Traffic analysis is based on the fact that every packet of data sent from your computer includes a header containing information about source, destination, size, timing, and other items. If you take a look at a packet header you can at the very least see who sent the the data packet. That's what traffic analysis in its simplest form is about: intercepting data packets and looking at their headers.
Tor tries to keep your packets private by distributing your transactions over several places on the Internet, so there is no direct connection to your destination. As Tor's Web site puts it: "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."
The Tor network consists of servers known as onion routers. Instead of sending data directly to a destination server, your computer uses these onion routers. To do this, the computer obtains a list of onion routers from a directory server and then selects a random path to the destination server. The clever part is that each onion router along the way knows only which server data is received by and which server data is being sent to -- as each layer in an onion touches only the ones on either side of it. In other words, none of the onion routers know where the data packet originated from.
To be able to use the Tor network you have to install a Tor client on your machine. The Tor software is available for Windows, Linux, and Mac OS X platforms and is pretty easy to install.
To protect your Web browser from leaking information via DNS requests, Tor client software relies on Privoxy, "a Web proxy with advanced filtering capabilities for protecting privacy, modifying Web page content, managing cookies, controlling access, and removing ads, banners, pop-ups, and other obnoxious Internet junk." This means that before you can use your Web browser with Tor software, you should install and configure Privoxy. Luckily, this is also an easy thing to do. Then add the following line to Privoxy's configuration file (on Windows right-click on the Privoxy icon in the System Tray and choose Edit > Main Configuration):
forward-socks4a / localhost:9050 .
Finally you have to "torify" your Web browser and other applications. This basically means that you have to specify proxy settings in the application. To configure, for example, a Firefox browser, choose Tools > Options, select the General section, and click the Connection settings button. Select the manual proxy configuration option, in the HTTP Proxy field enter localhost and in the Port field type 8118. Click OK, and you are done. If you need to configure other applications, check Tor's wiki, which provides detailed instructions on how to "torify" different software.
To begin preserving your online privacy, make sure that Tor and Privoxy are started, launch your Web browser, and point it to Junkbusters Web site. If Tor is working properly, the Web page will display an IP address that is different from your own.
Tor was initially designed and developed as part of the U.S. Naval Research Laboratory's Onion Routing program with support from DARPA. Today it is supported by Electronic Frontier Foundation, among others.
As any other open source project Tor needs help. If you are not a developer you can help by setting up an onion server, provided you have spare hardware and bandwidth. The installed Tor client can easily be turned into an onion router by simply editing its configuration file. However, doing so requires you have a working knowledge of server configuration, and it's a good idea to check Tor's documentation beforehand. If you are concerned about legal issues, check the Legal FAQ for Tor Server Operators as well.
Dmitri Popov is a freelance contributor, whose articles have appeared in Russian, British, and Danish computer magazines.