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1) <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
2) "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
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3) 
4) <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
5) <head>
6)   <title>Tor: Overview</title>
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7)   <meta name="Author" content="Roger Dingledine" />
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8)   <meta http-equiv="content-type" content="text/html; charset=ISO-8859-1" />
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9)   <link rel="stylesheet" type="text/css" href="stylesheet.css" />
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10)   <link rel="shortcut icon" type="image/x-icon" href="/favicon.ico" />
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11) </head>
12) 
13) <body>
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14) <div class="center">
15) 
16) <!-- TITLE BAR & NAVIGATION -->
17) <div class="banner">
18) <table class="table-banner" cellpadding=0 cellspacing=0>
19) <tr>
20) <td width="110">
21) <a href="http://tor.eff.org/"><img border="0" src="images/spacer.png"
22) width="110" height="79"></a>
23) </td>
24) <td class="links" valign="top">
25)       <a href="index.html">Home</a>  |
26)       <a href="howitworks.html">How It Works</a>  |
27)       <a href="download.html">Download</a>  |
28)       <a href="documentation.html">Documentation</a>  |
29)       <a href="http://wiki.noreply.org/wiki/TheOnionRouter/TorFAQ">FAQ</a>  |
30)       <a href="contribute.html">Contribute</a>  |
31)       <a href="developers.html">Developers</a>  |
32)       <a href="research.html">Research</a>  |
33)       <a href="people.html">People</a>
34) </td>
35) </tr>
36) </table>
37) </div>
38) <!-- END TITLE BAR & NAVIGATION -->
39) 
40) <div class="main-column">
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41) 
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42) <h2>Tor: Overview</h2>
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43) <hr>
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44) 
45) <p>
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46) Tor is a network-within-a-network that allows people and groups to
47) improve their privacy and security on the Internet.  It also enables
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48) software developers to create new communication tools
49) with built-in privacy features.  Tor provides the foundation for
50) a range of applications that allow organizations and individuals
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51) to share information over public networks without compromising their
52) privacy.
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53) </p>
54) 
55) <p>
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56) Individuals can use Tor to keep remote websites from tracking them and their
57) family members.
58) They can also use it to connect
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59) to resources such as news sites or instant messaging services that are
60) blocked by their local Internet service providers (ISPs).
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61) </p>
62) 
63) <p>
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64) Groups such as the German "Diabetes People" organization recommend Tor
65) for safeguarding their members' online privacy and security.  Activist
66) groups like the Electronic Frontier Foundation (EFF) are supporting
67) Tor's development as a mechanism for maintaining civil liberties online.
68) Corporations are investigating Tor as a safe way to conduct competitive
69) analysis, and are considering using Tor to test new experimental projects
70) without associating their names with these projects. A branch of the
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71) U.S. Navy uses Tor for open source intelligence gathering, and one of its
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72) teams used Tor while deployed in the Middle East recently.
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73) </p>
74) 
75) <p>
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76) The variety of people who use Tor is actually part of what makes it
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77) so secure.  Tor hides you among the other users on the network, so the more
78) populous and diverse the user base for Tor is,
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79) the more your anonymity will be protected.
80) </p>
81) 
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82) <h3>Why we need Tor</h3>
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83) 
84) <p>
85) Using Tor protects you against a common form of Internet surveillance
86) known as "traffic analysis."  Traffic analysis can be used to infer
87) who is talking to whom over a public network.  Knowing the source
88) and destination of your Internet traffic allows others to track your
89) behavior and interests.  This can impact your checkbook if, for example,
90) an e-commerce site uses price discrimination based on your country or
91) institution of origin.  It can even threaten your job and physical safety
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92) by revealing who and where you are. For example, if you're travelling
93) abroad and you connect to your employer's computers to check or send mail,
94) you can inadvertently reveal your national origin and professional
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95) affiliation to anyone observing the network, even if the connection
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96) is encrypted.
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97) </p>
98) 
99) <p>
100) How does traffic analysis work?  Internet data packets have two parts:
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101) a data payload and a header used for routing.  The data payload is
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102) whatever is being sent, whether that's an email message, a web page, or an
103) audio file.  Even if you encrypt the data payload of your communications,
104) traffic analysis still reveals a great deal about what you're doing and,
105) possibly, what you're saying.  That's because it focuses on the header,
106) which discloses source, destination, size, timing, and so on.
107) </p>
108) 
109) <p>
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110) A basic problem for the privacy minded is that the recipient of your
111) communications can see that you sent it by looking at headers.  So can
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112) authorized intermediaries like Internet service providers, and sometimes
113) unauthorized intermediaries as well.  A very simple form of traffic
114) analysis might involve sitting somewhere between sender and recipient on
115) the network, looking at headers.
116) </p>
117) 
118) <p>
119) But there are also more powerful kinds of traffic analysis.  Some
120) attackers spy on multiple parts of the Internet and use sophisticated
121) statistical techniques to track the communications patterns of many
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122) different organizations and individuals.  Encryption does not help against
123) these attackers, since it only hides the content of Internet traffic, not
124) the headers.
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125) </p>
126) 
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127) <h3>The solution: a distributed, anonymous network</h3>
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128) 
129) <p>
130) Tor helps to reduce the risks of both simple and sophisticated traffic
131) analysis by distributing your transactions over several places on the
132) Internet, so no single point can link you to your destination.  The idea
133) is similar to using a twisty, hard-to-follow route in order to throw off
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134) somebody who is tailing you&mdash;and then periodically erasing your
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135) footprints.  Instead of taking a direct route from source to
136) destination, data packets on the Tor network take a random pathway
137) through several servers that cover your tracks so no observer at any
138) single point can tell where the data came from or where it's going.
139) </p>
140) 
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141) <img alt="tor circuit step one" src="img/htw1.png" />
142) <img alt="tor circuit step two" src="img/htw2.png" />
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143) 
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144) <p>
145) To create a private network pathway with Tor, the user's software or
146) client incrementally builds a circuit of encrypted connections through
147) servers on the network.  The circuit is extended one hop at a time, and
148) each server along the way knows only which server gave it data and which
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149) server it is giving data to.  No individual server ever knows the
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150) complete path that a data packet has taken.  The client negotiates a
151) separate set of encryption keys for each hop along the circuit to ensure
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152) that each hop can't trace these connections as they pass through.
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153) </p>
154) 
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155) <img alt="tor circuit step three" src="img/htw3.png" />
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156) 
157) <p>
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158) Once a circuit has been established, many kinds of data can be exchanged
159) and several different sorts of software applications can be deployed
160) over the Tor network.  Because each server sees no more than one hop in
161) the circuit, neither an eavesdropper nor a compromised server can use
162) traffic analysis to link the connection's source and destination.  Tor
163) only works for TCP streams and can be used by any application with SOCKS
164) support.
165) </p>
166) 
167) <p>
168) For efficiency, the Tor software uses the same circuit for connections
169) that happen within the same minute or so.  Later requests are given a
170) new circuit, to keep people from linking your earlier actions to the new
171) ones.
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172) </p>
173) 
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174) <img alt="tor circuit step four" src="img/htw4.png" />
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175) 
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176) <h3>Hidden services</h3>
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177) 
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178) <p>
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179) Tor also makes it possible for users to hide their locations while
180) offering various kinds of services, such as web publishing or an instant
181) messaging server.  Using Tor "rendezvous points," other Tor users can
182) connect to these hidden services, each without knowing the other's
183) network identity.  This hidden service functionality could allow Tor
184) users to set up a website where people publish material without worrying
185) about censorship.  Nobody would be able to determine who was offering
186) the site, and nobody who offered the site would know who was posting to it.
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187) </p>
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188) 
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189) <h3>Staying anonymous</h3>
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190) 
191) <p>
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192) Tor can't solve all anonymity problems.  It focuses only on
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193) protecting the transport of data.  You need to use protocol-specific
194) support software if you don't want the sites you visit to see your
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195) identifying information. For example, you can use web proxies such as
196) Privoxy while web browsing to block cookies and withhold information
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197) about your browser type.
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198) </p>
199) 
200) <p>
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201) Also, to protect your anonymity, be smart.  Don't provide your name
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202) or other revealing information in web forms.  Be aware that, like all
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203) anonymizing networks that are fast enough for web browsing, Tor does not
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204) provide protection against end-to-end timing attacks: If your attacker
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205) can watch the traffic coming out of your computer, and also the traffic
206) arriving at your chosen destination, he can use statistical analysis to
207) discover that they are part of the same circuit.
208) </p>
209) 
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210) <h3>The future of Tor</h3>
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211) 
212) <p>
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213) Providing a usable anonymizing network on the Internet today is an
214) ongoing challenge. We want software that meets users' needs. We also
215) want to keep the network up and running in a way that handles as many
216) users as possible. Security and usability don't have to be at odds:
217) As Tor's usability increases, it will attract more users, which will
218) increase the possible sources and destinations of each communication,
219) thus increasing security for everyone.
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220) We're making progress, but we need your help.  Please consider
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221) <a href="cvs/tor/doc/tor-doc.html#installing">installing</a> a <a
222) href="cvs/tor/doc/tor-doc.html#server">server</a>
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223) or <a href="contribute.html">volunteering</a> as a <a
224) href="developers.html">developer</a>.
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225) </p>
226) 
227) <p>
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228) Ongoing trends in law, policy, and technology threaten anonymity as never
229) before, undermining our ability to speak and read freely online. These
230) trends also undermine national security and critical infrastructure by
231) making communication among individuals, organizations, corporations,
232) and governments more vulnerable to analysis. Each new user and server
233) provides additional diversity, enhancing Tor's ability to put control
234) over your security and privacy back into your hands.
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235) </p>
236) 
237)   </div><!-- #main -->
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238) </div>
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239)   <div class="bottom" id="bottom">
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240)      <i><a href="mailto:tor-webmaster@freehaven.net" class="smalllink">Webmaster</a></i> -
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241)      $Id$
242)   </div>