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2113 lines
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<title> Design of a blocking-resistant anonymity system\DRAFT</title>
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<h1 align="center">Design of a blocking-resistant anonymity system<br />DRAFT </h1>
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<div class="p"><!----></div>
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<h3 align="center">Roger Dingledine, Nick Mathewson </h3>
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<div class="p"><!----></div>
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<h2> Abstract</h2>
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Internet censorship is on the rise as websites around the world are
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increasingly blocked by government-level firewalls. Although popular
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anonymizing networks like Tor were originally designed to keep attackers from
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tracing people's activities, many people are also using them to evade local
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censorship. But if the censor simply denies access to the Tor network
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itself, blocked users can no longer benefit from the security Tor offers.
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<div class="p"><!----></div>
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Here we describe a design that builds upon the current Tor network
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to provide an anonymizing network that resists blocking
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by government-level attackers.
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<div class="p"><!----></div>
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<h2><a name="tth_sEc1">
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1</a> Introduction and Goals</h2>
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<div class="p"><!----></div>
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Anonymizing networks like Tor [<a href="#tor-design" name="CITEtor-design">11</a>] bounce traffic around a
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network of encrypting relays. Unlike encryption, which hides only <i>what</i>
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is said, these networks also aim to hide who is communicating with whom, which
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users are using which websites, and similar relations. These systems have a
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broad range of users, including ordinary citizens who want to avoid being
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profiled for targeted advertisements, corporations who don't want to reveal
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information to their competitors, and law enforcement and government
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intelligence agencies who need to do operations on the Internet without being
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noticed.
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<div class="p"><!----></div>
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Historical anonymity research has focused on an
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attacker who monitors the user (call her Alice) and tries to discover her
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activities, yet lets her reach any piece of the network. In more modern
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threat models such as Tor's, the adversary is allowed to perform active
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attacks such as modifying communications to trick Alice
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into revealing her destination, or intercepting some connections
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to run a man-in-the-middle attack. But these systems still assume that
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Alice can eventually reach the anonymizing network.
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<div class="p"><!----></div>
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An increasing number of users are using the Tor software
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less for its anonymity properties than for its censorship
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resistance properties — if they use Tor to access Internet sites like
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Wikipedia
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and Blogspot, they are no longer affected by local censorship
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and firewall rules. In fact, an informal user study
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showed China as the third largest user base
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for Tor clients, with perhaps ten thousand people accessing the Tor
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network from China each day.
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<div class="p"><!----></div>
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The current Tor design is easy to block if the attacker controls Alice's
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connection to the Tor network — by blocking the directory authorities,
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by blocking all the server IP addresses in the directory, or by filtering
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based on the fingerprint of the Tor TLS handshake. Here we describe an
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extended design that builds upon the current Tor network to provide an
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anonymizing
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network that resists censorship as well as anonymity-breaking attacks.
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In section <a href="#sec:adversary">2</a> we discuss our threat model — that is,
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the assumptions we make about our adversary. Section <a href="#sec:current-tor">3</a>
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describes the components of the current Tor design and how they can be
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leveraged for a new blocking-resistant design. Section <a href="#sec:related">4</a>
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explains the features and drawbacks of the currently deployed solutions.
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In sections <a href="#sec:bridges">5</a> through <a href="#sec:discovery">7</a>, we explore the
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components of our designs in detail. Section <a href="#sec:security">8</a> considers
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security implications and Section <a href="#sec:reachability">9</a> presents other
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issues with maintaining connectivity and sustainability for the design.
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Section <a href="#sec:future">10</a> speculates about future more complex designs,
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and finally Section <a href="#sec:conclusion">11</a> summarizes our next steps and
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recommendations.
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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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<div class="p"><!----></div>
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<h2><a name="tth_sEc2">
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<a name="sec:adversary">
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2</a> Adversary assumptions</h2>
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</a>
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<div class="p"><!----></div>
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To design an effective anti-censorship tool, we need a good model for the
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goals and resources of the censors we are evading. Otherwise, we risk
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spending our effort on keeping the adversaries from doing things they have no
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interest in doing, and thwarting techniques they do not use.
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The history of blocking-resistance designs is littered with conflicting
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assumptions about what adversaries to expect and what problems are
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in the critical path to a solution. Here we describe our best
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understanding of the current situation around the world.
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<div class="p"><!----></div>
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In the traditional security style, we aim to defeat a strong
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attacker — if we can defend against this attacker, we inherit protection
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against weaker attackers as well. After all, we want a general design
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that will work for citizens of China, Thailand, and other censored
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countries; for
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whistleblowers in firewalled corporate networks; and for people in
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unanticipated oppressive situations. In fact, by designing with
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a variety of adversaries in mind, we can take advantage of the fact that
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adversaries will be in different stages of the arms race at each location,
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so a server blocked in one locale can still be useful in others.
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<div class="p"><!----></div>
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We assume that the attackers' goals are somewhat complex.
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<dl compact="compact">
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<dt><b></b></dt>
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<dd><li>The attacker would like to restrict the flow of certain kinds of
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information, particularly when this information is seen as embarrassing to
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those in power (such as information about rights violations or corruption),
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or when it enables or encourages others to oppose them effectively (such as
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information about opposition movements or sites that are used to organize
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protests).</dd>
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<dt><b></b></dt>
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<dd><li>As a second-order effect, censors aim to chill citizens' behavior by
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creating an impression that their online activities are monitored.</dd>
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<dt><b></b></dt>
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<dd><li>In some cases, censors make a token attempt to block a few sites for
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obscenity, blasphemy, and so on, but their efforts here are mainly for
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show. In other cases, they really do try hard to block such content.</dd>
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<dt><b></b></dt>
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<dd><li>Complete blocking (where nobody at all can ever download censored
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content) is not a
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goal. Attackers typically recognize that perfect censorship is not only
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impossible, but unnecessary: if "undesirable" information is known only
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to a small few, further censoring efforts can be focused elsewhere.</dd>
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<dt><b></b></dt>
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<dd><li>Similarly, the censors are not attempting to shut down or block <i>
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every</i> anti-censorship tool — merely the tools that are popular and
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effective (because these tools impede the censors' information restriction
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goals) and those tools that are highly visible (thus making the censors
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look ineffectual to their citizens and their bosses).</dd>
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<dt><b></b></dt>
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<dd><li>Reprisal against <i>most</i> passive consumers of <i>most</i> kinds of
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blocked information is also not a goal, given the broadness of most
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censorship regimes. This seems borne out by fact.<a href="#tthFtNtAAB" name="tthFrefAAB"><sup>1</sup></a></dd>
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<dt><b></b></dt>
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<dd><li>Producers and distributors of targeted information are in much
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greater danger than consumers; the attacker would like to not only block
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their work, but identify them for reprisal.</dd>
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<dt><b></b></dt>
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<dd><li>The censors (or their governments) would like to have a working, useful
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Internet. There are economic, political, and social factors that prevent
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them from "censoring" the Internet by outlawing it entirely, or by
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blocking access to all but a tiny list of sites.
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Nevertheless, the censors <i>are</i> willing to block innocuous content
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(like the bulk of a newspaper's reporting) in order to censor other content
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distributed through the same channels (like that newspaper's coverage of
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the censored country).
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</dd>
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</dl>
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<div class="p"><!----></div>
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We assume there are three main technical network attacks in use by censors
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currently [<a href="#clayton:pet2006" name="CITEclayton:pet2006">7</a>]:
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<div class="p"><!----></div>
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<dl compact="compact">
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<dt><b></b></dt>
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<dd><li>Block a destination or type of traffic by automatically searching for
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certain strings or patterns in TCP packets. Offending packets can be
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dropped, or can trigger a response like closing the
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connection.</dd>
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<dt><b></b></dt>
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<dd><li>Block a destination by listing its IP address at a
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firewall or other routing control point.</dd>
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<dt><b></b></dt>
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<dd><li>Intercept DNS requests and give bogus responses for certain
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destination hostnames.
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</dd>
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</dl>
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<div class="p"><!----></div>
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We assume the network firewall has limited CPU and memory per
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connection [<a href="#clayton:pet2006" name="CITEclayton:pet2006">7</a>]. Against an adversary who could carefully
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examine the contents of every packet and correlate the packets in every
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stream on the network, we would need some stronger mechanism such as
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steganography, which introduces its own
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problems [<a href="#active-wardens" name="CITEactive-wardens">15</a>,<a href="#tcpstego" name="CITEtcpstego">26</a>]. But we make a "weak
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steganography" assumption here: to remain unblocked, it is necessary to
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remain unobservable only by computational resources on par with a modern
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router, firewall, proxy, or IDS.
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<div class="p"><!----></div>
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We assume that while various different regimes can coordinate and share
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notes, there will be a time lag between one attacker learning how to overcome
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a facet of our design and other attackers picking it up. (The most common
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vector of transmission seems to be commercial providers of censorship tools:
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once a provider adds a feature to meet one country's needs or requests, the
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feature is available to all of the provider's customers.) Conversely, we
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assume that insider attacks become a higher risk only after the early stages
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of network development, once the system has reached a certain level of
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success and visibility.
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<div class="p"><!----></div>
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We do not assume that government-level attackers are always uniform
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across the country. For example, users of different ISPs in China
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experience different censorship policies and mechanisms.
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<div class="p"><!----></div>
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We assume that the attacker may be able to use political and economic
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resources to secure the cooperation of extraterritorial or multinational
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corporations and entities in investigating information sources.
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For example, the censors can threaten the service providers of
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troublesome blogs with economic reprisals if they do not reveal the
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authors' identities.
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<div class="p"><!----></div>
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We assume that our users have control over their hardware and
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software — they don't have any spyware installed, there are no
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cameras watching their screens, etc. Unfortunately, in many situations
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these threats are real [<a href="#zuckerman-threatmodels" name="CITEzuckerman-threatmodels">28</a>]; yet
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software-based security systems like ours are poorly equipped to handle
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a user who is entirely observed and controlled by the adversary. See
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Section <a href="#subsec:cafes-and-livecds">8.4</a> for more discussion of what little
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we can do about this issue.
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<div class="p"><!----></div>
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Similarly, we assume that the user will be able to fetch a genuine
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version of Tor, rather than one supplied by the adversary; see
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Section <a href="#subsec:trust-chain">8.5</a> for discussion on helping the user
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confirm that he has a genuine version and that he can connect to the
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real Tor network.
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<div class="p"><!----></div>
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<h2><a name="tth_sEc3">
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<a name="sec:current-tor">
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3</a> Adapting the current Tor design to anti-censorship</h2>
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</a>
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<div class="p"><!----></div>
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Tor is popular and sees a lot of use — it's the largest anonymity
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network of its kind, and has
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attracted more than 800 volunteer-operated routers from around the
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world. Tor protects each user by routing their traffic through a multiply
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encrypted "circuit" built of a few randomly selected servers, each of which
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can remove only a single layer of encryption. Each server sees only the step
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before it and the step after it in the circuit, and so no single server can
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learn the connection between a user and her chosen communication partners.
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In this section, we examine some of the reasons why Tor has become popular,
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with particular emphasis to how we can take advantage of these properties
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for a blocking-resistance design.
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<div class="p"><!----></div>
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Tor aims to provide three security properties:
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<dl compact="compact">
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<dt><b></b></dt>
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<dd>1. A local network attacker can't learn, or influence, your
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destination.</dd>
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<dt><b></b></dt>
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<dd>2. No single router in the Tor network can link you to your
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destination.</dd>
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<dt><b></b></dt>
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<dd>3. The destination, or somebody watching the destination,
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can't learn your location.
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</dd>
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</dl>
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<div class="p"><!----></div>
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For blocking-resistance, we care most clearly about the first
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property. But as the arms race progresses, the second property
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will become important — for example, to discourage an adversary
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from volunteering a relay in order to learn that Alice is reading
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or posting to certain websites. The third property helps keep users safe from
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collaborating websites: consider websites and other Internet services
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that have been pressured
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recently into revealing the identity of bloggers
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or treating clients differently depending on their network
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location [<a href="#goodell-syverson06" name="CITEgoodell-syverson06">17</a>].
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<div class="p"><!----></div>
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The Tor design provides other features as well that are not typically
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present in manual or ad hoc circumvention techniques.
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<div class="p"><!----></div>
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First, Tor has a well-analyzed and well-understood way to distribute
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information about servers.
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Tor directory authorities automatically aggregate, test,
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and publish signed summaries of the available Tor routers. Tor clients
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can fetch these summaries to learn which routers are available and
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which routers are suitable for their needs. Directory information is cached
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throughout the Tor network, so once clients have bootstrapped they never
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need to interact with the authorities directly. (To tolerate a minority
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of compromised directory authorities, we use a threshold trust scheme —
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see Section <a href="#subsec:trust-chain">8.5</a> for details.)
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<div class="p"><!----></div>
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Second, the list of directory authorities is not hard-wired.
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Clients use the default authorities if no others are specified,
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but it's easy to start a separate (or even overlapping) Tor network just
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by running a different set of authorities and convincing users to prefer
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a modified client. For example, we could launch a distinct Tor network
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inside China; some users could even use an aggregate network made up of
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both the main network and the China network. (But we should not be too
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quick to create other Tor networks — part of Tor's anonymity comes from
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users behaving like other users, and there are many unsolved anonymity
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questions if different users know about different pieces of the network.)
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<div class="p"><!----></div>
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Third, in addition to automatically learning from the chosen directories
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which Tor routers are available and working, Tor takes care of building
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paths through the network and rebuilding them as needed. So the user
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never has to know how paths are chosen, never has to manually pick
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working proxies, and so on. More generally, at its core the Tor protocol
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is simply a tool that can build paths given a set of routers. Tor is
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quite flexible about how it learns about the routers and how it chooses
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the paths. Harvard's Blossom project [<a href="#blossom-thesis" name="CITEblossom-thesis">16</a>] makes this
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flexibility more concrete: Blossom makes use of Tor not for its security
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properties but for its reachability properties. It runs a separate set
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of directory authorities, its own set of Tor routers (called the Blossom
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network), and uses Tor's flexible path-building to let users view Internet
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resources from any point in the Blossom network.
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<div class="p"><!----></div>
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Fourth, Tor separates the role of <em>internal relay</em> from the
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role of <em>exit relay</em>. That is, some volunteers choose just to relay
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traffic between Tor users and Tor routers, and others choose to also allow
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connections to external Internet resources. Because we don't force all
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volunteers to play both roles, we end up with more relays. This increased
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diversity in turn is what gives Tor its security: the more options the
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user has for her first hop, and the more options she has for her last hop,
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the less likely it is that a given attacker will be watching both ends
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of her circuit [<a href="#tor-design" name="CITEtor-design">11</a>]. As a bonus, because our design attracts
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more internal relays that want to help out but don't want to deal with
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being an exit relay, we end up providing more options for the first
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hop — the one most critical to being able to reach the Tor network.
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<div class="p"><!----></div>
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Fifth, Tor is sustainable. Zero-Knowledge Systems offered the commercial
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but now defunct Freedom Network [<a href="#freedom21-security" name="CITEfreedom21-security">2</a>], a design with
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security comparable to Tor's, but its funding model relied on collecting
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money from users to pay relay operators. Modern commercial proxy systems
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similarly
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need to keep collecting money to support their infrastructure. On the
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other hand, Tor has built a self-sustaining community of volunteers who
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donate their time and resources. This community trust is rooted in Tor's
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open design: we tell the world exactly how Tor works, and we provide all
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the source code. Users can decide for themselves, or pay any security
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expert to decide, whether it is safe to use. Further, Tor's modularity
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as described above, along with its open license, mean that its impact
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will continue to grow.
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<div class="p"><!----></div>
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Sixth, Tor has an established user base of hundreds of
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thousands of people from around the world. This diversity of
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users contributes to sustainability as above: Tor is used by
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ordinary citizens, activists, corporations, law enforcement, and
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even government and military users,
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and they can
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only achieve their security goals by blending together in the same
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network [<a href="#econymics" name="CITEeconymics">1</a>,<a href="#usability:weis2006" name="CITEusability:weis2006">9</a>]. This user base also provides
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something else: hundreds of thousands of different and often-changing
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addresses that we can leverage for our blocking-resistance design.
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<div class="p"><!----></div>
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Finally and perhaps most importantly, Tor provides anonymity and prevents any
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single server from linking users to their communication partners. Despite
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initial appearances, <i>distributed-trust anonymity is critical for
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anti-censorship efforts</i>. If any single server can expose dissident bloggers
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or compile a list of users' behavior, the censors can profitably compromise
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that server's operator, perhaps by applying economic pressure to their
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employers,
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breaking into their computer, pressuring their family (if they have relatives
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in the censored area), or so on. Furthermore, in designs where any relay can
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expose its users, the censors can spread suspicion that they are running some
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of the relays and use this belief to chill use of the network.
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<div class="p"><!----></div>
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We discuss and adapt these components further in
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Section <a href="#sec:bridges">5</a>. But first we examine the strengths and
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weaknesses of other blocking-resistance approaches, so we can expand
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our repertoire of building blocks and ideas.
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|
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<div class="p"><!----></div>
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<h2><a name="tth_sEc4">
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<a name="sec:related">
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4</a> Current proxy solutions</h2>
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</a>
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<div class="p"><!----></div>
|
|
Relay-based blocking-resistance schemes generally have two main
|
|
components: a relay component and a discovery component. The relay part
|
|
encompasses the process of establishing a connection, sending traffic
|
|
back and forth, and so on — everything that's done once the user knows
|
|
where she's going to connect. Discovery is the step before that: the
|
|
process of finding one or more usable relays.
|
|
|
|
<div class="p"><!----></div>
|
|
For example, we can divide the pieces of Tor in the previous section
|
|
into the process of building paths and sending
|
|
traffic over them (relay) and the process of learning from the directory
|
|
servers about what routers are available (discovery). With this distinction
|
|
in mind, we now examine several categories of relay-based schemes.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.1">
|
|
4.1</a> Centrally-controlled shared proxies</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Existing commercial anonymity solutions (like Anonymizer.com) are based
|
|
on a set of single-hop proxies. In these systems, each user connects to
|
|
a single proxy, which then relays traffic between the user and her
|
|
destination. These public proxy
|
|
systems are typically characterized by two features: they control and
|
|
operate the proxies centrally, and many different users get assigned
|
|
to each proxy.
|
|
|
|
<div class="p"><!----></div>
|
|
In terms of the relay component, single proxies provide weak security
|
|
compared to systems that distribute trust over multiple relays, since a
|
|
compromised proxy can trivially observe all of its users' actions, and
|
|
an eavesdropper only needs to watch a single proxy to perform timing
|
|
correlation attacks against all its users' traffic and thus learn where
|
|
everyone is connecting. Worse, all users
|
|
need to trust the proxy company to have good security itself as well as
|
|
to not reveal user activities.
|
|
|
|
<div class="p"><!----></div>
|
|
On the other hand, single-hop proxies are easier to deploy, and they
|
|
can provide better performance than distributed-trust designs like Tor,
|
|
since traffic only goes through one relay. They're also more convenient
|
|
from the user's perspective — since users entirely trust the proxy,
|
|
they can just use their web browser directly.
|
|
|
|
<div class="p"><!----></div>
|
|
Whether public proxy schemes are more or less scalable than Tor is
|
|
still up for debate: commercial anonymity systems can use some of their
|
|
revenue to provision more bandwidth as they grow, whereas volunteer-based
|
|
anonymity systems can attract thousands of fast relays to spread the load.
|
|
|
|
<div class="p"><!----></div>
|
|
The discovery piece can take several forms. Most commercial anonymous
|
|
proxies have one or a handful of commonly known websites, and their users
|
|
log in to those websites and relay their traffic through them. When
|
|
these websites get blocked (generally soon after the company becomes
|
|
popular), if the company cares about users in the blocked areas, they
|
|
start renting lots of disparate IP addresses and rotating through them
|
|
as they get blocked. They notify their users of new addresses (by email,
|
|
for example). It's an arms race, since attackers can sign up to receive the
|
|
email too, but operators have one nice trick available to them: because they
|
|
have a list of paying subscribers, they can notify certain subscribers
|
|
about updates earlier than others.
|
|
|
|
<div class="p"><!----></div>
|
|
Access control systems on the proxy let them provide service only to
|
|
users with certain characteristics, such as paying customers or people
|
|
from certain IP address ranges.
|
|
|
|
<div class="p"><!----></div>
|
|
Discovery in the face of a government-level firewall is a complex and
|
|
unsolved
|
|
topic, and we're stuck in this same arms race ourselves; we explore it
|
|
in more detail in Section <a href="#sec:discovery">7</a>. But first we examine the
|
|
other end of the spectrum — getting volunteers to run the proxies,
|
|
and telling only a few people about each proxy.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.2">
|
|
4.2</a> Independent personal proxies</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Personal proxies such as Circumventor [<a href="#circumventor" name="CITEcircumventor">18</a>] and
|
|
CGIProxy [<a href="#cgiproxy" name="CITEcgiproxy">23</a>] use the same technology as the public ones as
|
|
far as the relay component goes, but they use a different strategy for
|
|
discovery. Rather than managing a few centralized proxies and constantly
|
|
getting new addresses for them as the old addresses are blocked, they
|
|
aim to have a large number of entirely independent proxies, each managing
|
|
its own (much smaller) set of users.
|
|
|
|
<div class="p"><!----></div>
|
|
As the Circumventor site explains, "You don't
|
|
actually install the Circumventor <em>on</em> the computer that is blocked
|
|
from accessing Web sites. You, or a friend of yours, has to install the
|
|
Circumventor on some <em>other</em> machine which is not censored."
|
|
|
|
<div class="p"><!----></div>
|
|
This tactic has great advantages in terms of blocking-resistance — recall
|
|
our assumption in Section <a href="#sec:adversary">2</a> that the attention
|
|
a system attracts from the attacker is proportional to its number of
|
|
users and level of publicity. If each proxy only has a few users, and
|
|
there is no central list of proxies, most of them will never get noticed by
|
|
the censors.
|
|
|
|
<div class="p"><!----></div>
|
|
On the other hand, there's a huge scalability question that so far has
|
|
prevented these schemes from being widely useful: how does the fellow
|
|
in China find a person in Ohio who will run a Circumventor for him? In
|
|
some cases he may know and trust some people on the outside, but in many
|
|
cases he's just out of luck. Just as hard, how does a new volunteer in
|
|
Ohio find a person in China who needs it?
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
This challenge leads to a hybrid design-centrally — distributed
|
|
personal proxies — which we will investigate in more detail in
|
|
Section <a href="#sec:discovery">7</a>.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.3">
|
|
4.3</a> Open proxies</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Yet another currently used approach to bypassing firewalls is to locate
|
|
open and misconfigured proxies on the Internet. A quick Google search
|
|
for "open proxy list" yields a wide variety of freely available lists
|
|
of HTTP, HTTPS, and SOCKS proxies. Many small companies have sprung up
|
|
providing more refined lists to paying customers.
|
|
|
|
<div class="p"><!----></div>
|
|
There are some downsides to using these open proxies though. First,
|
|
the proxies are of widely varying quality in terms of bandwidth and
|
|
stability, and many of them are entirely unreachable. Second, unlike
|
|
networks of volunteers like Tor, the legality of routing traffic through
|
|
these proxies is questionable: it's widely believed that most of them
|
|
don't realize what they're offering, and probably wouldn't allow it if
|
|
they realized. Third, in many cases the connection to the proxy is
|
|
unencrypted, so firewalls that filter based on keywords in IP packets
|
|
will not be hindered. Fourth, in many countries (including China), the
|
|
firewall authorities hunt for open proxies as well, to preemptively
|
|
block them. And last, many users are suspicious that some
|
|
open proxies are a little <em>too</em> convenient: are they run by the
|
|
adversary, in which case they get to monitor all the user's requests
|
|
just as single-hop proxies can?
|
|
|
|
<div class="p"><!----></div>
|
|
A distributed-trust design like Tor resolves each of these issues for
|
|
the relay component, but a constantly changing set of thousands of open
|
|
relays is clearly a useful idea for a discovery component. For example,
|
|
users might be able to make use of these proxies to bootstrap their
|
|
first introduction into the Tor network.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.4">
|
|
4.4</a> Blocking resistance and JAP</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Köpsell and Hilling's Blocking Resistance
|
|
design [<a href="#koepsell:wpes2004" name="CITEkoepsell:wpes2004">20</a>] is probably
|
|
the closest related work, and is the starting point for the design in this
|
|
paper. In this design, the JAP anonymity system [<a href="#web-mix" name="CITEweb-mix">3</a>] is used
|
|
as a base instead of Tor. Volunteers operate a large number of access
|
|
points that relay traffic to the core JAP
|
|
network, which in turn anonymizes users' traffic. The software to run these
|
|
relays is, as in our design, included in the JAP client software and enabled
|
|
only when the user decides to enable it. Discovery is handled with a
|
|
CAPTCHA-based mechanism; users prove that they aren't an automated process,
|
|
and are given the address of an access point. (The problem of a determined
|
|
attacker with enough manpower to launch many requests and enumerate all the
|
|
access points is not considered in depth.) There is also some suggestion
|
|
that information about access points could spread through existing social
|
|
networks.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.5">
|
|
4.5</a> Infranet</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
The Infranet design [<a href="#infranet" name="CITEinfranet">14</a>] uses one-hop relays to deliver web
|
|
content, but disguises its communications as ordinary HTTP traffic. Requests
|
|
are split into multiple requests for URLs on the relay, which then encodes
|
|
its responses in the content it returns. The relay needs to be an actual
|
|
website with plausible content and a number of URLs which the user might want
|
|
to access — if the Infranet software produced its own cover content, it would
|
|
be far easier for censors to identify. To keep the censors from noticing
|
|
that cover content changes depending on what data is embedded, Infranet needs
|
|
the cover content to have an innocuous reason for changing frequently: the
|
|
paper recommends watermarked images and webcams.
|
|
|
|
<div class="p"><!----></div>
|
|
The attacker and relay operators in Infranet's threat model are significantly
|
|
different than in ours. Unlike our attacker, Infranet's censor can't be
|
|
bypassed with encrypted traffic (presumably because the censor blocks
|
|
encrypted traffic, or at least considers it suspicious), and has more
|
|
computational resources to devote to each connection than ours (so it can
|
|
notice subtle patterns over time). Unlike our bridge operators, Infranet's
|
|
operators (and users) have more bandwidth to spare; the overhead in typical
|
|
steganography schemes is far higher than Tor's.
|
|
|
|
<div class="p"><!----></div>
|
|
The Infranet design does not include a discovery element. Discovery,
|
|
however, is a critical point: if whatever mechanism allows users to learn
|
|
about relays also allows the censor to do so, he can trivially discover and
|
|
block their addresses, even if the steganography would prevent mere traffic
|
|
observation from revealing the relays' addresses.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.6">
|
|
4.6</a> RST-evasion and other packet-level tricks</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
In their analysis of China's firewall's content-based blocking, Clayton,
|
|
Murdoch and Watson discovered that rather than blocking all packets in a TCP
|
|
streams once a forbidden word was noticed, the firewall was simply forging
|
|
RST packets to make the communicating parties believe that the connection was
|
|
closed [<a href="#clayton:pet2006" name="CITEclayton:pet2006">7</a>]. They proposed altering operating systems
|
|
to ignore forged RST packets. This approach might work in some cases, but
|
|
in practice it appears that many firewalls start filtering by IP address
|
|
once a sufficient number of RST packets have been sent.
|
|
|
|
<div class="p"><!----></div>
|
|
Other packet-level responses to filtering include splitting
|
|
sensitive words across multiple TCP packets, so that the censors'
|
|
firewalls can't notice them without performing expensive stream
|
|
reconstruction [<a href="#ptacek98insertion" name="CITEptacek98insertion">27</a>]. This technique relies on the
|
|
same insight as our weak steganography assumption.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.7">
|
|
4.7</a> Internal caching networks</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Freenet [<a href="#freenet-pets00" name="CITEfreenet-pets00">6</a>] is an anonymous peer-to-peer data store.
|
|
Analyzing Freenet's security can be difficult, as its design is in flux as
|
|
new discovery and routing mechanisms are proposed, and no complete
|
|
specification has (to our knowledge) been written. Freenet servers relay
|
|
requests for specific content (indexed by a digest of the content)
|
|
"toward" the server that hosts it, and then cache the content as it
|
|
follows the same path back to
|
|
the requesting user. If Freenet's routing mechanism is successful in
|
|
allowing nodes to learn about each other and route correctly even as some
|
|
node-to-node links are blocked by firewalls, then users inside censored areas
|
|
can ask a local Freenet server for a piece of content, and get an answer
|
|
without having to connect out of the country at all. Of course, operators of
|
|
servers inside the censored area can still be targeted, and the addresses of
|
|
external servers can still be blocked.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.8">
|
|
4.8</a> Skype</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
The popular Skype voice-over-IP software uses multiple techniques to tolerate
|
|
restrictive networks, some of which allow it to continue operating in the
|
|
presence of censorship. By switching ports and using encryption, Skype
|
|
attempts to resist trivial blocking and content filtering. Even if no
|
|
encryption were used, it would still be expensive to scan all voice
|
|
traffic for sensitive words. Also, most current keyloggers are unable to
|
|
store voice traffic. Nevertheless, Skype can still be blocked, especially at
|
|
its central login server.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc4.9">
|
|
4.9</a> Tor itself</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
And last, we include Tor itself in the list of current solutions
|
|
to firewalls. Tens of thousands of people use Tor from countries that
|
|
routinely filter their Internet. Tor's website has been blocked in most
|
|
of them. But why hasn't the Tor network been blocked yet?
|
|
|
|
<div class="p"><!----></div>
|
|
We have several theories. The first is the most straightforward: tens of
|
|
thousands of people are simply too few to matter. It may help that Tor is
|
|
perceived to be for experts only, and thus not worth attention yet. The
|
|
more subtle variant on this theory is that we've positioned Tor in the
|
|
public eye as a tool for retaining civil liberties in more free countries,
|
|
so perhaps blocking authorities don't view it as a threat. (We revisit
|
|
this idea when we consider whether and how to publicize a Tor variant
|
|
that improves blocking-resistance — see Section <a href="#subsec:publicity">9.5</a>
|
|
for more discussion.)
|
|
|
|
<div class="p"><!----></div>
|
|
The broader explanation is that the maintenance of most government-level
|
|
filters is aimed at stopping widespread information flow and appearing to be
|
|
in control, not by the impossible goal of blocking all possible ways to bypass
|
|
censorship. Censors realize that there will always
|
|
be ways for a few people to get around the firewall, and as long as Tor
|
|
has not publically threatened their control, they see no urgent need to
|
|
block it yet.
|
|
|
|
<div class="p"><!----></div>
|
|
We should recognize that we're <em>already</em> in the arms race. These
|
|
constraints can give us insight into the priorities and capabilities of
|
|
our various attackers.
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc5">
|
|
<a name="sec:bridges">
|
|
5</a> The relay component of our blocking-resistant design</h2>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Section <a href="#sec:current-tor">3</a> describes many reasons why Tor is
|
|
well-suited as a building block in our context, but several changes will
|
|
allow the design to resist blocking better. The most critical changes are
|
|
to get more relay addresses, and to distribute them to users differently.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc5.1">
|
|
5.1</a> Bridge relays</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Today, Tor servers operate on less than a thousand distinct IP addresses;
|
|
an adversary
|
|
could enumerate and block them all with little trouble. To provide a
|
|
means of ingress to the network, we need a larger set of entry points, most
|
|
of which an adversary won't be able to enumerate easily. Fortunately, we
|
|
have such a set: the Tor users.
|
|
|
|
<div class="p"><!----></div>
|
|
Hundreds of thousands of people around the world use Tor. We can leverage
|
|
our already self-selected user base to produce a list of thousands of
|
|
frequently-changing IP addresses. Specifically, we can give them a little
|
|
button in the GUI that says "Tor for Freedom", and users who click
|
|
the button will turn into <em>bridge relays</em> (or just <em>bridges</em>
|
|
for short). They can rate limit relayed connections to 10 KB/s (almost
|
|
nothing for a broadband user in a free country, but plenty for a user
|
|
who otherwise has no access at all), and since they are just relaying
|
|
bytes back and forth between blocked users and the main Tor network, they
|
|
won't need to make any external connections to Internet sites. Because
|
|
of this separation of roles, and because we're making use of software
|
|
that the volunteers have already installed for their own use, we expect
|
|
our scheme to attract and maintain more volunteers than previous schemes.
|
|
|
|
<div class="p"><!----></div>
|
|
As usual, there are new anonymity and security implications from running a
|
|
bridge relay, particularly from letting people relay traffic through your
|
|
Tor client; but we leave this discussion for Section <a href="#sec:security">8</a>.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc5.2">
|
|
5.2</a> The bridge directory authority</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
How do the bridge relays advertise their existence to the world? We
|
|
introduce a second new component of the design: a specialized directory
|
|
authority that aggregates and tracks bridges. Bridge relays periodically
|
|
publish server descriptors (summaries of their keys, locations, etc,
|
|
signed by their long-term identity key), just like the relays in the
|
|
"main" Tor network, but in this case they publish them only to the
|
|
bridge directory authorities.
|
|
|
|
<div class="p"><!----></div>
|
|
The main difference between bridge authorities and the directory
|
|
authorities for the main Tor network is that the main authorities provide
|
|
a list of every known relay, but the bridge authorities only give
|
|
out a server descriptor if you already know its identity key. That is,
|
|
you can keep up-to-date on a bridge's location and other information
|
|
once you know about it, but you can't just grab a list of all the bridges.
|
|
|
|
<div class="p"><!----></div>
|
|
The identity key, IP address, and directory port for each bridge
|
|
authority ship by default with the Tor software, so the bridge relays
|
|
can be confident they're publishing to the right location, and the
|
|
blocked users can establish an encrypted authenticated channel. See
|
|
Section <a href="#subsec:trust-chain">8.5</a> for more discussion of the public key
|
|
infrastructure and trust chain.
|
|
|
|
<div class="p"><!----></div>
|
|
Bridges use Tor to publish their descriptors privately and securely,
|
|
so even an attacker monitoring the bridge directory authority's network
|
|
can't make a list of all the addresses contacting the authority.
|
|
Bridges may publish to only a subset of the
|
|
authorities, to limit the potential impact of an authority compromise.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc5.3">
|
|
<a name="subsec:relay-together">
|
|
5.3</a> Putting them together</h3>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
If a blocked user knows the identity keys of a set of bridge relays, and
|
|
he has correct address information for at least one of them, he can use
|
|
that one to make a secure connection to the bridge authority and update
|
|
his knowledge about the other bridge relays. He can also use it to make
|
|
secure connections to the main Tor network and directory servers, so he
|
|
can build circuits and connect to the rest of the Internet. All of these
|
|
updates happen in the background: from the blocked user's perspective,
|
|
he just accesses the Internet via his Tor client like always.
|
|
|
|
<div class="p"><!----></div>
|
|
So now we've reduced the problem from how to circumvent the firewall
|
|
for all transactions (and how to know that the pages you get have not
|
|
been modified by the local attacker) to how to learn about a working
|
|
bridge relay.
|
|
|
|
<div class="p"><!----></div>
|
|
There's another catch though. We need to make sure that the network
|
|
traffic we generate by simply connecting to a bridge relay doesn't stand
|
|
out too much.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc6">
|
|
<a name="sec:network-fingerprint">
|
|
<a name="subsec:enclave-dirs">
|
|
6</a> Hiding Tor's network fingerprint</h2>
|
|
</a>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Currently, Tor uses two protocols for its network communications. The
|
|
main protocol uses TLS for encrypted and authenticated communication
|
|
between Tor instances. The second protocol is standard HTTP, used for
|
|
fetching directory information. All Tor servers listen on their "ORPort"
|
|
for TLS connections, and some of them opt to listen on their "DirPort"
|
|
as well, to serve directory information. Tor servers choose whatever port
|
|
numbers they like; the server descriptor they publish to the directory
|
|
tells users where to connect.
|
|
|
|
<div class="p"><!----></div>
|
|
One format for communicating address information about a bridge relay is
|
|
its IP address and DirPort. From there, the user can ask the bridge's
|
|
directory cache for an up-to-date copy of its server descriptor, and
|
|
learn its current circuit keys, its ORPort, and so on.
|
|
|
|
<div class="p"><!----></div>
|
|
However, connecting directly to the directory cache involves a plaintext
|
|
HTTP request. A censor could create a network fingerprint (known as a
|
|
<em>signature</em> in the intrusion detection field) for the request
|
|
and/or its response, thus preventing these connections. To resolve this
|
|
vulnerability, we've modified the Tor protocol so that users can connect
|
|
to the directory cache via the main Tor port — they establish a TLS
|
|
connection with the bridge as normal, and then send a special "begindir"
|
|
relay command to establish an internal connection to its directory cache.
|
|
|
|
<div class="p"><!----></div>
|
|
Therefore a better way to summarize a bridge's address is by its IP
|
|
address and ORPort, so all communications between the client and the
|
|
bridge will use ordinary TLS. But there are other details that need
|
|
more investigation.
|
|
|
|
<div class="p"><!----></div>
|
|
What port should bridges pick for their ORPort? We currently recommend
|
|
that they listen on port 443 (the default HTTPS port) if they want to
|
|
be most useful, because clients behind standard firewalls will have
|
|
the best chance to reach them. Is this the best choice in all cases,
|
|
or should we encourage some fraction of them pick random ports, or other
|
|
ports commonly permitted through firewalls like 53 (DNS) or 110
|
|
(POP)? Or perhaps we should use other ports where TLS traffic is
|
|
expected, like 993 (IMAPS) or 995 (POP3S). We need more research on our
|
|
potential users, and their current and anticipated firewall restrictions.
|
|
|
|
<div class="p"><!----></div>
|
|
Furthermore, we need to look at the specifics of Tor's TLS handshake.
|
|
Right now Tor uses some predictable strings in its TLS handshakes. For
|
|
example, it sets the X.509 organizationName field to "Tor", and it puts
|
|
the Tor server's nickname in the certificate's commonName field. We
|
|
should tweak the handshake protocol so it doesn't rely on any unusual details
|
|
in the certificate, yet it remains secure; the certificate itself
|
|
should be made to resemble an ordinary HTTPS certificate. We should also try
|
|
to make our advertised cipher-suites closer to what an ordinary web server
|
|
would support.
|
|
|
|
<div class="p"><!----></div>
|
|
Tor's TLS handshake uses two-certificate chains: one certificate
|
|
contains the self-signed identity key for
|
|
the router, and the second contains a current TLS key, signed by the
|
|
identity key. We use these to authenticate that we're talking to the right
|
|
router, and to limit the impact of TLS-key exposure. Most (though far from
|
|
all) consumer-oriented HTTPS services provide only a single certificate.
|
|
These extra certificates may help identify Tor's TLS handshake; instead,
|
|
bridges should consider using only a single TLS key certificate signed by
|
|
their identity key, and providing the full value of the identity key in an
|
|
early handshake cell. More significantly, Tor currently has all clients
|
|
present certificates, so that clients are harder to distinguish from servers.
|
|
But in a blocking-resistance environment, clients should not present
|
|
certificates at all.
|
|
|
|
<div class="p"><!----></div>
|
|
Last, what if the adversary starts observing the network traffic even
|
|
more closely? Even if our TLS handshake looks innocent, our traffic timing
|
|
and volume still look different than a user making a secure web connection
|
|
to his bank. The same techniques used in the growing trend to build tools
|
|
to recognize encrypted Bittorrent traffic
|
|
could be used to identify Tor communication and recognize bridge
|
|
relays. Rather than trying to look like encrypted web traffic, we may be
|
|
better off trying to blend with some other encrypted network protocol. The
|
|
first step is to compare typical network behavior for a Tor client to
|
|
typical network behavior for various other protocols. This statistical
|
|
cat-and-mouse game is made more complex by the fact that Tor transports a
|
|
variety of protocols, and we'll want to automatically handle web browsing
|
|
differently from, say, instant messaging.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc6.1">
|
|
<a name="subsec:id-address">
|
|
6.1</a> Identity keys as part of addressing information</h3>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
We have described a way for the blocked user to bootstrap into the
|
|
network once he knows the IP address and ORPort of a bridge. What about
|
|
local spoofing attacks? That is, since we never learned an identity
|
|
key fingerprint for the bridge, a local attacker could intercept our
|
|
connection and pretend to be the bridge we had in mind. It turns out
|
|
that giving false information isn't that bad — since the Tor client
|
|
ships with trusted keys for the bridge directory authority and the Tor
|
|
network directory authorities, the user can learn whether he's being
|
|
given a real connection to the bridge authorities or not. (After all,
|
|
if the adversary intercepts every connection the user makes and gives
|
|
him a bad connection each time, there's nothing we can do.)
|
|
|
|
<div class="p"><!----></div>
|
|
What about anonymity-breaking attacks from observing traffic, if the
|
|
blocked user doesn't start out knowing the identity key of his intended
|
|
bridge? The vulnerabilities aren't so bad in this case either — the
|
|
adversary could do similar attacks just by monitoring the network
|
|
traffic.
|
|
|
|
<div class="p"><!----></div>
|
|
Once the Tor client has fetched the bridge's server descriptor, it should
|
|
remember the identity key fingerprint for that bridge relay. Thus if
|
|
the bridge relay moves to a new IP address, the client can query the
|
|
bridge directory authority to look up a fresh server descriptor using
|
|
this fingerprint.
|
|
|
|
<div class="p"><!----></div>
|
|
So we've shown that it's <em>possible</em> to bootstrap into the network
|
|
just by learning the IP address and ORPort of a bridge, but are there
|
|
situations where it's more convenient or more secure to learn the bridge's
|
|
identity fingerprint as well as instead, while bootstrapping? We keep
|
|
that question in mind as we next investigate bootstrapping and discovery.
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc7">
|
|
<a name="sec:discovery">
|
|
7</a> Discovering working bridge relays</h2>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Tor's modular design means that we can develop a better relay component
|
|
independently of developing the discovery component. This modularity's
|
|
great promise is that we can pick any discovery approach we like; but the
|
|
unfortunate fact is that we have no magic bullet for discovery. We're
|
|
in the same arms race as all the other designs we described in
|
|
Section <a href="#sec:related">4</a>.
|
|
|
|
<div class="p"><!----></div>
|
|
In this section we describe a variety of approaches to adding discovery
|
|
components for our design.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc7.1">
|
|
<a name="subsec:first-bridge">
|
|
7.1</a> Bootstrapping: finding your first bridge.</h3>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
In Section <a href="#subsec:relay-together">5.3</a>, we showed that a user who knows
|
|
a working bridge address can use it to reach the bridge authority and
|
|
to stay connected to the Tor network. But how do new users reach the
|
|
bridge authority in the first place? After all, the bridge authority
|
|
will be one of the first addresses that a censor blocks.
|
|
|
|
<div class="p"><!----></div>
|
|
First, we should recognize that most government firewalls are not
|
|
perfect. That is, they may allow connections to Google cache or some
|
|
open proxy servers, or they let file-sharing traffic, Skype, instant
|
|
messaging, or World-of-Warcraft connections through. Different users will
|
|
have different mechanisms for bypassing the firewall initially. Second,
|
|
we should remember that most people don't operate in a vacuum; users will
|
|
hopefully know other people who are in other situations or have other
|
|
resources available. In the rest of this section we develop a toolkit
|
|
of different options and mechanisms, so that we can enable users in a
|
|
diverse set of contexts to bootstrap into the system.
|
|
|
|
<div class="p"><!----></div>
|
|
(For users who can't use any of these techniques, hopefully they know
|
|
a friend who can — for example, perhaps the friend already knows some
|
|
bridge relay addresses. If they can't get around it at all, then we
|
|
can't help them — they should go meet more people or learn more about
|
|
the technology running the firewall in their area.)
|
|
|
|
<div class="p"><!----></div>
|
|
By deploying all the schemes in the toolkit at once, we let bridges and
|
|
blocked users employ the discovery approach that is most appropriate
|
|
for their situation.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc7.2">
|
|
7.2</a> Independent bridges, no central discovery</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
The first design is simply to have no centralized discovery component at
|
|
all. Volunteers run bridges, and we assume they have some blocked users
|
|
in mind and communicate their address information to them out-of-band
|
|
(for example, through Gmail). This design allows for small personal
|
|
bridges that have only one or a handful of users in mind, but it can
|
|
also support an entire community of users. For example, Citizen Lab's
|
|
upcoming Psiphon single-hop proxy tool [<a href="#psiphon" name="CITEpsiphon">13</a>] plans to use this
|
|
<em>social network</em> approach as its discovery component.
|
|
|
|
<div class="p"><!----></div>
|
|
There are several ways to do bootstrapping in this design. In the simple
|
|
case, the operator of the bridge informs each chosen user about his
|
|
bridge's address information and/or keys. A different approach involves
|
|
blocked users introducing new blocked users to the bridges they know.
|
|
That is, somebody in the blocked area can pass along a bridge's address to
|
|
somebody else they trust. This scheme brings in appealing but complex game
|
|
theoretic properties: the blocked user making the decision has an incentive
|
|
only to delegate to trustworthy people, since an adversary who learns
|
|
the bridge's address and filters it makes it unavailable for both of them.
|
|
Also, delegating known bridges to members of your social network can be
|
|
dangerous: an the adversary who can learn who knows which bridges may
|
|
be able to reconstruct the social network.
|
|
|
|
<div class="p"><!----></div>
|
|
Note that a central set of bridge directory authorities can still be
|
|
compatible with a decentralized discovery process. That is, how users
|
|
first learn about bridges is entirely up to the bridges, but the process
|
|
of fetching up-to-date descriptors for them can still proceed as described
|
|
in Section <a href="#sec:bridges">5</a>. Of course, creating a central place that
|
|
knows about all the bridges may not be smart, especially if every other
|
|
piece of the system is decentralized. Further, if a user only knows
|
|
about one bridge and he loses track of it, it may be quite a hassle to
|
|
reach the bridge authority. We address these concerns next.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc7.3">
|
|
7.3</a> Families of bridges, no central discovery</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Because the blocked users are running our software too, we have many
|
|
opportunities to improve usability or robustness. Our second design builds
|
|
on the first by encouraging volunteers to run several bridges at once
|
|
(or coordinate with other bridge volunteers), such that some
|
|
of the bridges are likely to be available at any given time.
|
|
|
|
<div class="p"><!----></div>
|
|
The blocked user's Tor client would periodically fetch an updated set of
|
|
recommended bridges from any of the working bridges. Now the client can
|
|
learn new additions to the bridge pool, and can expire abandoned bridges
|
|
or bridges that the adversary has blocked, without the user ever needing
|
|
to care. To simplify maintenance of the community's bridge pool, each
|
|
community could run its own bridge directory authority — reachable via
|
|
the available bridges, and also mirrored at each bridge.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc7.4">
|
|
7.4</a> Public bridges with central discovery</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
What about people who want to volunteer as bridges but don't know any
|
|
suitable blocked users? What about people who are blocked but don't
|
|
know anybody on the outside? Here we describe how to make use of these
|
|
<em>public bridges</em> in a way that still makes it hard for the attacker
|
|
to learn all of them.
|
|
|
|
<div class="p"><!----></div>
|
|
The basic idea is to divide public bridges into a set of pools based on
|
|
identity key. Each pool corresponds to a <em>distribution strategy</em>:
|
|
an approach to distributing its bridge addresses to users. Each strategy
|
|
is designed to exercise a different scarce resource or property of
|
|
the user.
|
|
|
|
<div class="p"><!----></div>
|
|
How do we divide bridges between these strategy pools such that they're
|
|
evenly distributed and the allocation is hard to influence or predict,
|
|
but also in a way that's amenable to creating more strategies later
|
|
on without reshuffling all the pools? We assign a given bridge
|
|
to a strategy pool by hashing the bridge's identity key along with a
|
|
secret that only the bridge authority knows: the first n bits of this
|
|
hash dictate the strategy pool number, where n is a parameter that
|
|
describes how many strategy pools we want at this point. We choose n=3
|
|
to start, so we divide bridges between 8 pools; but as we later invent
|
|
new distribution strategies, we can increment n to split the 8 into
|
|
16. Since a bridge can't predict the next bit in its hash, it can't
|
|
anticipate which identity key will correspond to a certain new pool
|
|
when the pools are split. Further, since the bridge authority doesn't
|
|
provide any feedback to the bridge about which strategy pool it's in,
|
|
an adversary who signs up bridges with the goal of filling a certain
|
|
pool [<a href="#casc-rep" name="CITEcasc-rep">12</a>] will be hindered.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
The first distribution strategy (used for the first pool) publishes bridge
|
|
addresses in a time-release fashion. The bridge authority divides the
|
|
available bridges into partitions, and each partition is deterministically
|
|
available only in certain time windows. That is, over the course of a
|
|
given time slot (say, an hour), each requester is given a random bridge
|
|
from within that partition. When the next time slot arrives, a new set
|
|
of bridges from the pool are available for discovery. Thus some bridge
|
|
address is always available when a new
|
|
user arrives, but to learn about all bridges the attacker needs to fetch
|
|
all new addresses at every new time slot. By varying the length of the
|
|
time slots, we can make it harder for the attacker to guess when to check
|
|
back. We expect these bridges will be the first to be blocked, but they'll
|
|
help the system bootstrap until they <em>do</em> get blocked. Further,
|
|
remember that we're dealing with different blocking regimes around the
|
|
world that will progress at different rates — so this pool will still
|
|
be useful to some users even as the arms races progress.
|
|
|
|
<div class="p"><!----></div>
|
|
The second distribution strategy publishes bridge addresses based on the IP
|
|
address of the requesting user. Specifically, the bridge authority will
|
|
divide the available bridges in the pool into a bunch of partitions
|
|
(as in the first distribution scheme), hash the requester's IP address
|
|
with a secret of its own (as in the above allocation scheme for creating
|
|
pools), and give the requester a random bridge from the appropriate
|
|
partition. To raise the bar, we should discard the last octet of the
|
|
IP address before inputting it to the hash function, so an attacker
|
|
who only controls a single "/24" network only counts as one user. A
|
|
large attacker like China will still be able to control many addresses,
|
|
but the hassle of establishing connections from each network (or spoofing
|
|
TCP connections) may still slow them down. Similarly, as a special case,
|
|
we should treat IP addresses that are Tor exit nodes as all being on
|
|
the same network.
|
|
|
|
<div class="p"><!----></div>
|
|
The third strategy combines the time-based and location-based
|
|
strategies to further constrain and rate-limit the available bridge
|
|
addresses. Specifically, the bridge address provided in a given time
|
|
slot to a given network location is deterministic within the partition,
|
|
rather than chosen randomly each time from the partition. Thus, repeated
|
|
requests during that time slot from a given network are given the same
|
|
bridge address as the first request.
|
|
|
|
<div class="p"><!----></div>
|
|
The fourth strategy is based on Circumventor's discovery strategy.
|
|
The Circumventor project, realizing that its adoption will remain limited
|
|
if it has no central coordination mechanism, has started a mailing list to
|
|
distribute new proxy addresses every few days. From experimentation it
|
|
seems they have concluded that sending updates every three or four days
|
|
is sufficient to stay ahead of the current attackers.
|
|
|
|
<div class="p"><!----></div>
|
|
The fifth strategy provides an alternative approach to a mailing list:
|
|
users provide an email address and receive an automated response
|
|
listing an available bridge address. We could limit one response per
|
|
email address. To further rate limit queries, we could require a CAPTCHA
|
|
solution
|
|
in each case too. In fact, we wouldn't need to
|
|
implement the CAPTCHA on our side: if we only deliver bridge addresses
|
|
to Yahoo or GMail addresses, we can leverage the rate-limiting schemes
|
|
that other parties already impose for account creation.
|
|
|
|
<div class="p"><!----></div>
|
|
The sixth strategy ties in the social network design with public
|
|
bridges and a reputation system. We pick some seeds — trusted people in
|
|
blocked areas — and give them each a few dozen bridge addresses and a few
|
|
<em>delegation tokens</em>. We run a website next to the bridge authority,
|
|
where users can log in (they connect via Tor, and they don't need to
|
|
provide actual identities, just persistent pseudonyms). Users can delegate
|
|
trust to other people they know by giving them a token, which can be
|
|
exchanged for a new account on the website. Accounts in "good standing"
|
|
then accrue new bridge addresses and new tokens. As usual, reputation
|
|
schemes bring in a host of new complexities [<a href="#rep-anon" name="CITErep-anon">10</a>]: how do we
|
|
decide that an account is in good standing? We could tie reputation
|
|
to whether the bridges they're told about have been blocked — see
|
|
Section <a href="#subsec:geoip">7.7</a> below for initial thoughts on how to discover
|
|
whether bridges have been blocked. We could track reputation between
|
|
accounts (if you delegate to somebody who screws up, it impacts you too),
|
|
or we could use blinded delegation tokens [<a href="#chaum-blind" name="CITEchaum-blind">5</a>] to prevent
|
|
the website from mapping the seeds' social network. We put off deeper
|
|
discussion of the social network reputation strategy for future work.
|
|
|
|
<div class="p"><!----></div>
|
|
Pools seven and eight are held in reserve, in case our currently deployed
|
|
tricks all fail at once and the adversary blocks all those bridges — so
|
|
we can adapt and move to new approaches quickly, and have some bridges
|
|
immediately available for the new schemes. New strategies might be based
|
|
on some other scarce resource, such as relaying traffic for others or
|
|
other proof of energy spent. (We might also worry about the incentives
|
|
for bridges that sign up and get allocated to the reserve pools: will they
|
|
be unhappy that they're not being used? But this is a transient problem:
|
|
if Tor users are bridges by default, nobody will mind not being used yet.
|
|
See also Section <a href="#subsec:incentives">9.4</a>.)
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc7.5">
|
|
7.5</a> Public bridges with coordinated discovery</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
We presented the above discovery strategies in the context of a single
|
|
bridge directory authority, but in practice we will want to distribute the
|
|
operations over several bridge authorities — a single point of failure
|
|
or attack is a bad move. The first answer is to run several independent
|
|
bridge directory authorities, and bridges gravitate to one based on
|
|
their identity key. The better answer would be some federation of bridge
|
|
authorities that work together to provide redundancy but don't introduce
|
|
new security issues. We could even imagine designs where the bridge
|
|
authorities have encrypted versions of the bridge's server descriptors,
|
|
and the users learn a decryption key that they keep private when they
|
|
first hear about the bridge — this way the bridge authorities would not
|
|
be able to learn the IP address of the bridges.
|
|
|
|
<div class="p"><!----></div>
|
|
We leave this design question for future work.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc7.6">
|
|
7.6</a> Assessing whether bridges are useful</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Learning whether a bridge is useful is important in the bridge authority's
|
|
decision to include it in responses to blocked users. For example, if
|
|
we end up with a list of thousands of bridges and only a few dozen of
|
|
them are reachable right now, most blocked users will not end up knowing
|
|
about working bridges.
|
|
|
|
<div class="p"><!----></div>
|
|
There are three components for assessing how useful a bridge is. First,
|
|
is it reachable from the public Internet? Second, what proportion of
|
|
the time is it available? Third, is it blocked in certain jurisdictions?
|
|
|
|
<div class="p"><!----></div>
|
|
The first component can be tested just as we test reachability of
|
|
ordinary Tor servers. Specifically, the bridges do a self-test — connect
|
|
to themselves via the Tor network — before they are willing to
|
|
publish their descriptor, to make sure they're not obviously broken or
|
|
misconfigured. Once the bridges publish, the bridge authority also tests
|
|
reachability to make sure they're not confused or outright lying.
|
|
|
|
<div class="p"><!----></div>
|
|
The second component can be measured and tracked by the bridge authority.
|
|
By doing periodic reachability tests, we can get a sense of how often the
|
|
bridge is available. More complex tests will involve bandwidth-intensive
|
|
checks to force the bridge to commit resources in order to be counted as
|
|
available. We need to evaluate how the relationship of uptime percentage
|
|
should weigh into our choice of which bridges to advertise. We leave
|
|
this to future work.
|
|
|
|
<div class="p"><!----></div>
|
|
The third component is perhaps the trickiest: with many different
|
|
adversaries out there, how do we keep track of which adversaries have
|
|
blocked which bridges, and how do we learn about new blocks as they
|
|
occur? We examine this problem next.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc7.7">
|
|
<a name="subsec:geoip">
|
|
7.7</a> How do we know if a bridge relay has been blocked?</h3>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
There are two main mechanisms for testing whether bridges are reachable
|
|
from inside each blocked area: active testing via users, and passive
|
|
testing via bridges.
|
|
|
|
<div class="p"><!----></div>
|
|
In the case of active testing, certain users inside each area
|
|
sign up as testing relays. The bridge authorities can then use a
|
|
Blossom-like [<a href="#blossom-thesis" name="CITEblossom-thesis">16</a>] system to build circuits through them
|
|
to each bridge and see if it can establish the connection. But how do
|
|
we pick the users? If we ask random users to do the testing (or if we
|
|
solicit volunteers from the users), the adversary should sign up so he
|
|
can enumerate the bridges we test. Indeed, even if we hand-select our
|
|
testers, the adversary might still discover their location and monitor
|
|
their network activity to learn bridge addresses.
|
|
|
|
<div class="p"><!----></div>
|
|
Another answer is not to measure directly, but rather let the bridges
|
|
report whether they're being used.
|
|
Specifically, bridges should install a GeoIP database such as the public
|
|
IP-To-Country list [<a href="#ip-to-country" name="CITEip-to-country">19</a>], and then periodically report to the
|
|
bridge authorities which countries they're seeing use from. This data
|
|
would help us track which countries are making use of the bridge design,
|
|
and can also let us learn about new steps the adversary has taken in
|
|
the arms race. (The compressed GeoIP database is only several hundred
|
|
kilobytes, and we could even automate the update process by serving it
|
|
from the bridge authorities.)
|
|
More analysis of this passive reachability
|
|
testing design is needed to resolve its many edge cases: for example,
|
|
if a bridge stops seeing use from a certain area, does that mean the
|
|
bridge is blocked or does that mean those users are asleep?
|
|
|
|
<div class="p"><!----></div>
|
|
There are many more problems with the general concept of detecting whether
|
|
bridges are blocked. First, different zones of the Internet are blocked
|
|
in different ways, and the actual firewall jurisdictions do not match
|
|
country borders. Our bridge scheme could help us map out the topology
|
|
of the censored Internet, but this is a huge task. More generally,
|
|
if a bridge relay isn't reachable, is that because of a network block
|
|
somewhere, because of a problem at the bridge relay, or just a temporary
|
|
outage somewhere in between? And last, an attacker could poison our
|
|
bridge database by signing up already-blocked bridges. In this case,
|
|
if we're stingy giving out bridge addresses, users in that country won't
|
|
learn working bridges.
|
|
|
|
<div class="p"><!----></div>
|
|
All of these issues are made more complex when we try to integrate this
|
|
testing into our social network reputation system above.
|
|
Since in that case we punish or reward users based on whether bridges
|
|
get blocked, the adversary has new attacks to trick or bog down the
|
|
reputation tracking. Indeed, the bridge authority doesn't even know
|
|
what zone the blocked user is in, so do we blame him for any possible
|
|
censored zone, or what?
|
|
|
|
<div class="p"><!----></div>
|
|
Clearly more analysis is required. The eventual solution will probably
|
|
involve a combination of passive measurement via GeoIP and active
|
|
measurement from trusted testers. More generally, we can use the passive
|
|
feedback mechanism to track usage of the bridge network as a whole — which
|
|
would let us respond to attacks and adapt the design, and it would also
|
|
let the general public track the progress of the project.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc7.8">
|
|
7.8</a> Advantages of deploying all solutions at once</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
For once, we're not in the position of the defender: we don't have to
|
|
defend against every possible filtering scheme; we just have to defend
|
|
against at least one. On the flip side, the attacker is forced to guess
|
|
how to allocate his resources to defend against each of these discovery
|
|
strategies. So by deploying all of our strategies at once, we not only
|
|
increase our chances of finding one that the adversary has difficulty
|
|
blocking, but we actually make <em>all</em> of the strategies more robust
|
|
in the face of an adversary with limited resources.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc8">
|
|
<a name="sec:security">
|
|
8</a> Security considerations</h2>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc8.1">
|
|
8.1</a> Possession of Tor in oppressed areas</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Many people speculate that installing and using a Tor client in areas with
|
|
particularly extreme firewalls is a high risk — and the risk increases
|
|
as the firewall gets more restrictive. This notion certain has merit, but
|
|
there's
|
|
a counter pressure as well: as the firewall gets more restrictive, more
|
|
ordinary people behind it end up using Tor for more mainstream activities,
|
|
such as learning
|
|
about Wall Street prices or looking at pictures of women's ankles. So
|
|
as the restrictive firewall pushes up the number of Tor users, the
|
|
"typical" Tor user becomes more mainstream, and therefore mere
|
|
use or possession of the Tor software is not so surprising.
|
|
|
|
<div class="p"><!----></div>
|
|
It's hard to say which of these pressures will ultimately win out,
|
|
but we should keep both sides of the issue in mind.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc8.2">
|
|
<a name="subsec:upload-padding">
|
|
8.2</a> Observers can tell who is publishing and who is reading</h3>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Tor encrypts traffic on the local network, and it obscures the eventual
|
|
destination of the communication, but it doesn't do much to obscure the
|
|
traffic volume. In particular, a user publishing a home video will have a
|
|
different network fingerprint than a user reading an online news article.
|
|
Based on our assumption in Section <a href="#sec:adversary">2</a> that users who
|
|
publish material are in more danger, should we work to improve Tor's
|
|
security in this situation?
|
|
|
|
<div class="p"><!----></div>
|
|
In the general case this is an extremely challenging task:
|
|
effective <em>end-to-end traffic confirmation attacks</em>
|
|
are known where the adversary observes the origin and the
|
|
destination of traffic and confirms that they are part of the
|
|
same communication [<a href="#danezis:pet2004" name="CITEdanezis:pet2004">8</a>,<a href="#e2e-traffic" name="CITEe2e-traffic">24</a>]. Related are
|
|
<em>website fingerprinting attacks</em>, where the adversary downloads
|
|
a few hundred popular websites, makes a set of "fingerprints" for each
|
|
site, and then observes the target Tor client's traffic to look for
|
|
a match [<a href="#pet05-bissias" name="CITEpet05-bissias">4</a>,<a href="#defensive-dropping" name="CITEdefensive-dropping">21</a>]. But can we do better
|
|
against a limited adversary who just does coarse-grained sweeps looking
|
|
for unusually prolific publishers?
|
|
|
|
<div class="p"><!----></div>
|
|
One answer is for bridge users to automatically send bursts of padding
|
|
traffic periodically. (This traffic can be implemented in terms of
|
|
long-range drop cells, which are already part of the Tor specification.)
|
|
Of course, convincingly simulating an actual human publishing interesting
|
|
content is a difficult arms race, but it may be worthwhile to at least
|
|
start the race. More research remains.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc8.3">
|
|
8.3</a> Anonymity effects from acting as a bridge relay</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Against some attacks, relaying traffic for others can improve
|
|
anonymity. The simplest example is an attacker who owns a small number
|
|
of Tor servers. He will see a connection from the bridge, but he won't
|
|
be able to know whether the connection originated there or was relayed
|
|
from somebody else. More generally, the mere uncertainty of whether the
|
|
traffic originated from that user may be helpful.
|
|
|
|
<div class="p"><!----></div>
|
|
There are some cases where it doesn't seem to help: if an attacker can
|
|
watch all of the bridge's incoming and outgoing traffic, then it's easy
|
|
to learn which connections were relayed and which started there. (In this
|
|
case he still doesn't know the final destinations unless he is watching
|
|
them too, but in this case bridges are no better off than if they were
|
|
an ordinary client.)
|
|
|
|
<div class="p"><!----></div>
|
|
There are also some potential downsides to running a bridge. First, while
|
|
we try to make it hard to enumerate all bridges, it's still possible to
|
|
learn about some of them, and for some people just the fact that they're
|
|
running one might signal to an attacker that they place a higher value
|
|
on their anonymity. Second, there are some more esoteric attacks on Tor
|
|
relays that are not as well-understood or well-tested — for example, an
|
|
attacker may be able to "observe" whether the bridge is sending traffic
|
|
even if he can't actually watch its network, by relaying traffic through
|
|
it and noticing changes in traffic timing [<a href="#attack-tor-oak05" name="CITEattack-tor-oak05">25</a>]. On
|
|
the other hand, it may be that limiting the bandwidth the bridge is
|
|
willing to relay will allow this sort of attacker to determine if it's
|
|
being used as a bridge but not easily learn whether it is adding traffic
|
|
of its own.
|
|
|
|
<div class="p"><!----></div>
|
|
We also need to examine how entry guards fit in. Entry guards
|
|
(a small set of nodes that are always used for the first
|
|
step in a circuit) help protect against certain attacks
|
|
where the attacker runs a few Tor servers and waits for
|
|
the user to choose these servers as the beginning and end of her
|
|
circuit<a href="#tthFtNtAAC" name="tthFrefAAC"><sup>2</sup></a>.
|
|
If the blocked user doesn't use the bridge's entry guards, then the bridge
|
|
doesn't gain as much cover benefit. On the other hand, what design changes
|
|
are needed for the blocked user to use the bridge's entry guards without
|
|
learning what they are (this seems hard), and even if we solve that,
|
|
do they then need to use the guards' guards and so on down the line?
|
|
|
|
<div class="p"><!----></div>
|
|
It is an open research question whether the benefits of running a bridge
|
|
outweigh the risks. A lot of the decision rests on which attacks the
|
|
users are most worried about. For most users, we don't think running a
|
|
bridge relay will be that damaging, and it could help quite a bit.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc8.4">
|
|
<a name="subsec:cafes-and-livecds">
|
|
8.4</a> Trusting local hardware: Internet cafes and LiveCDs</h3>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Assuming that users have their own trusted hardware is not
|
|
always reasonable.
|
|
|
|
<div class="p"><!----></div>
|
|
For Internet cafe Windows computers that let you attach your own USB key,
|
|
a USB-based Tor image would be smart. There's Torpark, and hopefully
|
|
there will be more thoroughly analyzed and trustworthy options down the
|
|
road. Worries remain about hardware or software keyloggers and other
|
|
spyware, as well as physical surveillance.
|
|
|
|
<div class="p"><!----></div>
|
|
If the system lets you boot from a CD or from a USB key, you can gain
|
|
a bit more security by bringing a privacy LiveCD with you. (This
|
|
approach isn't foolproof either of course, since hardware
|
|
keyloggers and physical surveillance are still a worry).
|
|
|
|
<div class="p"><!----></div>
|
|
In fact, LiveCDs are also useful if it's your own hardware, since it's
|
|
easier to avoid leaving private data and logs scattered around the
|
|
system.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc8.5">
|
|
<a name="subsec:trust-chain">
|
|
8.5</a> The trust chain</h3>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Tor's "public key infrastructure" provides a chain of trust to
|
|
let users verify that they're actually talking to the right servers.
|
|
There are four pieces to this trust chain.
|
|
|
|
<div class="p"><!----></div>
|
|
First, when Tor clients are establishing circuits, at each step
|
|
they demand that the next Tor server in the path prove knowledge of
|
|
its private key [<a href="#tor-design" name="CITEtor-design">11</a>]. This step prevents the first node
|
|
in the path from just spoofing the rest of the path. Second, the
|
|
Tor directory authorities provide a signed list of servers along with
|
|
their public keys — so unless the adversary can control a threshold
|
|
of directory authorities, he can't trick the Tor client into using other
|
|
Tor servers. Third, the location and keys of the directory authorities,
|
|
in turn, is hard-coded in the Tor source code — so as long as the user
|
|
got a genuine version of Tor, he can know that he is using the genuine
|
|
Tor network. And last, the source code and other packages are signed
|
|
with the GPG keys of the Tor developers, so users can confirm that they
|
|
did in fact download a genuine version of Tor.
|
|
|
|
<div class="p"><!----></div>
|
|
In the case of blocked users contacting bridges and bridge directory
|
|
authorities, the same logic applies in parallel: the blocked users fetch
|
|
information from both the bridge authorities and the directory authorities
|
|
for the `main' Tor network, and they combine this information locally.
|
|
|
|
<div class="p"><!----></div>
|
|
How can a user in an oppressed country know that he has the correct
|
|
key fingerprints for the developers? As with other security systems, it
|
|
ultimately comes down to human interaction. The keys are signed by dozens
|
|
of people around the world, and we have to hope that our users have met
|
|
enough people in the PGP web of trust
|
|
that they can learn
|
|
the correct keys. For users that aren't connected to the global security
|
|
community, though, this question remains a critical weakness.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc9">
|
|
<a name="sec:reachability">
|
|
9</a> Maintaining reachability</h2>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc9.1">
|
|
9.1</a> How many bridge relays should you know about?</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
The strategies described in Section <a href="#sec:discovery">7</a> talked about
|
|
learning one bridge address at a time. But if most bridges are ordinary
|
|
Tor users on cable modem or DSL connection, many of them will disappear
|
|
and/or move periodically. How many bridge relays should a blocked user
|
|
know about so that she is likely to have at least one reachable at any
|
|
given point? This is already a challenging problem if we only consider
|
|
natural churn: the best approach is to see what bridges we attract in
|
|
reality and measure their churn. We may also need to factor in a parameter
|
|
for how quickly bridges get discovered and blocked by the attacker;
|
|
we leave this for future work after we have more deployment experience.
|
|
|
|
<div class="p"><!----></div>
|
|
A related question is: if the bridge relays change IP addresses
|
|
periodically, how often does the blocked user need to fetch updates in
|
|
order to keep from being cut out of the loop?
|
|
|
|
<div class="p"><!----></div>
|
|
Once we have more experience and intuition, we should explore technical
|
|
solutions to this problem too. For example, if the discovery strategies
|
|
give out k bridge addresses rather than a single bridge address, perhaps
|
|
we can improve robustness from the user perspective without significantly
|
|
aiding the adversary. Rather than giving out a new random subset of k
|
|
addresses at each point, we could bind them together into <em>bridge
|
|
families</em>, so all users that learn about one member of the bridge family
|
|
are told about the rest as well.
|
|
|
|
<div class="p"><!----></div>
|
|
This scheme may also help defend against attacks to map the set of
|
|
bridges. That is, if all blocked users learn a random subset of bridges,
|
|
the attacker should learn about a few bridges, monitor the country-level
|
|
firewall for connections to them, then watch those users to see what
|
|
other bridges they use, and repeat. By segmenting the bridge address
|
|
space, we can limit the exposure of other users.
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc9.2">
|
|
<a name="subsec:block-cable">
|
|
9.2</a> Cablemodem users don't usually provide important websites</h3>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Another attacker we might be concerned about is that the attacker could
|
|
just block all DSL and cablemodem network addresses, on the theory that
|
|
they don't run any important services anyway. If most of our bridges
|
|
are on these networks, this attack could really hurt.
|
|
|
|
<div class="p"><!----></div>
|
|
The first answer is to aim to get volunteers both from traditionally
|
|
"consumer" networks and also from traditionally "producer" networks.
|
|
Since bridges don't need to be Tor exit nodes, as we improve our usability
|
|
it seems quite feasible to get a lot of websites helping out.
|
|
|
|
<div class="p"><!----></div>
|
|
The second answer (not as practical) would be to encourage more use of
|
|
consumer networks for popular and useful Internet services.
|
|
|
|
<div class="p"><!----></div>
|
|
A related attack we might worry about is based on large countries putting
|
|
economic pressure on companies that want to expand their business. For
|
|
example, what happens if Verizon wants to sell services in China, and
|
|
China pressures Verizon to discourage its users in the free world from
|
|
running bridges?
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc9.3">
|
|
9.3</a> Scanning resistance: making bridges more subtle</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
If it's trivial to verify that a given address is operating as a bridge,
|
|
and most bridges run on a predictable port, then it's conceivable our
|
|
attacker could scan the whole Internet looking for bridges. (In fact,
|
|
he can just concentrate on scanning likely networks like cablemodem
|
|
and DSL services — see Section <a href="#subsec:block-cable">9.2</a>
|
|
above for
|
|
related attacks.) It would be nice to slow down this attack. It would
|
|
be even nicer to make it hard to learn whether we're a bridge without
|
|
first knowing some secret. We call this general property <em>scanning
|
|
resistance</em>, and it goes along with normalizing Tor's TLS handshake and
|
|
network fingerprint.
|
|
|
|
<div class="p"><!----></div>
|
|
We could provide a password to the blocked user, and she (or her Tor
|
|
client) provides a nonced hash of this password when she connects. We'd
|
|
need to give her an ID key for the bridge too (in addition to the IP
|
|
address and port — see Section <a href="#subsec:id-address">6.1</a>), and wait to
|
|
present the password until we've finished the TLS handshake, else it
|
|
would look unusual. If Alice can authenticate the bridge before she
|
|
tries to send her password, we can resist an adversary who pretends
|
|
to be the bridge and launches a man-in-the-middle attack to learn the
|
|
password. But even if she can't, we still resist against widespread
|
|
scanning.
|
|
|
|
<div class="p"><!----></div>
|
|
How should the bridge behave if accessed without the correct
|
|
authorization? Perhaps it should act like an unconfigured HTTPS server
|
|
("welcome to the default Apache page"), or maybe it should mirror
|
|
and act like common websites, or websites randomly chosen from Google.
|
|
|
|
<div class="p"><!----></div>
|
|
We might assume that the attacker can recognize HTTPS connections that
|
|
use self-signed certificates. (This process would be resource-intensive
|
|
but not out of the realm of possibility.) But even in this case, many
|
|
popular websites around the Internet use self-signed or just plain broken
|
|
SSL certificates.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc9.4">
|
|
<a name="subsec:incentives">
|
|
9.4</a> How to motivate people to run bridge relays</h3>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
One of the traditional ways to get people to run software that benefits
|
|
others is to give them motivation to install it themselves. An often
|
|
suggested approach is to install it as a stunning screensaver so everybody
|
|
will be pleased to run it. We take a similar approach here, by leveraging
|
|
the fact that these users are already interested in protecting their
|
|
own Internet traffic, so they will install and run the software.
|
|
|
|
<div class="p"><!----></div>
|
|
Eventually, we may be able to make all Tor users become bridges if they
|
|
pass their self-reachability tests — the software and installers need
|
|
more work on usability first, but we're making progress.
|
|
|
|
<div class="p"><!----></div>
|
|
In the mean time, we can make a snazzy network graph with
|
|
Vidalia<a href="#tthFtNtAAD" name="tthFrefAAD"><sup>3</sup></a> that
|
|
emphasizes the connections the bridge user is currently relaying.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc9.5">
|
|
<a name="subsec:publicity">
|
|
9.5</a> Publicity attracts attention</h3>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Many people working on this field want to publicize the existence
|
|
and extent of censorship concurrently with the deployment of their
|
|
circumvention software. The easy reason for this two-pronged push is
|
|
to attract volunteers for running proxies in their systems; but in many
|
|
cases their main goal is not to focus on actually allowing individuals
|
|
to circumvent the firewall, but rather to educate the world about the
|
|
censorship. The media also tries to do its part by broadcasting the
|
|
existence of each new circumvention system.
|
|
|
|
<div class="p"><!----></div>
|
|
But at the same time, this publicity attracts the attention of the
|
|
censors. We can slow down the arms race by not attracting as much
|
|
attention, and just spreading by word of mouth. If our goal is to
|
|
establish a solid social network of bridges and bridge users before
|
|
the adversary gets involved, does this extra attention work to our
|
|
disadvantage?
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc9.6">
|
|
9.6</a> The Tor website: how to get the software</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
One of the first censoring attacks against a system like ours is to
|
|
block the website and make the software itself hard to find. Our system
|
|
should work well once the user is running an authentic
|
|
copy of Tor and has found a working bridge, but to get to that point
|
|
we rely on their individual skills and ingenuity.
|
|
|
|
<div class="p"><!----></div>
|
|
Right now, most countries that block access to Tor block only the main
|
|
website and leave mirrors and the network itself untouched.
|
|
Falling back on word-of-mouth is always a good last resort, but we should
|
|
also take steps to make sure it's relatively easy for users to get a copy,
|
|
such as publicizing the mirrors more and making copies available through
|
|
other media. We might also mirror the latest version of the software on
|
|
each bridge, so users who hear about an honest bridge can get a good
|
|
copy.
|
|
See Section <a href="#subsec:first-bridge">7.1</a> for more discussion.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc10">
|
|
<a name="sec:future">
|
|
10</a> Future designs</h2>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
<h3><a name="tth_sEc10.1">
|
|
10.1</a> Bridges inside the blocked network too</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
Assuming actually crossing the firewall is the risky part of the
|
|
operation, can we have some bridge relays inside the blocked area too,
|
|
and more established users can use them as relays so they don't need to
|
|
communicate over the firewall directly at all? A simple example here is
|
|
to make new blocked users into internal bridges also — so they sign up
|
|
on the bridge authority as part of doing their query, and we give out
|
|
their addresses
|
|
rather than (or along with) the external bridge addresses. This design
|
|
is a lot trickier because it brings in the complexity of whether the
|
|
internal bridges will remain available, can maintain reachability with
|
|
the outside world, etc.
|
|
|
|
<div class="p"><!----></div>
|
|
More complex future designs involve operating a separate Tor network
|
|
inside the blocked area, and using <em>hidden service bridges</em> — bridges
|
|
that can be accessed by users of the internal Tor network but whose
|
|
addresses are not published or findable, even by these users — to get
|
|
from inside the firewall to the rest of the Internet. But this design
|
|
requires directory authorities to run inside the blocked area too,
|
|
and they would be a fine target to take down the network.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<h2><a name="tth_sEc11">
|
|
<a name="sec:conclusion">
|
|
11</a> Next Steps</h2>
|
|
</a>
|
|
|
|
<div class="p"><!----></div>
|
|
Technical solutions won't solve the whole censorship problem. After all,
|
|
the firewalls in places like China are <em>socially</em> very
|
|
successful, even if technologies and tricks exist to get around them.
|
|
However, having a strong technical solution is still necessary as one
|
|
important piece of the puzzle.
|
|
|
|
<div class="p"><!----></div>
|
|
In this paper, we have shown that Tor provides a great set of building
|
|
blocks to start from. The next steps are to deploy prototype bridges and
|
|
bridge authorities, implement some of the proposed discovery strategies,
|
|
and then observe the system in operation and get more intuition about
|
|
the actual requirements and adversaries we're up against.
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<h2>References</h2>
|
|
|
|
<dl compact="compact">
|
|
<dt><a href="#CITEeconymics" name="econymics">[1]</a></dt><dd>
|
|
Alessandro Acquisti, Roger Dingledine, and Paul Syverson.
|
|
On the economics of anonymity.
|
|
In Rebecca N. Wright, editor, <em>Financial Cryptography</em>.
|
|
Springer-Verlag, LNCS 2742, 2003.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEfreedom21-security" name="freedom21-security">[2]</a></dt><dd>
|
|
Adam Back, Ian Goldberg, and Adam 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="#CITEweb-mix" name="web-mix">[3]</a></dt><dd>
|
|
Oliver Berthold, Hannes Federrath, and Stefan Köpsell.
|
|
Web MIXes: A system for anonymous and unobservable Internet
|
|
access.
|
|
In H. Federrath, editor, <em>Designing Privacy Enhancing
|
|
Technologies: Workshop on Design Issue in Anonymity and Unobservability</em>.
|
|
Springer-Verlag, LNCS 2009, 2000.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEpet05-bissias" name="pet05-bissias">[4]</a></dt><dd>
|
|
George Dean Bissias, Marc Liberatore, and Brian Neil Levine.
|
|
Privacy vulnerabilities in encrypted http streams.
|
|
In <em>Proceedings of Privacy Enhancing Technologies workshop (PET
|
|
2005)</em>, May 2005.
|
|
|
|
<a href="http://prisms.cs.umass.edu/brian/pubs/bissias.liberatore.pet.2005.pdf"><tt>http://prisms.cs.umass.edu/brian/pubs/bissias.liberatore.pet.2005.pdf</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEchaum-blind" name="chaum-blind">[5]</a></dt><dd>
|
|
David Chaum.
|
|
Blind signatures for untraceable payments.
|
|
In D. Chaum, R.L. Rivest, and A.T. Sherman, editors, <em>Advances in
|
|
Cryptology: Proceedings of Crypto 82</em>, pages 199-203. Plenum Press, 1983.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEfreenet-pets00" name="freenet-pets00">[6]</a></dt><dd>
|
|
Ian Clarke, Oskar Sandberg, Brandon Wiley, and Theodore W. Hong.
|
|
Freenet: A distributed anonymous information storage and retrieval
|
|
system.
|
|
In H. Federrath, editor, <em>Designing Privacy Enhancing
|
|
Technologies: Workshop on Design Issue in Anonymity and Unobservability</em>,
|
|
pages 46-66. Springer-Verlag, LNCS 2009, July 2000.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEclayton:pet2006" name="clayton:pet2006">[7]</a></dt><dd>
|
|
Richard Clayton, Steven J. Murdoch, and Robert N. M. Watson.
|
|
Ignoring the great firewall of china.
|
|
In <em>Proceedings of the Sixth Workshop on Privacy Enhancing
|
|
Technologies (PET 2006)</em>, Cambridge, UK, June 2006. Springer.
|
|
<a href="http://www.cl.cam.ac.uk/~rnc1/ignoring.pdf"><tt>http://www.cl.cam.ac.uk/~rnc1/ignoring.pdf</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEdanezis:pet2004" name="danezis:pet2004">[8]</a></dt><dd>
|
|
George Danezis.
|
|
The traffic analysis of continuous-time mixes.
|
|
In David Martin and Andrei Serjantov, editors, <em>Privacy Enhancing
|
|
Technologies (PET 2004)</em>, LNCS, May 2004.
|
|
<a href="http://www.cl.cam.ac.uk/users/gd216/cmm2.pdf"><tt>http://www.cl.cam.ac.uk/users/gd216/cmm2.pdf</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEusability:weis2006" name="usability:weis2006">[9]</a></dt><dd>
|
|
Roger Dingledine and Nick Mathewson.
|
|
Anonymity loves company: Usability and the network effect.
|
|
In <em>Proceedings of the Fifth Workshop on the Economics of
|
|
Information Security (WEIS 2006)</em>, Cambridge, UK, June 2006.
|
|
<a href="http://freehaven.net/doc/wupss04/usability.pdf"><tt>http://freehaven.net/doc/wupss04/usability.pdf</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITErep-anon" name="rep-anon">[10]</a></dt><dd>
|
|
Roger Dingledine, Nick Mathewson, and Paul Syverson.
|
|
Reputation in P2P Anonymity Systems.
|
|
In <em>Proceedings of Workshop on Economics of Peer-to-Peer
|
|
Systems</em>, June 2003.
|
|
<a href="http://freehaven.net/doc/econp2p03/econp2p03.pdf"><tt>http://freehaven.net/doc/econp2p03/econp2p03.pdf</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEtor-design" name="tor-design">[11]</a></dt><dd>
|
|
Roger Dingledine, Nick Mathewson, and Paul Syverson.
|
|
Tor: The second-generation onion router.
|
|
In <em>Proceedings of the 13th USENIX Security Symposium</em>, August
|
|
2004.
|
|
<a href="http://tor.eff.org/tor-design.pdf"><tt>http://tor.eff.org/tor-design.pdf</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEcasc-rep" name="casc-rep">[12]</a></dt><dd>
|
|
Roger Dingledine and Paul Syverson.
|
|
Reliable MIX Cascade Networks through Reputation.
|
|
In Matt Blaze, editor, <em>Financial Cryptography</em>. Springer-Verlag,
|
|
LNCS 2357, 2002.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEpsiphon" name="psiphon">[13]</a></dt><dd>
|
|
Ronald Deibert et al.
|
|
Psiphon.
|
|
<a href="http://psiphon.civisec.org/"><tt>http://psiphon.civisec.org/</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEinfranet" name="infranet">[14]</a></dt><dd>
|
|
Nick Feamster, Magdalena Balazinska, Greg Harfst, Hari Balakrishnan, and David
|
|
Karger.
|
|
Infranet: Circumventing web censorship and surveillance.
|
|
In <em>Proceedings of the 11th USENIX Security Symposium</em>, August
|
|
2002.
|
|
<a href="http://nms.lcs.mit.edu/~feamster/papers/usenixsec2002.pdf"><tt>http://nms.lcs.mit.edu/~feamster/papers/usenixsec2002.pdf</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEactive-wardens" name="active-wardens">[15]</a></dt><dd>
|
|
Gina Fisk, Mike Fisk, Christos Papadopoulos, and Joshua Neil.
|
|
Eliminating steganography in internet traffic with active wardens.
|
|
In Fabien Petitcolas, editor, <em>Information Hiding Workshop (IH
|
|
2002)</em>. Springer-Verlag, LNCS 2578, October 2002.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEblossom-thesis" name="blossom-thesis">[16]</a></dt><dd>
|
|
Geoffrey Goodell.
|
|
<em>Perspective Access Networks</em>.
|
|
PhD thesis, Harvard University, July 2006.
|
|
<a href="http://afs.eecs.harvard.edu/~goodell/thesis.pdf"><tt>http://afs.eecs.harvard.edu/~goodell/thesis.pdf</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEgoodell-syverson06" name="goodell-syverson06">[17]</a></dt><dd>
|
|
Geoffrey Goodell and Paul Syverson.
|
|
The right place at the right time: The use of network location in
|
|
authentication and abuse prevention, 2006.
|
|
Submitted.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEcircumventor" name="circumventor">[18]</a></dt><dd>
|
|
Bennett Haselton.
|
|
How to install the Circumventor program.
|
|
|
|
<a href="http://www.peacefire.org/circumventor/simple-circumventor-instructions.html"><tt>http://www.peacefire.org/circumventor/simple-circumventor-instructions.html</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEip-to-country" name="ip-to-country">[19]</a></dt><dd>
|
|
Ip-to-country database.
|
|
<a href="http://ip-to-country.webhosting.info/"><tt>http://ip-to-country.webhosting.info/</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEkoepsell:wpes2004" name="koepsell:wpes2004">[20]</a></dt><dd>
|
|
Stefan Köpsell and Ulf Hilling.
|
|
How to achieve blocking resistance for existing systems enabling
|
|
anonymous web surfing.
|
|
In <em>Proceedings of the Workshop on Privacy in the Electronic
|
|
Society (WPES 2004)</em>, Washington, DC, USA, October 2004.
|
|
<a href="http://freehaven.net/anonbib/papers/p103-koepsell.pdf"><tt>http://freehaven.net/anonbib/papers/p103-koepsell.pdf</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEdefensive-dropping" name="defensive-dropping">[21]</a></dt><dd>
|
|
Brian N. Levine, Michael K. Reiter, Chenxi Wang, and Matthew Wright.
|
|
Timing analysis in low-latency mix-based systems.
|
|
In Ari Juels, editor, <em>Financial Cryptography</em>. Springer-Verlag,
|
|
LNCS (forthcoming), 2004.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEmackinnon-personal" name="mackinnon-personal">[22]</a></dt><dd>
|
|
Rebecca MacKinnon.
|
|
Private communication, 2006.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEcgiproxy" name="cgiproxy">[23]</a></dt><dd>
|
|
James Marshall.
|
|
CGIProxy: HTTP/FTP Proxy in a CGI Script.
|
|
<a href="http://www.jmarshall.com/tools/cgiproxy/"><tt>http://www.jmarshall.com/tools/cgiproxy/</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEe2e-traffic" name="e2e-traffic">[24]</a></dt><dd>
|
|
Nick Mathewson and Roger Dingledine.
|
|
Practical traffic analysis: Extending and resisting statistical
|
|
disclosure.
|
|
In David Martin and Andrei Serjantov, editors, <em>Privacy Enhancing
|
|
Technologies (PET 2004)</em>, LNCS, May 2004.
|
|
<a href="http://freehaven.net/doc/e2e-traffic/e2e-traffic.pdf"><tt>http://freehaven.net/doc/e2e-traffic/e2e-traffic.pdf</tt></a>.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEattack-tor-oak05" name="attack-tor-oak05">[25]</a></dt><dd>
|
|
Steven J. Murdoch and George Danezis.
|
|
Low-cost traffic analysis of tor.
|
|
In <em>IEEE Symposium on Security and Privacy</em>. IEEE CS, May 2005.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEtcpstego" name="tcpstego">[26]</a></dt><dd>
|
|
Steven J. Murdoch and Stephen Lewis.
|
|
Embedding covert channels into TCP/IP.
|
|
In Mauro Barni, Jordi Herrera-Joancomartí, Stefan Katzenbeisser,
|
|
and Fernando Pérez-González, editors, <em>Information Hiding: 7th
|
|
International Workshop</em>, volume 3727 of <em>LNCS</em>, pages 247-261,
|
|
Barcelona, Catalonia (Spain), June 2005. Springer-Verlag.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEptacek98insertion" name="ptacek98insertion">[27]</a></dt><dd>
|
|
Thomas H. Ptacek and Timothy N. Newsham.
|
|
Insertion, evasion, and denial of service: Eluding network intrusion
|
|
detection.
|
|
Technical report, Secure Networks, Inc., Suite 330, 1201 5th Street
|
|
S.W, Calgary, Alberta, Canada, T2R-0Y6, 1998.
|
|
|
|
<div class="p"><!----></div>
|
|
</dd>
|
|
<dt><a href="#CITEzuckerman-threatmodels" name="zuckerman-threatmodels">[28]</a></dt><dd>
|
|
Ethan Zuckerman.
|
|
We've got to adjust some of our threat models.
|
|
<a href="http://www.ethanzuckerman.com/blog/?p=1019"><tt>http://www.ethanzuckerman.com/blog/?p=1019</tt></a>.</dd>
|
|
</dl>
|
|
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
|
|
<div class="p"><!----></div>
|
|
<hr /><h3>Footnotes:</h3>
|
|
|
|
<div class="p"><!----></div>
|
|
<a name="tthFtNtAAB"></a><a href="#tthFrefAAB"><sup>1</sup></a>So far in places
|
|
like China, the authorities mainly go after people who publish materials
|
|
and coordinate organized movements [<a href="#mackinnon-personal" name="CITEmackinnon-personal">22</a>].
|
|
If they find that a
|
|
user happens to be reading a site that should be blocked, the typical
|
|
response is simply to block the site. Of course, even with an encrypted
|
|
connection, the adversary may be able to distinguish readers from
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publishers by observing whether Alice is mostly downloading bytes or mostly
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uploading them — we discuss this issue more in
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Section <a href="#subsec:upload-padding">8.2</a>.
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<div class="p"><!----></div>
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<a name="tthFtNtAAC"></a><a href="#tthFrefAAC"><sup>2</sup></a><a href="http://wiki.noreply.org/noreply/TheOnionRouter/TorFAQ\#EntryGuards"><tt>http://wiki.noreply.org/noreply/TheOnionRouter/TorFAQ#EntryGuards</tt></a>
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<div class="p"><!----></div>
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<a name="tthFtNtAAD"></a><a href="#tthFrefAAD"><sup>3</sup></a><a href="http://vidalia-project.net/"><tt>http://vidalia-project.net/</tt></a>
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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 3.77.<br />On 11 May 2007, 21:49.</small>
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</html>
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