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en/overview.wml   2) # Revision: $Revision$

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en/overview.wml   4) #include "head.wmi" TITLE="Overview"

overview.html     5) 
overview.html     6) <div class="main-column">

overview.html     7) 

overview.html     8) <h2>Tor: Overview</h2>

overview.html     9) <hr />

overview.html    10) 
overview.html    11) <p>

overview.html    12) Tor is a network of virtual tunnels that allows people and groups to

overview.html    13) improve their privacy and security on the Internet.  It also enables

overview.html    14) software developers to create new communication tools
overview.html    15) with built-in privacy features.  Tor provides the foundation for
overview.html    16) a range of applications that allow organizations and individuals

overview.html    17) to share information over public networks without compromising their
overview.html    18) privacy.

overview.html    19) </p>
overview.html    20) 
overview.html    21) <p>

overview.html    22) Individuals use Tor to keep websites from tracking them and their family
overview.html    23) members, or to connect to news sites, instant messaging services, or the
overview.html    24) like when these are blocked by their local Internet providers.  Tor's <a

en/overview.wml  25) href="<cvssandbox>tor/doc/tor-hidden-service.html">hidden services</a>

overview.html    26) let users publish web sites and other services without needing to reveal
overview.html    27) the location of the site. Individuals also use Tor for socially sensitive
overview.html    28) communication: chat rooms and web forums for rape and abuse survivors,
overview.html    29) or people with illnesses.

overview.html    30) </p>
overview.html    31) 
overview.html    32) <p>

overview.html    33) Journalists use Tor to communicate more safely with whistleblowers and
overview.html    34) dissidents. Non-governmental organizations (NGOs) use Tor to allow their
overview.html    35) workers to connect to their home website while they're in a foreign
overview.html    36) country, without notifying everybody nearby that they're working with
overview.html    37) that organization.

overview.html    38) </p>
overview.html    39) 
overview.html    40) <p>

overview.html    41) Groups such as Indymedia recommend Tor for safeguarding their members'
overview.html    42) online privacy and security. Activist groups like the Electronic Frontier
overview.html    43) Foundation (EFF) are supporting Tor's development as a mechanism for
overview.html    44) maintaining civil liberties online. Corporations use Tor as a safe way
overview.html    45) to conduct competitive analysis, and to protect sensitive procurement

overview.html    46) patterns from eavesdroppers. They also use it to replace traditional
overview.html    47) VPNs, which reveal the exact amount and timing of communication. Which
overview.html    48) locations have employees working late? Which locations have employees
overview.html    49) consulting job-hunting websites? Which research divisions are communicating
overview.html    50) with the company's patent lawyers?
overview.html    51) </p>
overview.html    52) 
overview.html    53) <p>
overview.html    54) A branch of the U.S. Navy uses Tor for open source intelligence
overview.html    55) gathering, and one of its teams used Tor while deployed in the Middle
overview.html    56) East recently. Law enforcement uses Tor for visiting or surveilling
overview.html    57) web sites without leaving government IP addresses in their web logs,
overview.html    58) and for security during sting operations.

overview.html    59) </p>
overview.html    60) 
overview.html    61) <p>
overview.html    62) The variety of people who use Tor is actually <a
overview.html    63) href="http://freehaven.net/doc/fc03/econymics.pdf">part of what makes
overview.html    64) it so secure</a>.  Tor hides you among the other users on the network,
overview.html    65) so the more populous and diverse the user base for Tor is, the more your
overview.html    66) anonymity will be protected.

overview.html    67) </p>
overview.html    68) 

overview.html    69) <h3>Why we need Tor</h3>

overview.html    70) 
overview.html    71) <p>
overview.html    72) Using Tor protects you against a common form of Internet surveillance
overview.html    73) known as "traffic analysis."  Traffic analysis can be used to infer
overview.html    74) who is talking to whom over a public network.  Knowing the source
overview.html    75) and destination of your Internet traffic allows others to track your
overview.html    76) behavior and interests.  This can impact your checkbook if, for example,
overview.html    77) an e-commerce site uses price discrimination based on your country or
overview.html    78) institution of origin.  It can even threaten your job and physical safety

overview.html    79) by revealing who and where you are. For example, if you're travelling
overview.html    80) abroad and you connect to your employer's computers to check or send mail,
overview.html    81) you can inadvertently reveal your national origin and professional

overview.html    82) affiliation to anyone observing the network, even if the connection

overview.html    83) is encrypted.

overview.html    84) </p>
overview.html    85) 
overview.html    86) <p>
overview.html    87) How does traffic analysis work?  Internet data packets have two parts:

overview.html    88) a data payload and a header used for routing.  The data payload is

overview.html    89) whatever is being sent, whether that's an email message, a web page, or an
overview.html    90) audio file.  Even if you encrypt the data payload of your communications,
overview.html    91) traffic analysis still reveals a great deal about what you're doing and,
overview.html    92) possibly, what you're saying.  That's because it focuses on the header,
overview.html    93) which discloses source, destination, size, timing, and so on.
overview.html    94) </p>
overview.html    95) 
overview.html    96) <p>

overview.html    97) A basic problem for the privacy minded is that the recipient of your
overview.html    98) communications can see that you sent it by looking at headers.  So can

overview.html    99) authorized intermediaries like Internet service providers, and sometimes
overview.html   100) unauthorized intermediaries as well.  A very simple form of traffic
overview.html   101) analysis might involve sitting somewhere between sender and recipient on
overview.html   102) the network, looking at headers.
overview.html   103) </p>
overview.html   104) 
overview.html   105) <p>
overview.html   106) But there are also more powerful kinds of traffic analysis.  Some
overview.html   107) attackers spy on multiple parts of the Internet and use sophisticated
overview.html   108) statistical techniques to track the communications patterns of many

overview.html   109) different organizations and individuals.  Encryption does not help against
overview.html   110) these attackers, since it only hides the content of Internet traffic, not
overview.html   111) the headers.

overview.html   112) </p>
overview.html   113) 

overview.html   114) <h3>The solution: a distributed, anonymous network</h3>

overview.html   115) 
overview.html   116) <p>
overview.html   117) Tor helps to reduce the risks of both simple and sophisticated traffic
overview.html   118) analysis by distributing your transactions over several places on the
overview.html   119) Internet, so no single point can link you to your destination.  The idea
overview.html   120) is similar to using a twisty, hard-to-follow route in order to throw off

overview.html   121) somebody who is tailing you—and then periodically erasing your

overview.html   122) footprints.  Instead of taking a direct route from source to
overview.html   123) destination, data packets on the Tor network take a random pathway
overview.html   124) through several servers that cover your tracks so no observer at any
overview.html   125) single point can tell where the data came from or where it's going.
overview.html   126) </p>
overview.html   127) 

en/overview.wml 128) <img alt="Tor circuit step one" src="$(IMGROOT)/htw1.png" />

overview.html   129) 

overview.html   130) <p>
overview.html   131) To create a private network pathway with Tor, the user's software or
overview.html   132) client incrementally builds a circuit of encrypted connections through
overview.html   133) servers on the network.  The circuit is extended one hop at a time, and
overview.html   134) each server along the way knows only which server gave it data and which

overview.html   135) server it is giving data to.  No individual server ever knows the

overview.html   136) complete path that a data packet has taken.  The client negotiates a
overview.html   137) separate set of encryption keys for each hop along the circuit to ensure

overview.html   138) that each hop can't trace these connections as they pass through.

overview.html   139) </p>
overview.html   140) 

en/overview.wml 141) <img alt="Tor circuit step two" src="$(IMGROOT)/htw2.png" />

overview.html   142) 
overview.html   143) <p>

overview.html   144) Once a circuit has been established, many kinds of data can be exchanged
overview.html   145) and several different sorts of software applications can be deployed
overview.html   146) over the Tor network.  Because each server sees no more than one hop in
overview.html   147) the circuit, neither an eavesdropper nor a compromised server can use
overview.html   148) traffic analysis to link the connection's source and destination.  Tor
overview.html   149) only works for TCP streams and can be used by any application with SOCKS
overview.html   150) support.
overview.html   151) </p>
overview.html   152) 
overview.html   153) <p>
overview.html   154) For efficiency, the Tor software uses the same circuit for connections
overview.html   155) that happen within the same minute or so.  Later requests are given a
overview.html   156) new circuit, to keep people from linking your earlier actions to the new
overview.html   157) ones.

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

en/overview.wml 160) <img alt="Tor circuit step three" src="$(IMGROOT)/htw3.png" />

overview.html   161) 

overview.html   162) <h3>Hidden services</h3>

overview.html   163) 

overview.html   164) <p>

overview.html   165) Tor also makes it possible for users to hide their locations while
overview.html   166) offering various kinds of services, such as web publishing or an instant
overview.html   167) messaging server.  Using Tor "rendezvous points," other Tor users can

en/overview.wml 168) connect to these <a
en/overview.wml 169) href="<cvssandbox>tor/doc/tor-hidden-service.html">hidden services</a>,
en/overview.wml 170) each without knowing the other's

overview.html   171) network identity.  This hidden service functionality could allow Tor
overview.html   172) users to set up a website where people publish material without worrying
overview.html   173) about censorship.  Nobody would be able to determine who was offering
overview.html   174) the site, and nobody who offered the site would know who was posting to it.

overview.html   175) </p>

overview.html   176) 

overview.html   177) <h3>Staying anonymous</h3>

overview.html   178) 
overview.html   179) <p>

overview.html   180) Tor can't solve all anonymity problems.  It focuses only on

overview.html   181) protecting the transport of data.  You need to use protocol-specific
overview.html   182) support software if you don't want the sites you visit to see your

overview.html   183) identifying information. For example, you can use web proxies such as
overview.html   184) Privoxy while web browsing to block cookies and withhold information

overview.html   185) about your browser type.

overview.html   186) </p>
overview.html   187) 
overview.html   188) <p>

overview.html   189) Also, to protect your anonymity, be smart.  Don't provide your name

overview.html   190) or other revealing information in web forms.  Be aware that, like all

overview.html   191) anonymizing networks that are fast enough for web browsing, Tor does not

overview.html   192) provide protection against end-to-end timing attacks: If your attacker

overview.html   193) can watch the traffic coming out of your computer, and also the traffic
overview.html   194) arriving at your chosen destination, he can use statistical analysis to
overview.html   195) discover that they are part of the same circuit.
overview.html   196) </p>
overview.html   197) 

overview.html   198) <h3>The future of Tor</h3>

overview.html   199) 
overview.html   200) <p>

overview.html   201) Providing a usable anonymizing network on the Internet today is an
overview.html   202) ongoing challenge. We want software that meets users' needs. We also
overview.html   203) want to keep the network up and running in a way that handles as many
overview.html   204) users as possible. Security and usability don't have to be at odds:
overview.html   205) As Tor's usability increases, it will attract more users, which will
overview.html   206) increase the possible sources and destinations of each communication,
overview.html   207) thus increasing security for everyone.

overview.html   208) We're making progress, but we need your help.  Please consider

en/overview.wml 209) <a href="<cvssandbox>tor/doc/tor-doc-server.html">running a server</a>

en/overview.wml 210) or <a href="<page volunteer>">volunteering</a> as a
en/overview.wml 211) <a href="<page developers>">developer</a>.

overview.html   212) </p>
overview.html   213) 
overview.html   214) <p>

overview.html   215) Ongoing trends in law, policy, and technology threaten anonymity as never
overview.html   216) before, undermining our ability to speak and read freely online. These
overview.html   217) trends also undermine national security and critical infrastructure by
overview.html   218) making communication among individuals, organizations, corporations,
overview.html   219) and governments more vulnerable to analysis. Each new user and server
overview.html   220) provides additional diversity, enhancing Tor's ability to put control
overview.html   221) over your security and privacy back into your hands.

overview.html   222) </p>
overview.html   223) 
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