tor introduction

The tor network (the onion router) is a network of server computers called "nodes" on the internet that allow secure and anonymous comminucations. This has proved resistent to attack from even the vast combined resources of the NSA and GCHQ. What follows is a simplified approximation of how tor work. For more detaled and accurate description please visit:

• wikipedia article on tor
• tor website

There are three types of tor nodes:

• Entry nodes
• Routing nodes
• Exit nodes

Several types of communications can be sent through the tor network e.g. browsing a website. When a tor user visits a website, the tor software creates a "circuit" composed of:

• the user's PC
• a randomly selected tor entry node
• a randomly selected tor routing node
• a randomly selected tor exit node
• the website

This means that:

• the tor entry node:
• can't read the data
• can't read the destination
• knows the origin of the message
• knows to pass the message to the tor routing node
• the tor routing node:
• can't read the data
• can't read the destination
• does not know the origin of the message
• knows to pass the message to the tor exit node
• the tor exit node:
• can read the data
• can read the destination
• passes the data unencrypted to the destination
• does not know the origin of the message
• knows to pass the message to the destination

For the return message:

• the tor exit node:
• can read the data
• can't read the destination
• knows to pass the message to the tor routing node
• the tor routing node:
• can't read the data
• can't read the destination
• knows to pass the message to the tor entry node
• the tor entry node:
• can't read the data
• knows to pass the message to the user's PC

The tor network ensures that:

• no tor node knows both the source and destinmation of the message - indeed the tor routing node knows neither
• only the tor exit node says see the content of the meesage and the reply

Security can be increased further if the application using the tor network adds its own encryption e.g. if you were to use HTTPS to comminicate with the website. The message would then be encrypted 4 times before leaving the PC and would remain encrypted all the way to the website. The reply would also remain encrypted on the entire return path. Thus, none of the tor nodes can read the content of the message.

Vulnerabilities

The design of the tor is sound, but there are some things that tor users can do that may compromise their anonymity:

• Using applications that iclude the user's IP address or other identifying data in the messages sent. The Tails operating system specifically excludes all such applications
• Visiting sites with very little traffic may allow identification through traffic analysis

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