology should the network use?
|
|
(Restricted-route topologies promise comparable anonymity with better
|
|
scalability [<a href="#danezis-pets03" name="CITEdanezis-pets03">13</a>], but whatever topology we choose, we
|
|
need some way to keep attackers from manipulating their position within
|
|
it [<a href="#casc-rep" name="CITEcasc-rep">21</a>].) Fourth, if no central authority is tracking
|
|
server reliability, how do we stop unreliable servers from making
|
|
the network unusable? Fifth, do clients receive so much anonymity
|
|
from running their own ORs that we should expect them all to do
|
|
so [<a href="#econymics" name="CITEeconymics">1</a>], or do we need another incentive structure to
|
|
motivate them? Tarzan and MorphMix present possible solutions.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
When a Tor node goes down, all its circuits (and thus streams) must break.
|
|
Will users abandon the system because of this brittleness? How well
|
|
does the method in Section <a href="#subsec:dos">6.1</a> allow streams to survive
|
|
node failure? If affected users rebuild circuits immediately, how much
|
|
anonymity is lost? It seems the problem is even worse in a peer-to-peer
|
|
environment-such systems don't yet provide an incentive for peers to
|
|
stay connected when they're done retrieving content, so we would expect
|
|
a higher churn rate.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc10">
|
|
10</a> Future Directions</h2>
|
|
<a name="sec:conclusion">
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Tor brings together many innovations into a unified deployable system. The
|
|
next immediate steps include:
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Scalability:</em> Tor's emphasis on deployability and design simplicity
|
|
has led us to adopt a clique topology, semi-centralized
|
|
directories, and a full-network-visibility model for client
|
|
knowledge. These properties will not scale past a few hundred servers.
|
|
Section <a href="#sec:maintaining-anonymity">9</a> describes some promising
|
|
approaches, but more deployment experience will be helpful in learning
|
|
the relative importance of these bottlenecks.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Bandwidth classes:</em> This paper assumes that all ORs have
|
|
good bandwidth and latency. We should instead adopt the MorphMix model,
|
|
where nodes advertise their bandwidth level (DSL, T1, T3), and
|
|
Alice avoids bottlenecks by choosing nodes that match or
|
|
exceed her bandwidth. In this way DSL users can usefully join the Tor
|
|
network.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Incentives:</em> Volunteers who run nodes are rewarded with publicity
|
|
and possibly better anonymity [<a href="#econymics" name="CITEeconymics">1</a>]. More nodes means increased
|
|
scalability, and more users can mean more anonymity. We need to continue
|
|
examining the incentive structures for participating in Tor. Further,
|
|
we need to explore more approaches to limiting abuse, and understand
|
|
why most people don't bother using privacy systems.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Cover traffic:</em> Currently Tor omits cover traffic-its costs
|
|
in performance and bandwidth are clear but its security benefits are
|
|
not well understood. We must pursue more research on link-level cover
|
|
traffic and long-range cover traffic to determine whether some simple padding
|
|
method offers provable protection against our chosen adversary.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Caching at exit nodes:</em> Perhaps each exit node should run a
|
|
caching web proxy [<a href="#shsm03" name="CITEshsm03">47</a>], to improve anonymity for cached pages
|
|
(Alice's request never
|
|
leaves the Tor network), to improve speed, and to reduce bandwidth cost.
|
|
On the other hand, forward security is weakened because caches
|
|
constitute a record of retrieved files. We must find the right
|
|
balance between usability and security.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Better directory distribution:</em>
|
|
Clients currently download a description of
|
|
the entire network every 15 minutes. As the state grows larger
|
|
and clients more numerous, we may need a solution in which
|
|
clients receive incremental updates to directory state.
|
|
More generally, we must find more
|
|
scalable yet practical ways to distribute up-to-date snapshots of
|
|
network status without introducing new attacks.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Further specification review:</em> Our public
|
|
byte-level specification [<a href="#tor-spec" name="CITEtor-spec">20</a>] needs
|
|
external review. We hope that as Tor
|
|
is deployed, more people will examine its
|
|
specification.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Multisystem interoperability:</em> We are currently working with the
|
|
designer of MorphMix to unify the specification and implementation of
|
|
the common elements of our two systems. So far, this seems
|
|
to be relatively straightforward. Interoperability will allow testing
|
|
and direct comparison of the two designs for trust and scalability.
|
|
|
|
<div class="p"><!----></div>
|
|
<em>Wider-scale deployment:</em> The original goal of Tor was to
|
|
gain experience in deploying an anonymizing overlay network, and
|
|
learn from having actual users. We are now at a point in design
|
|
and development where we can start deploying a wider network. Once
|
|
we have many actual users, we will doubtlessly be better
|
|
able to evaluate some of our design decisions, including our
|
|
robustness/latency tradeoffs, our performance tradeoffs (including
|
|
cell size), our abuse-prevention mechanisms, and
|
|
our overall usability.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<h2>Acknowledgments</h2>
|
|
We thank Peter Palfrader, Geoff Goodell, Adam Shostack, Joseph Sokol-Margolis,
|
|
John Bashinski, and Zack Brown
|
|
for editing and comments;
|
|
Matej Pfajfar, Andrei Serjantov, Marc Rennhard for design discussions;
|
|
Bram Cohen for congestion control discussions;
|
|
Adam Back for suggesting telescoping circuits; and
|
|
Cathy Meadows for formal analysis of the <em>extend</em> protocol.
|
|
This work has been supported by ONR and DARPA.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h2>References</h2>
|
|
|
|
<dl compact="compact">
|
|
<font size="-1"></font> <dt><a href="#CITEeconymics" name="econymics">[1]</a></dt><dd>
|
|
A. Acquisti, R. Dingledine, and P. Syverson.
|
|
On the economics of anonymity.
|
|
In R. N. Wright, editor, <em>Financial Cryptography</em>.
|
|
Springer-Verlag, LNCS 2742, 2003.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEeternity" name="eternity">[2]</a></dt><dd>
|
|
R. Anderson.
|
|
The eternity service.
|
|
In <em>Pragocrypt '96</em>, 1996.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEanonymizer" name="anonymizer">[3]</a></dt><dd>
|
|
The Anonymizer.
|
|
<tt><http://anonymizer.com/>.
|
|
|
|
<div class="p"><!----></div>
|
|
</tt></dd>
|
|
<dt><a href="#CITEfreedom21-security" name="freedom21-security">[4]</a></dt><dd>
|
|
A. Back, I. Goldberg, and A. Shostack.
|
|
Freedom systems 2.1 security issues and analysis.
|
|
White paper, Zero Knowledge Systems, Inc., May 2001.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEback01" name="back01">[5]</a></dt><dd>
|
|
A. Back, U. Möller, and A. Stiglic.
|
|
Traffic analysis attacks and trade-offs in anonymity providing
|
|
systems.
|
|
In I. S. Moskowitz, editor, <em>Information Hiding (IH 2001)</em>, pages
|
|
245-257. Springer-Verlag, LNCS 2137, 2001.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEeax" name="eax">[6]</a></dt><dd>
|
|
M. Bellare, P. Rogaway, and D. Wagner.
|
|
The EAX mode of operation: A two-pass authenticated-encryption
|
|
scheme optimized for simplicity and efficiency.
|
|
In <em>Fast Software Encryption 2004</em>, February 2004.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEweb-mix" name="web-mix">[7]</a></dt><dd>
|
|
O. Berthold, H. Federrath, and S. Köpsell.
|
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Web MIXes: A system for anonymous and unobservable Internet
|
|
access.
|
|
In H. Federrath, editor, <em>Designing Privacy Enhancing
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Technologies: Workshop on Design Issue in Anonymity and Unobservability</em>.
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Springer-Verlag, LNCS 2009, 2000.
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|
<div class="p"><!----></div>
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|
</dd>
|
|
<dt><a href="#CITEfreedom2-arch" name="freedom2-arch">[8]</a></dt><dd>
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|
P. Boucher, A. Shostack, and I. Goldberg.
|
|
Freedom systems 2.0 architecture.
|
|
White paper, Zero Knowledge Systems, Inc., December 2000.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
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|
<dt><a href="#CITEcebolla" name="cebolla">[9]</a></dt><dd>
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|
Z. Brown.
|
|
Cebolla: Pragmatic IP Anonymity.
|
|
In <em>Ottawa Linux Symposium</em>, June 2002.
|
|
|
|
<div class="p"><!----></div>
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|
</dd>
|
|
<dt><a href="#CITEchaum-mix" name="chaum-mix">[10]</a></dt><dd>
|
|
D. Chaum.
|
|
Untraceable electronic mail, return addresses, and digital
|
|
pseudo-nyms.
|
|
<em>Communications of the ACM</em>, 4(2), February 1981.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEcfs:sosp01" name="cfs:sosp01">[11]</a></dt><dd>
|
|
F. Dabek, M. F. Kaashoek, D. Karger, R. Morris, and I. Stoica.
|
|
Wide-area cooperative storage with CFS.
|
|
In <em>18th ACM Symposium on Operating Systems Principles
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(SOSP '01)</em>, Chateau Lake Louise, Banff, Canada, October 2001.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEpipenet" name="pipenet">[12]</a></dt><dd>
|
|
W. Dai.
|
|
Pipenet 1.1.
|
|
Usenet post, August 1996.
|
|
<tt><http://www.eskimo.com/ weidai/pipenet.txt> First mentioned in a
|
|
post to the cypherpunks list, Feb. 1995.
|
|
|
|
<div class="p"><!----></div>
|
|
</tt></dd>
|
|
<dt><a href="#CITEdanezis-pets03" name="danezis-pets03">[13]</a></dt><dd>
|
|
G. Danezis.
|
|
Mix-networks with restricted routes.
|
|
In R. Dingledine, editor, <em>Privacy Enhancing Technologies (PET
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|
2003)</em>. Springer-Verlag LNCS 2760, 2003.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEstatistical-disclosure" name="statistical-disclosure">[14]</a></dt><dd>
|
|
G. Danezis.
|
|
Statistical disclosure attacks.
|
|
In <em>Security and Privacy in the Age of Uncertainty (SEC2003)</em>,
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|
pages 421-426, Athens, May 2003. IFIP TC11, Kluwer.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEminion-design" name="minion-design">[15]</a></dt><dd>
|
|
G. Danezis, R. Dingledine, and N. Mathewson.
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|
Mixminion: Design of a type III anonymous remailer protocol.
|
|
In <em>2003 IEEE Symposium on Security and Privacy</em>, pages 2-15.
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|
IEEE CS, May 2003.
|
|
|
|
<div class="p"><!----></div>
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|
</dd>
|
|
<dt><a href="#CITEpuzzles-tls" name="puzzles-tls">[16]</a></dt><dd>
|
|
D. Dean and A. Stubblefield.
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Using Client Puzzles to Protect TLS.
|
|
In <em>Proceedings of the 10th USENIX Security Symposium</em>. USENIX,
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Aug. 2001.
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|
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|
<div class="p"><!----></div>
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|
</dd>
|
|
<dt><a href="#CITETLS" name="TLS">[17]</a></dt><dd>
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|
T. Dierks and C. Allen.
|
|
The TLS Protocol - Version 1.0.
|
|
IETF RFC 2246, January 1999.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEmix-acc" name="mix-acc">[18]</a></dt><dd>
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|
R. Dingledine, M. J. Freedman, D. Hopwood, and D. Molnar.
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A Reputation System to Increase MIX-net Reliability.
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126-141. Springer-Verlag, LNCS 2137, 2001.
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|
<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEfreehaven-berk" name="freehaven-berk">[19]</a></dt><dd>
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R. Dingledine, M. J. Freedman, and D. Molnar.
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The free haven project: Distributed anonymous storage service.
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In H. Federrath, editor, <em>Designing Privacy Enhancing
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Springer-Verlag, LNCS 2009, July 2000.
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|
<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEtor-spec" name="tor-spec">[20]</a></dt><dd>
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R. Dingledine and N. Mathewson.
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|
Tor protocol specifications.
|
|
<tt><http://freehaven.net/tor/tor-spec.txt>.
|
|
|
|
<div class="p"><!----></div>
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|
</tt></dd>
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|
<dt><a href="#CITEcasc-rep" name="casc-rep">[21]</a></dt><dd>
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R. Dingledine and P. Syverson.
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Reliable MIX Cascade Networks through Reputation.
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LNCS 2357, 2002.
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|
<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEsybil" name="sybil">[22]</a></dt><dd>
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J. Douceur.
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The Sybil Attack.
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<div class="p"><!----></div>
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</dd>
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|
<dt><a href="#CITEfederrath-ih96" name="federrath-ih96">[23]</a></dt><dd>
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H. Federrath, A. Jerichow, and A. Pfitzmann.
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MIXes in mobile communication systems: Location management with
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privacy.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEtarzan:ccs02" name="tarzan:ccs02">[24]</a></dt><dd>
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M. J. Freedman and R. Morris.
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Tarzan: A peer-to-peer anonymizing network layer.
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In <em>9th ACM Conference on Computer and Communications
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|
<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEherbivore" name="herbivore">[25]</a></dt><dd>
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S. Goel, M. Robson, M. Polte, and E. G. Sirer.
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Herbivore: A scalable and efficient protocol for anonymous
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communication.
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Information Science, February 2003.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEian-thesis" name="ian-thesis">[26]</a></dt><dd>
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I. Goldberg.
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<em>A Pseudonymous Communications Infrastructure for the Internet</em>.
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|
PhD thesis, UC Berkeley, Dec 2000.
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|
<div class="p"><!----></div>
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|
</dd>
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<dt><a href="#CITEor-ih96" name="or-ih96">[27]</a></dt><dd>
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D. M. Goldschlag, M. G. Reed, and P. F. Syverson.
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Hiding routing information.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEbabel" name="babel">[28]</a></dt><dd>
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C. Gülcü and G. Tsudik.
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Mixing E-mail with Babel.
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In <em>Network and Distributed Security Symposium (NDSS 96)</em>,
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEhintz-pet02" name="hintz-pet02">[29]</a></dt><dd>
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A. Hintz.
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Fingerprinting websites using traffic analysis.
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In R. Dingledine and P. Syverson, editors, <em>Privacy Enhancing
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEjerichow-jsac98" name="jerichow-jsac98">[30]</a></dt><dd>
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A. Jerichow, J. Müller, A. Pfitzmann, B. Pfitzmann, and M. Waidner.
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Real-time mixes: A bandwidth-efficient anonymity protocol.
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<em>IEEE Journal on Selected Areas in Communications</em>,
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16(4):495-509, May 1998.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITElimits-open" name="limits-open">[31]</a></dt><dd>
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D. Kesdogan, D. Agrawal, and S. Penz.
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Limits of anonymity in open environments.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEsocks4" name="socks4">[32]</a></dt><dd>
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D. Koblas and M. R. Koblas.
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SOCKS.
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In <em>UNIX Security III Symposium (1992 USENIX Security
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEdefensive-dropping" name="defensive-dropping">[33]</a></dt><dd>
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B. N. Levine, M. K. Reiter, C. Wang, and M. Wright.
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Timing analysis in low-latency mix-based systems.
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In A. Juels, editor, <em>Financial Cryptography</em>. Springer-Verlag,
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LNCS (forthcoming), 2004.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEhordes-jcs" name="hordes-jcs">[34]</a></dt><dd>
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B. N. Levine and C. Shields.
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Hordes: A multicast-based protocol for anonymity.
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<em>Journal of Computer Security</em>, 10(3):213-240, 2002.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEmeadows96" name="meadows96">[35]</a></dt><dd>
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C. Meadows.
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The NRL protocol analyzer: An overview.
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<em>Journal of Logic Programming</em>, 26(2):113-131, 1996.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEmixmaster-spec" name="mixmaster-spec">[36]</a></dt><dd>
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U. Möller, L. Cottrell, P. Palfrader, and L. Sassaman.
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Mixmaster Protocol - Version 2.
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Draft, July 2003.
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<tt><http://www.abditum.com/mixmaster-spec.txt>.
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<div class="p"><!----></div>
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</tt></dd>
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<dt><a href="#CITEdarkside" name="darkside">[37]</a></dt><dd>
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V. S. Pai, L. Wang, K. Park, R. Pang, and L. Peterson.
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The Dark Side of the Web: An Open Proxy's View.
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<tt><http://codeen.cs.princeton.edu/>.
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<div class="p"><!----></div>
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</tt></dd>
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<dt><a href="#CITEisdn-mixes" name="isdn-mixes">[38]</a></dt><dd>
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A. Pfitzmann, B. Pfitzmann, and M. Waidner.
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ISDN-mixes: Untraceable communication with very small bandwidth
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overhead.
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In <em>GI/ITG Conference on Communication in Distributed Systems</em>,
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pages 451-463, February 1991.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEprivoxy" name="privoxy">[39]</a></dt><dd>
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Privoxy.
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<tt><http://www.privoxy.org/>.
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|
<div class="p"><!----></div>
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</tt></dd>
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<dt><a href="#CITEreed-protocols97" name="reed-protocols97">[40]</a></dt><dd>
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M. G. Reed, P. F. Syverson, and D. M. Goldschlag.
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Protocols using anonymous connections: Mobile applications.
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In B. Christianson, B. Crispo, M. Lomas, and M. Roe, editors, <em>
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Security Protocols: 5th International Workshop</em>, pages 13-23.
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Springer-Verlag, LNCS 1361, April 1997.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEor-jsac98" name="or-jsac98">[41]</a></dt><dd>
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M. G. Reed, P. F. Syverson, and D. M. Goldschlag.
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Anonymous connections and onion routing.
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<em>IEEE Journal on Selected Areas in Communications</em>,
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16(4):482-494, May 1998.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEcrowds-tissec" name="crowds-tissec">[42]</a></dt><dd>
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M. K. Reiter and A. D. Rubin.
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Crowds: Anonymity for web transactions.
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<em>ACM TISSEC</em>, 1(1):66-92, June 1998.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEmorphmix:fc04" name="morphmix:fc04">[43]</a></dt><dd>
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M. Rennhard and B. Plattner.
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Practical anonymity for the masses with morphmix.
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In A. Juels, editor, <em>Financial Cryptography</em>. Springer-Verlag,
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LNCS (forthcoming), 2004.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEanonnet" name="anonnet">[44]</a></dt><dd>
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M. Rennhard, S. Rafaeli, L. Mathy, B. Plattner, and D. Hutchison.
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Analysis of an Anonymity Network for Web Browsing.
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In <em>IEEE 7th Intl. Workshop on Enterprise Security (WET ICE
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITESS03" name="SS03">[45]</a></dt><dd>
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A. Serjantov and P. Sewell.
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Passive attack analysis for connection-based anonymity systems.
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In <em>Computer Security - ESORICS 2003</em>. Springer-Verlag, LNCS
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2808, October 2003.
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|
<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEp5" name="p5">[46]</a></dt><dd>
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R. Sherwood, B. Bhattacharjee, and A. Srinivasan.
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p<sup>5</sup>: A protocol for scalable anonymous communication.
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In <em>IEEE Symposium on Security and Privacy</em>, pages 58-70. IEEE
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CS, 2002.
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEshsm03" name="shsm03">[47]</a></dt><dd>
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A. Shubina and S. Smith.
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|
Using caching for browsing anonymity.
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<em>ACM SIGEcom Exchanges</em>, 4(2), Sept 2003.
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|
<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEor-discex00" name="or-discex00">[48]</a></dt><dd>
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P. Syverson, M. Reed, and D. Goldschlag.
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Onion Routing access configurations.
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In <em>DARPA Information Survivability Conference and Exposition
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(DISCEX 2000)</em>, volume 1, pages 34-40. IEEE CS Press, 2000.
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|
<div class="p"><!----></div>
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|
</dd>
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<dt><a href="#CITEor-pet00" name="or-pet00">[49]</a></dt><dd>
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P. Syverson, G. Tsudik, M. Reed, and C. Landwehr.
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Towards an Analysis of Onion Routing Security.
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In H. Federrath, editor, <em>Designing Privacy Enhancing
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Technologies: Workshop on Design Issue in Anonymity and Unobservability</em>,
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|
pages 96-114. Springer-Verlag, LNCS 2009, July 2000.
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|
|
<div class="p"><!----></div>
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</dd>
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|
<dt><a href="#CITEtannenbaum96" name="tannenbaum96">[50]</a></dt><dd>
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|
A. Tannenbaum.
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|
Computer networks, 1996.
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|
<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEjap-backdoor" name="jap-backdoor">[51]</a></dt><dd>
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|
The AN.ON Project.
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|
German police proceeds against anonymity service.
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|
Press release, September 2003.
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|
<tt><http://www.datenschutzzentrum.de/material/themen/presse/anon-bka_e.htm>.
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|
|
|
<div class="p"><!----></div>
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|
</tt></dd>
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<dt><a href="#CITEtangler" name="tangler">[52]</a></dt><dd>
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M. Waldman and D. Mazières.
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Tangler: A censorship-resistant publishing system based on document
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entanglements.
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In <em>8<sup>th</sup> ACM Conference on Computer and Communications
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<div class="p"><!----></div>
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</dd>
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<dt><a href="#CITEpublius" name="publius">[53]</a></dt><dd>
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M. Waldman, A. Rubin, and L. Cranor.
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Publius: A robust, tamper-evident, censorship-resistant and
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|
source-anonymous web publishing system.
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In <em>Proc. 9th USENIX Security Symposium</em>, pages 59-72, August
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2000.
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|
<div class="p"><!----></div>
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|
</dd>
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|
<dt><a href="#CITEwright03" name="wright03">[54]</a></dt><dd>
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|
M. Wright, M. Adler, B. N. Levine, and C. Shields.
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|
Defending anonymous communication against passive logging attacks.
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|
In <em>IEEE Symposium on Security and Privacy</em>, pages 28-41. IEEE
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|
CS, May 2003.</dd>
|
|
</dl>
|
|
|
|
|
|
<div class="p"><!----></div>
|
|
<hr /><h3>Footnotes:</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
<a name="tthFtNtAAB"></a><a href="#tthFrefAAB"><sup>1</sup></a>Actually, the negotiated key is used to derive two
|
|
symmetric keys: one for each direction.
|
|
<div class="p"><!----></div>
|
|
<a name="tthFtNtAAC"></a><a href="#tthFrefAAC"><sup>2</sup></a>
|
|
With 48 bits of digest per cell, the probability of an accidental
|
|
collision is far lower than the chance of hardware failure.
|
|
<div class="p"><!----></div>
|
|
<a name="tthFtNtAAD"></a><a href="#tthFrefAAD"><sup>3</sup></a>
|
|
Rather than rely on an external infrastructure, the Onion Routing network
|
|
can run the lookup service itself. Our current implementation provides a
|
|
simple lookup system on the
|
|
directory servers.
|
|
<div class="p"><!----></div>
|
|
<a name="tthFtNtAAE"></a><a href="#tthFrefAAE"><sup>4</sup></a>Note that this fingerprinting
|
|
attack should not be confused with the much more complicated latency
|
|
attacks of [<a href="#back01" name="CITEback01">5</a>], which require a fingerprint of the latencies
|
|
of all circuits through the network, combined with those from the
|
|
network edges to the target user and the responder website.
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<br /><br /><hr /><small>File translated from
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T<sub><font size="-1">E</font></sub>X
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by <a href="http://hutchinson.belmont.ma.us/tth/">
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T<sub><font size="-1">T</font></sub>H</a>,
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version 3.59.<br />On 18 May 2004, 10:45.</small>
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</body></html>
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