1. Tor – The Onion Router
Presented By
Ashly Liza Philip
S5 MCA
Roll No. 06
MACE

2. Privacy in public networks
 Internet is designed as a public network.
 Routing information is public.
 Encryption does not hide identities.
 Prone to „traffic analysis‟

3. Contd..
Internet data packet
Header Data
payload
Source,
Whatever being sent
destination, time
Encryption can‟t Hidden by
hide encryption

4. What is Anonymity?
 State of being not identifiable within a set of
subjects.
 Unlink-ability of action and identity
 Unobserve-ability (hard to achieve)

5. Applications of Anonymity
Privacy
Untraceable electronic mail
Law enforcement and
intelligence
Anonymous electronic voting
Censorship-resistant
publishing

6. Anonymous networks
PipeNet
Garlic Routing
Onion Routing
Anonymizer
Crowds
Java Anon Proxy

7. Onion Routing
 By David Goldschlag, Michael Reed, Paul Syverson
 ONION; special data structure.
 Uses public key cryptography
 Hides source and destination
 Anonymize TCP-based applications
 Protects anonymity of a user over network
 Resistant to eavesdropping and traffic analysis

8. Working of Onion Routing
 Uses an Onion Proxy
 Routes data randomly
 A router knows only its predecessor and successor
 No correspondence between data layers
 Different onion at each hop

9. R R4 R
R
R3
R1 R
R
Alice R2
R Bob
 Sender chooses a random sequence of routers
-Some routers are honest, some controlled by attacker
-Sender controls the length of the path
slide 9

10. Decrypting an Onion

11. Contd..

12. Tor – Implementation of Onion Routing
 TOR stands for „The Onion Router.‟
 Second generation onion router.
 Low-latency anonymous network
 By Paul Syverson, Nick Mathewson, Roger
Dingledine in 2004
 Maintained by Free Haven Project
 Hundreds of nodes on all continents

13. Contd..
 “Easy-to-use” client proxy
 Freely available
 Supports only TCP
 Uses SOCKS interface
 No observer can tell where data comes from/going

14. Advancements over Onion Routing
Perfect forward secrecy
Variable exit policies
Directory servers
Leaky-pipe circuit topology
End-to-end integrity checking
SOCKS interface

15. How Tor works?
 Same as Onion Routing
 Uses Diffie-Hellman key exchange
 Distributes data over several places
 Takes random pathway
 Used with Privoxy

16. Tor Circuit
 Each circuit shared by multiple TCP streams
 Circuit created preemptively
 Can recover from failure

17. Circuit setup
Built from entry point one step at a time
Circuit ID is chosen randomly
Diffie-Hellman process initiated
Negotiate a symmetric session key
Circuit extension and Relay packet

18. Tor Circuit setup(1)
Client proxy establish a symmetric session key and circuit with
Onion Router #1

19. Tor Circuit setup(2)
Circuit extension

20. Tor Circuit setup(3)
Client proxy extends the circuit by establishing a symmetric
session key with Onion Router #3

21. Using circuit

22. Cells in Tor
Cell (512bytes)
Header Payload
CircID Data
Command

23. Contd..
Based on commands in cells:-
Control cells Relay cells
Cells

24. Commands in cells
Control cells
Relay Cells
• -padding • - relay data
• -create/created • -relay begin
• -destroy • -relay end
• -relay connected
• -relay teardown
• -relay connected
• -relay extend/extended
• -relay truncate/truncated

25. E.g. Fetching a webpage

26. Congestion control
OR keeps track of PACKAGE WINDOW &
DELIVERY WINDOW
• Window initialized to 1000cells
• Decremented accordingly
Circuit throttling • Forward nothing on reaching 0
• Window initialized to 500cells
• Use relay sendme cells
Stream throttling • Incremented on receiving relay
sendme

27. Hidden services
 Allow location hidden services
 Both end-points remain anonymous
 Both parties connect via a third party, the rendezvous
point
 Introduction points act as contact points
 Server can be operated from inside a firewall

28. Creating and connecting to a
Location hidden service

29. Attacks on Tor
Passive attacks
Active attacks
Attacks against Rendezvous
points
Directory attacks

30. Users of Tor
Law
Militaries enforcement Bloggers
officers
Journalists High profile
Activists
and audience people
NGO‟s

31. Weaknesses
End-to-end
Slow network Illegal use
correlation

32. Real world deployment
The Tor download page.

33. The Tor/Privoxy installer.
Tor in the Windows system tray.

34. Setting up a proxy for
Internet Explorer
The Tor detector

35. Future enhancements
 Usability and Integration
 Scalability
 Caching at exit nodes

36. Conclusion
 Tor becomes most widely used anonymous network
with its speed
 Tor supports mutual anonymity with the help of
Rendezvous Point
 Tor resists local adversary effectively
 Tor anonymizes TCP streams, providing a high-
throughput and low-latency network compared to the
onion routing