tor-webwml.git

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2) # Revision: $Revision: 22308 $
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5) #include "head.wmi" TITLE="Tor Project: Overview" CHARSET="UTF-8"
6) <div id="content" class="clearfix">
7)   <div id="breadcrumbs">

8)     <a href="<page index>">Home &raquo; </a>

9)     <a href="<page about/overview>">About &raquo; </a>

10)   </div>
11)   <div id="maincol">
12)     <a name="overview"></a>
13)     <h2><a class="anchor" href="#overview">Tor: Overview</a></h2>
14)     <!-- BEGIN SIDEBAR -->
15)     <div class="sidebar-left">
16)       <h3>Topics</h3>
17)       <ul>
18)         <li><a href="<page about/overview>#overview">Overview</a></li>
19)         <li><a href="<page about/overview>#whyweneedtor">Why we need Tor</a></li>
20)         <li><a href="<page about/overview>#thesolution">The Solution</a></li>
21)         <li><a href="<page about/overview>#hiddenservices">Hidden services</a></li>
22)         <li><a href="<page about/overview>#stayinganonymous">Staying anonymous</a></li>
23)         <li><a href="<page about/overview>#thefutureoftor">The future of Tor</a></li>
24)       </ul>
25)     </div>
26)     <!-- END SIDEBAR -->
27)     

28)     <hr>

29)     
30)     <p>
31)     Tor is a network of virtual tunnels that allows people and groups to
32)     improve their privacy and security on the Internet.  It also enables
33)     software developers to create new communication tools
34)     with built-in privacy features.  Tor provides the foundation for
35)     a range of applications that allow organizations and individuals
36)     to share information over public networks without compromising their
37)     privacy.
38)     </p>
39)     
40)     <p>
41)     Individuals use Tor to keep websites from tracking them and their family
42)     members, or to connect to news sites, instant messaging services, or the
43)     like when these are blocked by their local Internet providers.  Tor's <a
44)     href="<page docs/hidden-services>">hidden services</a>
45)     let users publish web sites and other services without needing to reveal
46)     the location of the site. Individuals also use Tor for socially sensitive
47)     communication: chat rooms and web forums for rape and abuse survivors,
48)     or people with illnesses.
49)     </p>
50)     
51)     <p>
52)     Journalists use Tor to communicate more safely with whistleblowers and
53)     dissidents. Non-governmental organizations (NGOs) use Tor to allow their
54)     workers to connect to their home website while they're in a foreign
55)     country, without notifying everybody nearby that they're working with
56)     that organization.
57)     </p>
58)     
59)     <p>
60)     Groups such as Indymedia recommend Tor for safeguarding their members'
61)     online privacy and security. Activist groups like the Electronic Frontier
62)     Foundation (EFF) recommend Tor as a mechanism for
63)     maintaining civil liberties online. Corporations use Tor as a safe way
64)     to conduct competitive analysis, and to protect sensitive procurement
65)     patterns from eavesdroppers. They also use it to replace traditional
66)     VPNs, which reveal the exact amount and timing of communication. Which
67)     locations have employees working late? Which locations have employees
68)     consulting job-hunting websites? Which research divisions are communicating
69)     with the company's patent lawyers?
70)     </p>
71)     
72)     <p>
73)     A branch of the U.S. Navy uses Tor for open source intelligence
74)     gathering, and one of its teams used Tor while deployed in the Middle
75)     East recently. Law enforcement uses Tor for visiting or surveilling
76)     web sites without leaving government IP addresses in their web logs,
77)     and for security during sting operations.
78)     </p>
79)     
80)     <p>
81)     The variety of people who use Tor is actually <a
82)     href="http://freehaven.net/doc/fc03/econymics.pdf">part of what makes
83)     it so secure</a>.  Tor hides you among <a href="<page about/torusers>">the
84)     other users on the network</a>,
85)     so the more populous and diverse the user base for Tor is, the more your
86)     anonymity will be protected.
87)     </p>
88)     
89)     <a name="whyweneedtor"></a>
90)     <h3><a class="anchor" href="#whyweneedtor">Why we need Tor</a></h3>
91)     
92)     <p>
93)     Using Tor protects you against a common form of Internet surveillance
94)     known as "traffic analysis."  Traffic analysis can be used to infer
95)     who is talking to whom over a public network.  Knowing the source
96)     and destination of your Internet traffic allows others to track your
97)     behavior and interests.  This can impact your checkbook if, for example,
98)     an e-commerce site uses price discrimination based on your country or
99)     institution of origin.  It can even threaten your job and physical safety
100)     by revealing who and where you are. For example, if you're travelling
101)     abroad and you connect to your employer's computers to check or send mail,
102)     you can inadvertently reveal your national origin and professional
103)     affiliation to anyone observing the network, even if the connection
104)     is encrypted.
105)     </p>
106)     
107)     <p>
108)     How does traffic analysis work?  Internet data packets have two parts:
109)     a data payload and a header used for routing.  The data payload is
110)     whatever is being sent, whether that's an email message, a web page, or an
111)     audio file.  Even if you encrypt the data payload of your communications,
112)     traffic analysis still reveals a great deal about what you're doing and,
113)     possibly, what you're saying.  That's because it focuses on the header,
114)     which discloses source, destination, size, timing, and so on.
115)     </p>
116)     
117)     <p>
118)     A basic problem for the privacy minded is that the recipient of your
119)     communications can see that you sent it by looking at headers.  So can
120)     authorized intermediaries like Internet service providers, and sometimes
121)     unauthorized intermediaries as well.  A very simple form of traffic
122)     analysis might involve sitting somewhere between sender and recipient on
123)     the network, looking at headers.
124)     </p>
125)     
126)     <p>
127)     But there are also more powerful kinds of traffic analysis.  Some
128)     attackers spy on multiple parts of the Internet and use sophisticated
129)     statistical techniques to track the communications patterns of many
130)     different organizations and individuals.  Encryption does not help against
131)     these attackers, since it only hides the content of Internet traffic, not
132)     the headers.
133)     </p>
134)     
135)     <a name="thesolution"></a>
136)     <h3><a class="anchor" href="#thesolution">The solution: a distributed, anonymous network</a></h3>

137)     <img src="$(IMGROOT)/htw1.png" alt="How Tor works">

138)     
139)     <p>
140)     Tor helps to reduce the risks of both simple and sophisticated traffic
141)     analysis by distributing your transactions over several places on the
142)     Internet, so no single point can link you to your destination.  The idea
143)     is similar to using a twisty, hard-to-follow route in order to throw off
144)     somebody who is tailing you &mdash; and then periodically erasing your
145)     footprints.  Instead of taking a direct route from source to
146)     destination, data packets on the Tor network take a random pathway
147)     through several relays that cover your tracks so no observer at any
148)     single point can tell where the data came from or where it's going.
149)     </p>
150)     

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

152)     
153)     <p>
154)     To create a private network pathway with Tor, the user's software or
155)     client incrementally builds a circuit of encrypted connections through
156)     relays on the network.  The circuit is extended one hop at a time, and
157)     each relay along the way knows only which relay gave it data and which
158)     relay it is giving data to.  No individual relay ever knows the
159)     complete path that a data packet has taken.  The client negotiates a
160)     separate set of encryption keys for each hop along the circuit to ensure
161)     that each hop can't trace these connections as they pass through.
162)     </p>
163)     

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

165)     
166)     <p>
167)     Once a circuit has been established, many kinds of data can be exchanged
168)     and several different sorts of software applications can be deployed
169)     over the Tor network.  Because each relay sees no more than one hop in
170)     the circuit, neither an eavesdropper nor a compromised relay can use
171)     traffic analysis to link the connection's source and destination.  Tor
172)     only works for TCP streams and can be used by any application with SOCKS
173)     support.
174)     </p>
175)     
176)     <p>
177)     For efficiency, the Tor software uses the same circuit for connections
178)     that happen within the same ten minutes or so.  Later requests are given a
179)     new circuit, to keep people from linking your earlier actions to the new
180)     ones.
181)     </p>
182)     

183)     <p><img alt="Tor circuit step three" src="$(IMGROOT)/htw3.png"></p>