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<h2>Tor: an anonymizing overlay network for TCP</h2>

Traffic analysis can be used to infer who is talking to whom over a
public network. For example, Internet packets have a header used for
routing, and a payload that carries the data. The header, which must be
visible to the network (and to observers of the network), reveals the
source and destination of the packet. Even if the header were obscured
in some way, the packet could still be tracked as it moves through the
network. Encrypting the payload is similarly ineffective, because the
routing information is all an observer needs.

Knowing the source and destination of your Internet traffic allows
somebody to track your behavior and interests, impacting your checkbook or
even threatening your job or physical safety.

Individuals, corporations, and governments all have an interest in
traffic analysis protection. Individuals want to protect themselves and
their family members from remote websites, or connect to resources such
as news sites or instant messaging services that are blocked locally.
User groups such as the German "Diabetes People" organization recommend
Tor for their members' online privacy and security. Activist groups such
as the Electronic Frontier Foundation are publicizing Tor as a mechanism
for maintaining civil liberties online. Corporations such as Google and
Wal-Mart are investigating Tor as a safe avenue for competitive analysis
or to try out new experimental projects without associating their name
with the project. A branch of the U.S. Navy uses Tor for open source
intelligence gathering at its headquarters, and one of their teams used
Tor while deployed in the Middle East recently.

Tor helps to reduce the traffic analysis risk by distributing your
transactions over several places on the Internet, so no single point can
link you to your destination. To make private connections in Tor, users
incrementally build a path or circuit of encrypted connections
through servers on the network, extending it one step at a time so that
each server in the circuit only learns which server extended to it and
which server it has been asked to extend to. The client negotiates a
separate set of encryption keys for each step along the circuit.

[Insert snazzy onion diagram here.]

Once a circuit has been established, the client software waits for
applications to request TCP connections, and directs these application
streams along the circuit. Many streams can be multiplexed along a single
circuit, so applications don't need to wait for keys to be negotiated
every time they open a connection. Because each server sees no
more than one end of the connection, a local eavesdropper or a compromised
server cannot use traffic analysis to link the connection's source and
destination. The Tor client software rotates circuits periodically
to prevent long-term linkability between different actions by a
single user.

Tor also makes it possible for the clients to be hidden. Using Tor
"rendezvous points," other Tor clients can connect to these hidden
services, each without knowing the other's network identity. These hidden
websites let users publish material without worrying about censorship.

Of course, Tor can't solve all privacy problems itself. Tor focuses on
protecting the transport. You need to use other protocol-specific
software, such as Privoxy for web browsing, to clean identifying
information such as browser type and characteristics, and you need
to use other common sense such as not providing your name or other
revealing information in web forms. Also, 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 simple statistics to discover that they are part
of the same circuit.

Providing a usable anonymizing network on the Internet today is hard,
both in terms of providing usable software that meets users' needs,
and also in terms of solving the research challenges. But we're making
progress at finding a good balance to provide both usability and security.
Our goal is to supplement laws by giving people the power to make their
own decisions about their privacy.

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