Tor is a free tool that allows users to browse the internet anonymously. It works by routing internet traffic through a network of volunteers around the world, preventing others from seeing what sites a user visits. Originally created by the U.S. Naval Research Laboratory to protect government communications, Tor has since been adopted by normal users, journalists, activists, and others seeking privacy online or wanting to access censored sites. It encrypts data and routes it through several volunteer nodes, removing layers of encryption at each stop to hide a message's origin, destination, and contents from intermediaries.

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Tor project: Anonymity online

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Overview
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What is Tor?
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• Tor was originally designed, implemented, and deployed as a third-
generation onion routing project of the U.S. Naval Research Laboratory
[1], for the primary purpose of protecting government communications.
• Tor is a free tool that allows people to use the internet anonymously.
• Basically, Tor protects you by bouncing your communications
around a distributed network of relays run by volunteers all around
the world: it prevents somebody watching your Internet connection
from learning what sites you visit, it prevents the sites you visit from
learning your physical location, and it lets you access sites which
are blocked.
• Tor anonymizes the origin of your traffic!

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What is Tor?
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IP address that appears
via other browsers at
the same time
IP address that appears
via other browsers at
the same time
IP address that appears
via the Tor browser
IP address that appears
via the Tor browser

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What is under the hood?
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Basic knowledge from networks
•Internet data packet consists of two kinds of data:
– control information (header): provides data the network needs to deliver the user data
(e.g. source and destination addresses, error detection codes like checksums, and
sequencing information);
– user data (also known as payload): whatever is being sent, whether that's an email
message, a web page, or an audio file.
•Even if you encrypt the data payload of your communications, traffic
analysis still reveals a great deal about what you're doing and, possibly,
what you're saying. That's because it focuses on the header, which
discloses source, destination, size, timing, and so on.
Figure from http://www.totalsem.com/files/MMP_N_ch1.pdf

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What is under the hood? (cnt’d)
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Tor is based on Onion Routing, a technique for anonymous communication
over a computer network.
http://en.wikipedia.org/wiki/Onion_routing
Steps
•Messages are repeatedly encrypted and then
sent through several network nodes called onion
routers.
•Each onion router removes a layer of encryption
to uncover routing instructions, and sends the
message to the next router where this is
repeated. This prevents these intermediary
nodes from knowing the origin, destination, and
contents of the message.
Onions

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What is under the hood? (cnt’d)
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User's software or client
incrementally builds a circuit of
encrypted connections through
relays on the network.

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Who is using Tor?
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• Normal people (e.g. protect their browsing records)
• Militaries (e.g. military field agents)
• Journalists and their audiences
(e.g. citizen journalists encouraging social change)
• Law enforcement officers
(e.g. for online “undercover” operations)
• Activists and Whilstblowers
(e.g. avoid persecution while still raising a voice)
• Bloggers
• IT professionals (e.g. during development and operational testing, access internet
resources while leaving security policies in place)

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Tor project
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Software and Services under the Tor project umbrella:
•Torbutton
•Tor Browser Bundle
•Vidalia
•Arm
•Orbot
•Tails
•Onionoo
•Metrics Portal
•Tor Cloud
•Obfsproxy
•Shadow
•Tor2web
https://www.torproject.org/projects/projects.html.en

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Hands on
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• Demo
• Metrics: https://metrics.torproject.org/ http://tigerpa.ws/tor_metrics/

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References
1. Onion Routing http://www.onion-router.net/
2. Tor project: http://www.torproject.org
3. Roger Dingledine, Nick Mathewson, Paul SyversonTor: The Second-Generation Onion
Router
4. Len Sassaman: The Faithless Endpoint How Tor puts certain users at greater risk
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