The Tor Project [1] has spawned a whole global community dedicated to the concept of anonymous browsing. The project's Tor Browser [2], which lets the user surf the web without leaving a trail or trace, was originally intended to help dissidents in totalitarian countries communicate without surveillance, but since then, it has become popular with whistle-blowers, drug buyers, and millions of everyday users who simply don't want to submit to the culture of tracking and targeting that exists on the mainstream Internet.

The core technology behind the Tor Browser is a technique known as onion routing. Onion routing routes a message through a message of participating routers that could be anywhere on the Internet. A data packet takes a random path through a series of the routers. The packet is encapsulated in multiple layers of encrypted routing information. Each router receives the packet and uses its private key to decrypt the outermost layer, which contains a destination address for where to forward the packet, and then sends the data on to the next link in the chain.

The power of onion routing is that no single router on the network has complete knowledge of where the packet came from and where it is going. Each router can only read the single layer specifically encrypted and addressed to it. This technique is known as onion routing, because the many layers of encrypted routing information resemble the layers of an onion that are gradually peeled away as the packet makes its way through the network. (For more information, see the "How Private?" box.)

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