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    <a href="<page index>">Home &raquo; </a>
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    <h1>Tor: Pluggable Transports</h1>

    <hr>

    <h3>Sometimes the Tor network is censored, and you can't connect to
    it.</h3>

    <br />

    <p>
    An increasing number of censoring countries are using Deep Packet
    Inspection (DPI) to classify Internet traffic flows by protocol.
    While Tor uses <a href="<page docs/bridges>">bridge relays</a> to
    get around a censor that blocks by IP address, the censor can use
    DPI to recognize and filter Tor traffic flows even when they connect
    to unexpected IP addresses.
    </p>

    <h3>Pluggable Transports help you bypass censorship against Tor.</h3>

    <br />

    <p>
    Pluggable Transports (PT) transform the Tor traffic flow between the client
    and the bridge. This way, censors who monitor traffic between the client
    and the bridge will see innocent-looking transformed traffic instead of the
    actual Tor traffic.
    External programs can talk to Tor clients and Tor bridges using the <a
    href="https://gitweb.torproject.org/torspec.git/tree/pt-spec.txt">pluggable
    transport API</a>, to make it easier to build interoperable programs.
    </p>


    <h3>Learn more:</h3>

    <br />

    <ul>
        <li><a href="#user">How to use a Pluggable Transport</a></li>
        <li><a href="#operator">How to become a PT bridge operator</a></li>
        <li><a href="#developer">How to become a PT developer</a></li>
        <li><a href="#list-of-pts">List of PTs organized by their status</a></li>
    </ul>
    <br /><hr>

    <h2 id="user">How to use PTs to bypass censorship</h2>
    <p>If connections to the Tor network are being blocked by your ISP or country, follow these instructions:
    </p>

    <a href="$(IMGROOT)/PT/2016-07-how-to-use-PT.png"> <img src="$(IMGROOT)/PT/2016-07-how-to-use-PT.png" width="830" alt="How to use PTs: 1-download tor (send email to gettor@torproject.org); 2 select configure 3; check my isp blocks tor option; 4 select obfs4; 5 press connect" /></a>
    <!-- TODO: move alt to instructions in plain text for visually impaird users -->

    <br /><br />

    <hr>

    <h2 id="operator">Become a PT bridge operator:</h2>

    <h3>How to run PTs to help censored users</h3>

    <br />

    <p>
    Anyone can set up a PT bridge server and help provide bandwidth to users
    who needs it. Once you set up a transport type, your bridge will
    automatically advertise support for the transport in its descriptor.
    </p>

    <p>
    <strong>obfs4</strong> is currently the most effective transport to bypass
    censorship. We are asking volunteers to run bridges for it.<br /> To learn
    how to run this transport, please visit the <a
    href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports/obfs4proxy">obfs4proxy
    wiki page</a>.
    </p>

    <p>
    <a
    href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports#BecomeaPTbridgeoperator">Go
    to our wiki</a> to learn how to set up other types of PTs.
    </p>

    <hr>

    <h2 id="developer">Become a PT developer:</h2>

    <p> The links below are the main documentation for PT developers</p>

    <ul>
        <li><a href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports/GuidelinesForDeployingPTs">Guidelines for deploying Pluggable Transports on Tor Browser</a></li>
        <li><a href="https://gitweb.torproject.org/torspec.git/tree/pt-spec.txt">PT technical spec</a></li>
        <li><a href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports/PTEvaluationCriteria">Pluggable Transports Evaluation Criteria</a></li>
    </ul>

    <p><a href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports">Our wiki</a> is also a great source of information, such as how to <a href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports#Waystofollowandjointheconversation">get in touch with the community</a>, <a href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports/ideas">ideas for new PTs</a>, how to <a href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports/WorkListForDevsToHelpOutWith">help with PTs already deployed</a> and much more.</p>

    <hr>

    <h2 id="list-of-pts">List of PTs organized by status:</h2>

    <h3>Currently deployed PTs</h3>

    <p>These Pluggable Transports are currently deployed in Tor Browser, and you can start using them by <a href="<page download/download-easy>">downloading and using Tor Browser</a>.
    </p>

      <ul>
        <li><a href="https://github.com/Yawning/obfs4/blob/master/doc/obfs4-spec.txt"><b>obfs4</b></a></li>
            <ul>
            <li><strong>Description:</strong> Is a transport with the same features as <a href="http://www.cs.kau.se/philwint/scramblesuit/"><b>ScrambleSuit</b></a> but utilizing Dan Bernstein's <a href="http://elligator.cr.yp.to/elligator-20130828.pdf">elligator2</b></a> technique for public key obfuscation, and the <a href="https://gitweb.torproject.org/torspec.git/tree/proposals/216-ntor-handshake.txt">ntor protocol</a> for one-way authentication. This results in a faster protocol.</li>
            <li><strong>Language:</strong> Go</li>
            <li><strong>Maintainer:</strong> Yawning Angel</li>
            <li><strong>Evaluation:</strong> <a href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports/Obfs4Evaluation">obfs4 Evaluation</a></li>
            </ul>
        </li>
        <!-- TODO: update the link with repo hosted on git.tpo. and make a note that this client supports obfs3 -->

        <li><a href="https://trac.torproject.org/projects/tor/wiki/doc/meek"><b>meek</b></a></li>
            <ul>
                <li><strong>Description:</strong> Is a transport that uses HTTP for carrying bytes and TLS for obfuscation. Traffic is relayed through a third-party server (Google App Engine). It uses a trick to talk to the third party so that it looks like it is talking to an unblocked server.</li>
                <li><strong>Language:</strong> Go</li>
                <li><strong>Maintainer:</strong> David Fifield</li>
                <li><strong>Evaluation:</strong> <a href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports/MeekEvaluation">meek Evaluation</a></li>
            </ul>
        </li>
        <!-- TODO: add more info about meek. include amazon and azure and maybe remove google for now -->

        <li><a href="https://fteproxy.org/"><b>Format-Transforming Encryption</b></a> (FTE)</li>
            <ul>
                <li><strong>Description:</strong> It transforms Tor traffic to arbitrary
        formats using their language descriptions. See the <a href="https://kpdyer.com/publications/ccs2013-fte.pdf">research paper</a>.</li>
                <li><strong>Language:</strong> Python/C++</li>
                <li><strong>Maintainer:</strong> Kevin Dyer</li>
                <li><strong>Evaluation:</strong> <a href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports/FteEvaluation">FTE Evaluation</a></li>
            </ul> 
        </li>

        <li><a href="http://www.cs.kau.se/philwint/scramblesuit/"><b>ScrambleSuit</b></a></li>
            <ul>
                <li><strong>Description:</strong> Is a pluggable transport that protects against follow-up probing attacks and is also capable of changing its network fingerprint (packet length distribution, inter-arrival times, etc.).</li>
                <li><strong>Language:</strong> Python</li>
                <li><strong>Maintainer:</strong> Philipp Winter</li>
                <li><strong>Evaluation:</strong> <a href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports/ScrambleSuitEvaluation"> ScrambleSuit Evaluation</a></li>
            </ul>
        </li>

        <!-- TODO: it's unclear whether orbot still uses obfsclient or not;
                   commenting out untill furthure notice -->
        <!-- <li><a href="https://github.com/yawning/obfsclient"><b>obfsclient</b></a>
        is a multi-transport pluggable transport proxy (like obfsproxy),
        written in C++ that implements the client-side of <em>obfs2</em>,
        <em>obfs3</em> and <em>scramblesuit</em>. It's used by
        <a href="https://guardianproject.info/apps/orbot/">Orbot</a> on
        Android because of the difficulties of using Python applications.
        Maintained by Yawning Angel. <br>
        </li> -->

      </ul>

    <h3>Undeployed PTs</h3>
    <br />
      <!-- TODO: move this section to wiki -->
      <!-- TODO: add snowflake -->
    <ul>
    <li><b>StegoTorus</b></li>
        <ul>
            <li><strong>Description:</strong>is an Obfsproxy fork that extends it to a) split Tor streams across multiple connections to avoid packet size signatures, and b) embed the traffic flows in traces that look like HTML, JavasCript, or PDF. See its <a href="https://gitweb.torproject.org/stegotorus.git">git repository</a>.</li>
            <li><strong>Language:</strong> C++</li>
            <li><strong>Maintainer:</strong> Zack Weinberg</li>
            <li><strong>Evaluation:</strong> none</li>            
        </ul> 
    </li>

            <li><b>SkypeMorph</b></li>
        <ul>
            <li><strong>Description:</strong> It transforms Tor traffic flows so they look like Skype Video. See its <a href="http://crysp.uwaterloo.ca/software/SkypeMorph-0.5.1.tar.gz">source code</a> and <a href="http://cacr.uwaterloo.ca/techreports/2012/cacr2012-08.pdf">design paper</a>.</li>
            <li><strong>Language:</strong></li>
            <li><strong>Maintainer:</strong> Ian Goldberg.</li>
            <li><strong>Evaluation:</strong> none</li>            
        </ul>
    </li>

    <li><b>Dust</b></li>
        <ul>
            <li><strong>Description:</strong> It aims to provide a packet-based (rather than connection-based) DPI-resistant protocol. See its <a href="https://github.com/blanu/Dust">git repository</a>.</li>
            <li><strong>Language:</strong> Python</li>
            <li><strong>Maintainer:</strong> Brandon Wiley.</li>
            <li><strong>Evaluation:</strong> none</li>            
        </ul>
    </li>

    </ul>

    <br /><br />

    <p>Our goal is to have a wide variety of Pluggable Transport designs. You can check out a <a href="https://trac.torproject.org/projects/tor/wiki/doc/PluggableTransports/list">full list of Pluggables Transports here</a>.</p>

    <p>Many are at the research phase now, so it's a perfect time to play with them or suggest new designs. Please let us know if you find or start other projects that could be useful for making Tor's traffic flows more DPI-resistant!
    </p>

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