Tor is a network of virtual tunnels that allows people and groups to
improve their privacy and security on the Internet. It also enables
software developers to create new communication tools with built-in
privacy features. Tor provides the foundation for a range of
applications that allow organizations and individuals to share
information over public networks without compromising their privacy.
Individuals use Tor to keep websites from tracking them and their
family members, or to connect to news sites, instant messaging
services, or the like when these are blocked by their local Internet
providers. Tor's hidden services let users publish web sites and
other services without needing to reveal the location of the site.
Individuals also use Tor for socially sensitive communication: chat
rooms and web forums for rape and abuse survivors, or people with
illnesses.
Journalists use Tor to communicate more safely with whistleblowers
and dissidents. Non-governmental organizations (NGOs) use Tor to
allow their workers to connect to their home website while they're in
a foreign country, without notifying everybody nearby that they're
working with that organization.
Groups such as Indymedia recommend Tor for safeguarding their
members' online privacy and security. Activist groups like the
Electronic Frontier Foundation (EFF) are supporting Tor's development
as a mechanism for maintaining civil liberties online. Corporations
use Tor as a safe way to conduct competitive analysis, and to protect
sensitive procurement patterns from eavesdroppers. They also use it
to replace traditional VPNs, which reveal the exact amount and timing
of communication. Which locations have employees working late? Which
locations have employees consulting job-hunting websites? Which
research divisions are communicating with the company's patent
lawyers?
A branch of the U.S. Navy uses Tor for open source intelligence
gathering, and one of its teams used Tor while deployed in the Middle
East recently. Law enforcement uses Tor for visiting or surveilling
web sites without leaving government IP addresses in their web logs,
and for security during sting operations.
The variety of people who use Tor is actually part of what makes it
so secure. Tor hides you among the other users on the network, so the
more populous and diverse the user base for Tor is, the more your
anonymity will be protected.
Why we need Tor
Using Tor protects you against a common form of Internet surveillance
known as "traffic analysis." Traffic analysis can be used to infer
who is talking to whom over a public network. Knowing the source and
destination of your Internet traffic allows others to track your
behavior and interests. This can impact your checkbook if, for
example, an e-commerce site uses price discrimination based on your
country or institution of origin. It can even threaten your job and
physical safety by revealing who and where you are. For example, if
you're travelling abroad and you connect to your employer's computers
to check or send mail, you can inadvertently reveal your national
origin and professional affiliation to anyone observing the network,
even if the connection is encrypted.
How does traffic analysis work? Internet data packets have two parts:
a data payload and a header used for routing. The data payload is
whatever is being sent, whether that's an email message, a web page,
or an audio file. Even if you encrypt the data payload of your
communications, traffic analysis still reveals a great deal about
what you're doing and, possibly, what you're saying. That's because
it focuses on the header, which discloses source, destination, size,
timing, and so on.
A basic problem for the privacy minded is that the recipient of your
communications can see that you sent it by looking at headers. So can
authorized intermediaries like Internet service providers, and
sometimes unauthorized intermediaries as well. A very simple form of
traffic analysis might involve sitting somewhere between sender and
recipient on the network, looking at headers.
But there are also more powerful kinds of traffic analysis. Some
attackers spy on multiple parts of the Internet and use sophisticated
statistical techniques to track the communications patterns of many
different organizations and individuals. Encryption does not help
against these attackers, since it only hides the content of Internet
traffic, not the headers.
The solution: a distributed, anonymous network
Tor helps to reduce the risks of both simple and sophisticated
traffic analysis by distributing your transactions over several
places on the Internet, so no single point can link you to your
destination. The idea is similar to using a twisty, hard-to-follow
route in order to throw off somebody who is tailing you-and then
periodically erasing your footprints. Instead of taking a direct
route from source to destination, data packets on the Tor network
take a random pathway through several servers that cover your tracks
so no observer at any single point can tell where the data came from
or where it's going.
To create a private network pathway with Tor, the user's software or
client incrementally builds a circuit of encrypted connections
through servers on the network. The circuit is extended one hop at a
time, and each server along the way knows only which server gave it
data and which server it is giving data to. No individual server ever
knows the complete path that a data packet has taken. The client
negotiates a separate set of encryption keys for each hop along the
circuit to ensure that each hop can't trace these connections as they
pass through.
Once a circuit has been established, many kinds of data can be
exchanged and several different sorts of software applications can be
deployed over the Tor network. Because each server sees no more than
one hop in the circuit, neither an eavesdropper nor a compromised
server can use traffic analysis to link the connection's source and
destination. Tor only works for TCP streams and can be used by any
application with SOCKS support.
For efficiency, the Tor software uses the same circuit for
connections that happen within the same minute or so. Later requests
are given a new circuit, to keep people from linking your earlier
actions to the new ones.
Hidden services
Tor also makes it possible for users to hide their locations while
offering various kinds of services, such as web publishing or an
instant messaging server. Using Tor "rendezvous points," other Tor
users can connect to these hidden services, each without knowing the
other's network identity. This hidden service functionality could
allow Tor users to set up a website where people publish material
without worrying about censorship. Nobody would be able to determine
who was offering the site, and nobody who offered the site would know
who was posting to it.
Staying anonymous
Tor can't solve all anonymity problems. It focuses only on protecting
the transport of data. You need to use protocol-specific support
software if you don't want the sites you visit to see your
identifying information. For example, you can use web proxies such as
Privoxy while web browsing to block cookies and withhold information
about your browser type.
Also, to protect your anonymity, be smart. Don't provide your name or
other revealing information in web forms. Be aware that, like all
anonymizing networks that are fast enough for web browsing, Tor does
not provide protection against end-to-end timing attacks: If your
attacker can watch the traffic coming out of your computer, and also
the traffic arriving at your chosen destination, he can use
statistical analysis to discover that they are part of the same
circuit.
The future of Tor
Providing a usable anonymizing network on the Internet today is an
ongoing challenge. We want software that meets users' needs. We also
want to keep the network up and running in a way that handles as many
users as possible. Security and usability don't have to be at odds:
As Tor's usability increases, it will attract more users, which will
increase the possible sources and destinations of each communication,
thus increasing security for everyone. We're making progress, but we
need your help. Please consider running a server or volunteering as a
developer.
Ongoing trends in law, policy, and technology threaten anonymity as
never before, undermining our ability to speak and read freely
online. These trends also undermine national security and critical
infrastructure by making communication among individuals,
organizations, corporations, and governments more vulnerable to
analysis. Each new user and server provides additional diversity,
enhancing Tor's ability to put control over your security and privacy
back into your hands.
