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en/overview.wml   5) #include "head.wmi" TITLE="Tor: Overview" CHARSET="UTF-8"

overview.html     6) 
overview.html     7) <div class="main-column">

overview.html     8) 

en/overview.wml   9) <a name="overview"></a>
en/overview.wml  10) <h2><a class="anchor" href="#overview">Tor: Overview</a></h2>
en/overview.wml  11) <!-- BEGIN SIDEBAR -->
en/overview.wml  12) <div class="sidebar-left">
en/overview.wml  13) <h3>Topics</h3>
en/overview.wml  14) <ul>
en/overview.wml  15) <li><a href="<page overview>#overview">Overview</a></li>
en/overview.wml  16) <li><a href="<page overview>#whyweneedtor">Why we need Tor</a></li>
en/overview.wml  17) <li><a href="<page overview>#thesolution">The Solution</a></li>
en/overview.wml  18) <li><a href="<page overview>#hiddenservices">Hidden services</a></li>
en/overview.wml  19) <li><a href="<page overview>#stayinganonymous">Staying anonymous</a></li>
en/overview.wml  20) <li><a href="<page overview>#thefutureoftor">The future of Tor</a></li>
en/overview.wml  21) </ul>
en/overview.wml  22) </div>
en/overview.wml  23) <!-- END SIDEBAR -->
en/overview.wml  24) 

overview.html    25) <hr />

overview.html    26) 
overview.html    27) <p>

overview.html    28) Tor is a network of virtual tunnels that allows people and groups to

overview.html    29) improve their privacy and security on the Internet.  It also enables

overview.html    30) software developers to create new communication tools
overview.html    31) with built-in privacy features.  Tor provides the foundation for
overview.html    32) a range of applications that allow organizations and individuals

overview.html    33) to share information over public networks without compromising their
overview.html    34) privacy.

overview.html    35) </p>
overview.html    36) 
overview.html    37) <p>

overview.html    38) Individuals use Tor to keep websites from tracking them and their family
overview.html    39) members, or to connect to news sites, instant messaging services, or the
overview.html    40) like when these are blocked by their local Internet providers.  Tor's <a

en/overview.wml  41) href="<page hidden-services>">hidden services</a>

overview.html    42) let users publish web sites and other services without needing to reveal
overview.html    43) the location of the site. Individuals also use Tor for socially sensitive
overview.html    44) communication: chat rooms and web forums for rape and abuse survivors,
overview.html    45) or people with illnesses.

overview.html    46) </p>
overview.html    47) 
overview.html    48) <p>

overview.html    49) Journalists use Tor to communicate more safely with whistleblowers and
overview.html    50) dissidents. Non-governmental organizations (NGOs) use Tor to allow their
overview.html    51) workers to connect to their home website while they're in a foreign
overview.html    52) country, without notifying everybody nearby that they're working with
overview.html    53) that organization.

overview.html    54) </p>
overview.html    55) 
overview.html    56) <p>

overview.html    57) Groups such as Indymedia recommend Tor for safeguarding their members'
overview.html    58) online privacy and security. Activist groups like the Electronic Frontier

en/overview.wml  59) Foundation (EFF) recommend Tor as a mechanism for

overview.html    60) maintaining civil liberties online. Corporations use Tor as a safe way
overview.html    61) to conduct competitive analysis, and to protect sensitive procurement

overview.html    62) patterns from eavesdroppers. They also use it to replace traditional
overview.html    63) VPNs, which reveal the exact amount and timing of communication. Which
overview.html    64) locations have employees working late? Which locations have employees
overview.html    65) consulting job-hunting websites? Which research divisions are communicating
overview.html    66) with the company's patent lawyers?
overview.html    67) </p>
overview.html    68) 
overview.html    69) <p>
overview.html    70) A branch of the U.S. Navy uses Tor for open source intelligence
overview.html    71) gathering, and one of its teams used Tor while deployed in the Middle
overview.html    72) East recently. Law enforcement uses Tor for visiting or surveilling
overview.html    73) web sites without leaving government IP addresses in their web logs,
overview.html    74) and for security during sting operations.

overview.html    75) </p>
overview.html    76) 
overview.html    77) <p>
overview.html    78) The variety of people who use Tor is actually <a
overview.html    79) href="http://freehaven.net/doc/fc03/econymics.pdf">part of what makes

en/overview.wml  80) it so secure</a>.  Tor hides you among <a href="<page torusers>">the
en/overview.wml  81) other users on the network</a>,

overview.html    82) so the more populous and diverse the user base for Tor is, the more your
overview.html    83) anonymity will be protected.

overview.html    84) </p>
overview.html    85) 

en/overview.wml  86) <a name="whyweneedtor"></a>
en/overview.wml  87) <h3><a class="anchor" href="#whyweneedtor">Why we need Tor</a></h3>

overview.html    88) 
overview.html    89) <p>
overview.html    90) Using Tor protects you against a common form of Internet surveillance
overview.html    91) known as "traffic analysis."  Traffic analysis can be used to infer
overview.html    92) who is talking to whom over a public network.  Knowing the source
overview.html    93) and destination of your Internet traffic allows others to track your
overview.html    94) behavior and interests.  This can impact your checkbook if, for example,
overview.html    95) an e-commerce site uses price discrimination based on your country or
overview.html    96) institution of origin.  It can even threaten your job and physical safety

overview.html    97) by revealing who and where you are. For example, if you're travelling
overview.html    98) abroad and you connect to your employer's computers to check or send mail,
overview.html    99) you can inadvertently reveal your national origin and professional

overview.html   100) affiliation to anyone observing the network, even if the connection

overview.html   101) is encrypted.

overview.html   102) </p>
overview.html   103) 
overview.html   104) <p>
overview.html   105) How does traffic analysis work?  Internet data packets have two parts:

overview.html   106) a data payload and a header used for routing.  The data payload is

overview.html   107) whatever is being sent, whether that's an email message, a web page, or an
overview.html   108) audio file.  Even if you encrypt the data payload of your communications,
overview.html   109) traffic analysis still reveals a great deal about what you're doing and,
overview.html   110) possibly, what you're saying.  That's because it focuses on the header,
overview.html   111) which discloses source, destination, size, timing, and so on.
overview.html   112) </p>
overview.html   113) 
overview.html   114) <p>

overview.html   115) A basic problem for the privacy minded is that the recipient of your
overview.html   116) communications can see that you sent it by looking at headers.  So can

overview.html   117) authorized intermediaries like Internet service providers, and sometimes
overview.html   118) unauthorized intermediaries as well.  A very simple form of traffic
overview.html   119) analysis might involve sitting somewhere between sender and recipient on
overview.html   120) the network, looking at headers.
overview.html   121) </p>
overview.html   122) 
overview.html   123) <p>
overview.html   124) But there are also more powerful kinds of traffic analysis.  Some
overview.html   125) attackers spy on multiple parts of the Internet and use sophisticated
overview.html   126) statistical techniques to track the communications patterns of many

overview.html   127) different organizations and individuals.  Encryption does not help against
overview.html   128) these attackers, since it only hides the content of Internet traffic, not
overview.html   129) the headers.

overview.html   130) </p>
overview.html   131) 

en/overview.wml 132) <a name="thesolution"></a>
en/overview.wml 133) <h3><a class="anchor" href="#thesolution">The solution: a distributed, anonymous network</a></h3>

overview.html   134) 
overview.html   135) <p>
overview.html   136) Tor helps to reduce the risks of both simple and sophisticated traffic
overview.html   137) analysis by distributing your transactions over several places on the
overview.html   138) Internet, so no single point can link you to your destination.  The idea
overview.html   139) is similar to using a twisty, hard-to-follow route in order to throw off

en/overview.wml 140) somebody who is tailing you — and then periodically erasing your

overview.html   141) footprints.  Instead of taking a direct route from source to
overview.html   142) destination, data packets on the Tor network take a random pathway

en/overview.wml 143) through several relays that cover your tracks so no observer at any

overview.html   144) single point can tell where the data came from or where it's going.
overview.html   145) </p>
overview.html   146) 

en/overview.wml 147) <p><img alt="Tor circuit step one" src="$(IMGROOT)/htw1.png" /></p>

overview.html   148) 

overview.html   149) <p>
overview.html   150) To create a private network pathway with Tor, the user's software or
overview.html   151) client incrementally builds a circuit of encrypted connections through

en/overview.wml 152) relays on the network.  The circuit is extended one hop at a time, and
en/overview.wml 153) each relay along the way knows only which relay gave it data and which
en/overview.wml 154) relay it is giving data to.  No individual relay ever knows the

overview.html   155) complete path that a data packet has taken.  The client negotiates a
overview.html   156) separate set of encryption keys for each hop along the circuit to ensure

overview.html   157) that each hop can't trace these connections as they pass through.

overview.html   158) </p>
overview.html   159) 

en/overview.wml 160) <p><img alt="Tor circuit step two" src="$(IMGROOT)/htw2.png" /></p>

overview.html   161) 
overview.html   162) <p>

overview.html   163) Once a circuit has been established, many kinds of data can be exchanged
overview.html   164) and several different sorts of software applications can be deployed

en/overview.wml 165) over the Tor network.  Because each relay sees no more than one hop in
en/overview.wml 166) the circuit, neither an eavesdropper nor a compromised relay can use

overview.html   167) traffic analysis to link the connection's source and destination.  Tor
overview.html   168) only works for TCP streams and can be used by any application with SOCKS
overview.html   169) support.
overview.html   170) </p>
overview.html   171) 
overview.html   172) <p>
overview.html   173) For efficiency, the Tor software uses the same circuit for connections

en/overview.wml 174) that happen within the same ten minutes or so.  Later requests are given a

overview.html   175) new circuit, to keep people from linking your earlier actions to the new
overview.html   176) ones.

overview.html   177) </p>
overview.html   178) 

en/overview.wml 179) <p><img alt="Tor circuit step three" src="$(IMGROOT)/htw3.png" /></p>
en/overview.wml 180) 

overview.html   181) 

en/overview.wml 182) <a name="hiddenservices"></a>
en/overview.wml 183) <h3><a class="anchor" href="#hiddenservices">Hidden services</a></h3>

overview.html   184) 

overview.html   185) <p>

overview.html   186) Tor also makes it possible for users to hide their locations while
overview.html   187) offering various kinds of services, such as web publishing or an instant
overview.html   188) messaging server.  Using Tor "rendezvous points," other Tor users can

en/overview.wml 189) connect to these hidden services, each without knowing the other's

overview.html   190) network identity.  This hidden service functionality could allow Tor
overview.html   191) users to set up a website where people publish material without worrying
overview.html   192) about censorship.  Nobody would be able to determine who was offering
overview.html   193) the site, and nobody who offered the site would know who was posting to it.

en/overview.wml 194) Learn more about <a href="<page docs/tor-hidden-service>">configuring
en/overview.wml 195) hidden services</a> and how the <a href="<page hidden-services>">hidden
en/overview.wml 196) service protocol</a> works.

overview.html   197) </p>

overview.html   198) 

en/overview.wml 199) <a name="stayinganonymous"></a>
en/overview.wml 200) <h3><a class="anchor" href="#stayinganonymous">Staying anonymous</a></h3>

overview.html   201) 
overview.html   202) <p>

overview.html   203) Tor can't solve all anonymity problems.  It focuses only on

overview.html   204) protecting the transport of data.  You need to use protocol-specific
overview.html   205) support software if you don't want the sites you visit to see your

overview.html   206) identifying information. For example, you can use web proxies such as
overview.html   207) Privoxy while web browsing to block cookies and withhold information

overview.html   208) about your browser type.

overview.html   209) </p>
overview.html   210) 
overview.html   211) <p>

overview.html   212) Also, to protect your anonymity, be smart.  Don't provide your name

overview.html   213) or other revealing information in web forms.  Be aware that, like all

overview.html   214) anonymizing networks that are fast enough for web browsing, Tor does not

overview.html   215) provide protection against end-to-end timing attacks: If your attacker

overview.html   216) can watch the traffic coming out of your computer, and also the traffic
overview.html   217) arriving at your chosen destination, he can use statistical analysis to
overview.html   218) discover that they are part of the same circuit.
overview.html   219) </p>
overview.html   220) 

en/overview.wml 221) <a name="thefutureoftor"></a>
en/overview.wml 222) <h3><a class="anchor" href="#thefutureoftor">The future of Tor</a></h3>

overview.html   223) 
overview.html   224) <p>

overview.html   225) Providing a usable anonymizing network on the Internet today is an
overview.html   226) ongoing challenge. We want software that meets users' needs. We also
overview.html   227) want to keep the network up and running in a way that handles as many
overview.html   228) users as possible. Security and usability don't have to be at odds:
overview.html   229) As Tor's usability increases, it will attract more users, which will
overview.html   230) increase the possible sources and destinations of each communication,
overview.html   231) thus increasing security for everyone.

overview.html   232) We're making progress, but we need your help.  Please consider

en/overview.wml 233) <a href="<page docs/tor-doc-relay>">running a relay</a>

en/overview.wml 234) or <a href="<page volunteer>">volunteering</a> as a

en/overview.wml 235) <a href="<page documentation>#Developers">developer</a>.

overview.html   236) </p>
overview.html   237) 
overview.html   238) <p>

overview.html   239) Ongoing trends in law, policy, and technology threaten anonymity as never
overview.html   240) before, undermining our ability to speak and read freely online. These
overview.html   241) trends also undermine national security and critical infrastructure by
overview.html   242) making communication among individuals, organizations, corporations,

en/overview.wml 243) and governments more vulnerable to analysis. Each new user and relay

overview.html   244) provides additional diversity, enhancing Tor's ability to put control
overview.html   245) over your security and privacy back into your hands.

overview.html   246) </p>
overview.html   247) 
overview.html   248)   </div><!-- #main -->