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    <h2>Tor: Pluggable Transports</h2>

    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.

    Pluggable transports 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
transport API</a>, to make it easier to build interoperable programs.


    <li><b>Obfsproxy</b> is a framework for implementing new transport
    protocols in C using libevent. See its
    <a href="<page projects/obfsproxy>">web page</a>,
    <a href="https://gitweb.torproject.org/obfsproxy.git">git repository</a>,
    <a href="https://gitweb.torproject.org/obfsproxy.git/blob/HEAD:/doc/obfs2/protocol-spec.txt">obfs2 protocol description</a>,
    <a href="https://blog.torproject.org/blog/obfsproxy-next-step-censorship-arms-race">blog post</a>.
    Maintained by George Kadianakis.

    <li><b>Flashproxy</b> turns ordinary web browsers into bridges using
    websockets, and has a little python stub to hook Tor clients to the
    websocket connection. See its
    <a href="http://crypto.stanford.edu/flashproxy/">web page</a>,
    <a href="https://gitweb.torproject.org/flashproxy.git">git repository</a>,
    <a href="http://crypto.stanford.edu/flashproxy/flashproxy.pdf">design paper</a>.
    Maintained by David Fifield.

    <li><b>StegoTorus</b> 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>.
    Maintained by Zack Weinberg.

    <li><b>SkypeMorph</b> 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>
    <a href="http://cacr.uwaterloo.ca/techreports/2012/cacr2012-08.pdf">design paper</a>.
    Maintained by Ian Goldberg.

    <li><b>Dust</b> 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>.
    Maintained by Brandon Wiley.

    <li><b>Format-Transforming Encryption</b> transforms Tor traffic
    to arbitrary formats using their language descriptions. See
    the <a href="http://eprint.iacr.org/2012/494">research
    paper</a>. The scheme is not deployed yet.


    Our goal is to have a wide variety of pluggable transport designs.
    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!

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