Cryptography for Anonymity

We now look at a very different use of cryptography—to support anonymity. While the concept of anonymity at a basic level (the hiding of identity) feels very intuitive, anonymity is surprisingly difficult to define precisely. This is partly because anonymity has many facets and is generally hard to provide in digital environments. For example, by using encryption it is fairly easy to scramble a piece of data containing an identity. However, if this ciphertext is sent over the Internet, the sender’s IP address might give away the identity contained within the data. We will discuss Tor, the most widely used tool for providing anonymity on the Internet. We will examine how Tor uses cryptography to support a specific, although limited, notion of anonymity.

There are many reasons why someone might wish to remain anonymous on the Internet. Some intentions are good, and, of course, some are bad. However, the number of positive applications is considerable, making a compelling case for establishing effective tools to support anonymity. These include:

• Ordinary Internet users who are concerned about preventing their private Internet activities from being known by commercial companies and government surveillance programs.

• Internet users in countries with a low tolerance for freedom of speech, especially those where opposition to the political regime has severe consequences.

• Journalists and other investigators, including law enforcement and national security officers, conducting sensitive projects.

Tor background

Tor is open-source software developed by the nonprofit Tor Project. Tor primarily consists of a distributed network of volunteer network entities through which Internet communications can be routed to provide anonymity. Here, we will only consider Tor’s support for anonymous Internet browsing, but note that Tor supports other features. There is no doubt that Tor has been enormously successful since its 2002 launch, with well over one million users now using it every day.