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

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

overview.html     8) 

overview.html     9) <h2>Tor: Overview</h2>

overview.html    10) <hr />

overview.html    11) 
overview.html    12) <p>

overview.html    13) Tor is a network of virtual tunnels that allows people and groups to

overview.html    14) improve their privacy and security on the Internet.  It also enables

overview.html    15) software developers to create new communication tools
overview.html    16) with built-in privacy features.  Tor provides the foundation for
overview.html    17) a range of applications that allow organizations and individuals

overview.html    18) to share information over public networks without compromising their
overview.html    19) privacy.

overview.html    20) </p>
overview.html    21) 
overview.html    22) <p>

overview.html    23) Individuals use Tor to keep websites from tracking them and their family
overview.html    24) members, or to connect to news sites, instant messaging services, or the
overview.html    25) like when these are blocked by their local Internet providers.  Tor's <a

en/overview.wml  26) href="<page docs/tor-hidden-service>">hidden services</a>

overview.html    27) let users publish web sites and other services without needing to reveal
overview.html    28) the location of the site. Individuals also use Tor for socially sensitive
overview.html    29) communication: chat rooms and web forums for rape and abuse survivors,
overview.html    30) or people with illnesses.

overview.html    31) </p>
overview.html    32) 
overview.html    33) <p>

overview.html    34) Journalists use Tor to communicate more safely with whistleblowers and
overview.html    35) dissidents. Non-governmental organizations (NGOs) use Tor to allow their
overview.html    36) workers to connect to their home website while they're in a foreign
overview.html    37) country, without notifying everybody nearby that they're working with
overview.html    38) that organization.

overview.html    39) </p>
overview.html    40) 
overview.html    41) <p>

overview.html    42) Groups such as Indymedia recommend Tor for safeguarding their members'
overview.html    43) online privacy and security. Activist groups like the Electronic Frontier

en/overview.wml  44) Foundation (EFF) recommend Tor as a mechanism for

overview.html    45) maintaining civil liberties online. Corporations use Tor as a safe way
overview.html    46) to conduct competitive analysis, and to protect sensitive procurement

overview.html    47) patterns from eavesdroppers. They also use it to replace traditional
overview.html    48) VPNs, which reveal the exact amount and timing of communication. Which
overview.html    49) locations have employees working late? Which locations have employees
overview.html    50) consulting job-hunting websites? Which research divisions are communicating
overview.html    51) with the company's patent lawyers?
overview.html    52) </p>
overview.html    53) 
overview.html    54) <p>
overview.html    55) A branch of the U.S. Navy uses Tor for open source intelligence
overview.html    56) gathering, and one of its teams used Tor while deployed in the Middle
overview.html    57) East recently. Law enforcement uses Tor for visiting or surveilling
overview.html    58) web sites without leaving government IP addresses in their web logs,
overview.html    59) and for security during sting operations.

overview.html    60) </p>
overview.html    61) 
overview.html    62) <p>
overview.html    63) The variety of people who use Tor is actually <a
overview.html    64) href="http://freehaven.net/doc/fc03/econymics.pdf">part of what makes

en/overview.wml  65) it so secure</a>.  Tor hides you among <a href="<page torusers>">the
en/overview.wml  66) other users on the network</a>,

overview.html    67) so the more populous and diverse the user base for Tor is, the more your
overview.html    68) anonymity will be protected.

overview.html    69) </p>
overview.html    70) 

overview.html    71) <h3>Why we need Tor</h3>

overview.html    72) 
overview.html    73) <p>
overview.html    74) Using Tor protects you against a common form of Internet surveillance
overview.html    75) known as "traffic analysis."  Traffic analysis can be used to infer
overview.html    76) who is talking to whom over a public network.  Knowing the source
overview.html    77) and destination of your Internet traffic allows others to track your
overview.html    78) behavior and interests.  This can impact your checkbook if, for example,
overview.html    79) an e-commerce site uses price discrimination based on your country or
overview.html    80) institution of origin.  It can even threaten your job and physical safety

overview.html    81) by revealing who and where you are. For example, if you're travelling
overview.html    82) abroad and you connect to your employer's computers to check or send mail,
overview.html    83) you can inadvertently reveal your national origin and professional

overview.html    84) affiliation to anyone observing the network, even if the connection

overview.html    85) is encrypted.

overview.html    86) </p>
overview.html    87) 
overview.html    88) <p>
overview.html    89) How does traffic analysis work?  Internet data packets have two parts:

overview.html    90) a data payload and a header used for routing.  The data payload is

overview.html    91) whatever is being sent, whether that's an email message, a web page, or an
overview.html    92) audio file.  Even if you encrypt the data payload of your communications,
overview.html    93) traffic analysis still reveals a great deal about what you're doing and,
overview.html    94) possibly, what you're saying.  That's because it focuses on the header,
overview.html    95) which discloses source, destination, size, timing, and so on.
overview.html    96) </p>
overview.html    97) 
overview.html    98) <p>

overview.html    99) A basic problem for the privacy minded is that the recipient of your
overview.html   100) communications can see that you sent it by looking at headers.  So can

overview.html   101) authorized intermediaries like Internet service providers, and sometimes
overview.html   102) unauthorized intermediaries as well.  A very simple form of traffic
overview.html   103) analysis might involve sitting somewhere between sender and recipient on
overview.html   104) the network, looking at headers.
overview.html   105) </p>
overview.html   106) 
overview.html   107) <p>
overview.html   108) But there are also more powerful kinds of traffic analysis.  Some
overview.html   109) attackers spy on multiple parts of the Internet and use sophisticated
overview.html   110) statistical techniques to track the communications patterns of many

overview.html   111) different organizations and individuals.  Encryption does not help against
overview.html   112) these attackers, since it only hides the content of Internet traffic, not
overview.html   113) the headers.

overview.html   114) </p>
overview.html   115) 

overview.html   116) <h3>The solution: a distributed, anonymous network</h3>

overview.html   117) 
overview.html   118) <p>
overview.html   119) Tor helps to reduce the risks of both simple and sophisticated traffic
overview.html   120) analysis by distributing your transactions over several places on the
overview.html   121) Internet, so no single point can link you to your destination.  The idea
overview.html   122) is similar to using a twisty, hard-to-follow route in order to throw off

en/overview.wml 123) somebody who is tailing you — and then periodically erasing your

overview.html   124) footprints.  Instead of taking a direct route from source to
overview.html   125) destination, data packets on the Tor network take a random pathway

en/overview.wml 126) through several relays that cover your tracks so no observer at any

overview.html   127) single point can tell where the data came from or where it's going.
overview.html   128) </p>
overview.html   129) 

en/overview.wml 130) <img alt="Tor circuit step one" src="$(IMGROOT)/htw1.png" />

overview.html   131) 

overview.html   132) <p>
overview.html   133) To create a private network pathway with Tor, the user's software or
overview.html   134) client incrementally builds a circuit of encrypted connections through

en/overview.wml 135) relays on the network.  The circuit is extended one hop at a time, and
en/overview.wml 136) each relay along the way knows only which relay gave it data and which
en/overview.wml 137) relay it is giving data to.  No individual relay ever knows the

overview.html   138) complete path that a data packet has taken.  The client negotiates a
overview.html   139) separate set of encryption keys for each hop along the circuit to ensure

overview.html   140) that each hop can't trace these connections as they pass through.

overview.html   141) </p>
overview.html   142) 

en/overview.wml 143) <img alt="Tor circuit step two" src="$(IMGROOT)/htw2.png" />

overview.html   144) 
overview.html   145) <p>

overview.html   146) Once a circuit has been established, many kinds of data can be exchanged
overview.html   147) and several different sorts of software applications can be deployed

en/overview.wml 148) over the Tor network.  Because each relay sees no more than one hop in
en/overview.wml 149) the circuit, neither an eavesdropper nor a compromised relay can use

overview.html   150) traffic analysis to link the connection's source and destination.  Tor
overview.html   151) only works for TCP streams and can be used by any application with SOCKS
overview.html   152) support.
overview.html   153) </p>
overview.html   154) 
overview.html   155) <p>
overview.html   156) For efficiency, the Tor software uses the same circuit for connections

en/overview.wml 157) that happen within the same ten minutes or so.  Later requests are given a

overview.html   158) new circuit, to keep people from linking your earlier actions to the new
overview.html   159) ones.

overview.html   160) </p>
overview.html   161) 

en/overview.wml 162) <img alt="Tor circuit step three" src="$(IMGROOT)/htw3.png" />

overview.html   163) 

overview.html   164) <h3>Hidden services</h3>

overview.html   165) 

overview.html   166) <p>

overview.html   167) Tor also makes it possible for users to hide their locations while
overview.html   168) offering various kinds of services, such as web publishing or an instant
overview.html   169) messaging server.  Using Tor "rendezvous points," other Tor users can

en/overview.wml 170) connect to these hidden services, 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.

en/overview.wml 175) Learn more about <a href="<page docs/tor-hidden-service>">configuring
en/overview.wml 176) hidden services</a> and how the <a href="<page hidden-services>">hidden
en/overview.wml 177) service protocol</a> works.

overview.html   178) </p>

overview.html   179) 

overview.html   180) <h3>Staying anonymous</h3>

overview.html   181) 
overview.html   182) <p>

overview.html   183) Tor can't solve all anonymity problems.  It focuses only on

overview.html   184) protecting the transport of data.  You need to use protocol-specific
overview.html   185) support software if you don't want the sites you visit to see your

overview.html   186) identifying information. For example, you can use web proxies such as
overview.html   187) Privoxy while web browsing to block cookies and withhold information

overview.html   188) about your browser type.

overview.html   189) </p>
overview.html   190) 
overview.html   191) <p>

overview.html   192) Also, to protect your anonymity, be smart.  Don't provide your name

overview.html   193) or other revealing information in web forms.  Be aware that, like all

overview.html   194) anonymizing networks that are fast enough for web browsing, Tor does not

overview.html   195) provide protection against end-to-end timing attacks: If your attacker

overview.html   196) can watch the traffic coming out of your computer, and also the traffic
overview.html   197) arriving at your chosen destination, he can use statistical analysis to
overview.html   198) discover that they are part of the same circuit.
overview.html   199) </p>
overview.html   200) 

overview.html   201) <h3>The future of Tor</h3>

overview.html   202) 
overview.html   203) <p>

overview.html   204) Providing a usable anonymizing network on the Internet today is an
overview.html   205) ongoing challenge. We want software that meets users' needs. We also
overview.html   206) want to keep the network up and running in a way that handles as many
overview.html   207) users as possible. Security and usability don't have to be at odds:
overview.html   208) As Tor's usability increases, it will attract more users, which will
overview.html   209) increase the possible sources and destinations of each communication,
overview.html   210) thus increasing security for everyone.

overview.html   211) We're making progress, but we need your help.  Please consider

en/overview.wml 212) <a href="<page docs/tor-doc-relay>">running a relay</a>

en/overview.wml 213) or <a href="<page volunteer>">volunteering</a> as a

en/overview.wml 214) <a href="<page documentation>#Developers">developer</a>.

overview.html   215) </p>
overview.html   216) 
overview.html   217) <p>

overview.html   218) Ongoing trends in law, policy, and technology threaten anonymity as never
overview.html   219) before, undermining our ability to speak and read freely online. These
overview.html   220) trends also undermine national security and critical infrastructure by
overview.html   221) making communication among individuals, organizations, corporations,

en/overview.wml 222) and governments more vulnerable to analysis. Each new user and relay

overview.html   223) provides additional diversity, enhancing Tor's ability to put control
overview.html   224) over your security and privacy back into your hands.

overview.html   225) </p>
overview.html   226) 
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