Onion routing - an application independent infrastructure for private communication over a public network.

1. Onion Routing

2. 2
Goals
 Introduction
 Overview of onion routing
 Explanation of security goals
 Description of network model &
assumptions
 Discussion of adversary types
 Comparison with Crowds
 Tor
 Future Works

3. 3
Why anonymous communication?
 Public Networks are Vulnerable to
Traffic Analysis
 Encryption does not hide routing
information.
 The simple fact that two parties are
communicating can be used to link the two
parties.

4. 4
Who Needs Anonymity?
 Political Dissidents, Whistleblowers
 Censorship resistant publishers
 Socially sensitive communicants
- Chat rooms and web forums
 Law Enforcement:
− Anonymous tips or crime reporting
− Surveillance and honeypots
 Corporations

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Onion routing
 Onion routing - an application independent
infrastructure for private communication over a
public network.
 Provides anonymous
connections that are
resistant to both traffic
analysis and evesdropping

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2.Onion Routing Overview
 Onion router ≈ real time Chaum mix
 Store and forward with minimal delays

7. 7
 Onion routing connection phases
 Connection Setup
 Data Movement (Transmission)
 Connection Termination (Teardown)

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Setup Phase
 Connection initiator builds an onion
 Layered cryptographic structure, specifying:
 Path through network
 Point-to-point symmetric encryption algorithms
 Cryptographic keys
 At each step
 Router decrypts entire structure
 Sets up encrypted channels to predecessor and
successor nodes
 Forwards new onion on to successor

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Transmission Phase
 When connection initiator wants to send data
 Break data into uniform (128 bit) blocks
Encrypt each block once for each router in the
path
-Uses symmetric encryption here
 Send data to first onion router
 All onion routers connected by persistent TCP
thick pipes which add another layer of
encryption on top of all of this encryption!

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Onion Routing Overview

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Connection Termination
 By the initiator or the destination server
 Since onion routers may fail, however,
any onion router involved in a
connection can cause that connection to
be terminated.

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3.Security Goals
 The goal is to hide
 Sender activity
 Receiver activity
 Sender content
 Receiver content
 Source-destination pairs

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4.Network Assumptions
1. Onion routers are all fully connected
2. Links are padded or bandwidth-limited
to a constant rate
3. Unrestricted exit policies
4. For each route, each hop is chosen at
random
5. Number of nodes in a route is chosen
at random

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5.Adversary Model
 4 Types of adversaries
 Observer
 Disrupter
 Hostile user
 Compromised COR
 Adversary
distributions
 Single
 Multiple
 Roving (realistic)
 Global

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6.Comparison with other
anonymity mechanisms
 Mixes
 Proxies
 Crowds

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 Any compromised node on the path
compromises both receiver activity and
receiver content.
 Request contents are exposed to
intermediate nodes as decryption keys
are available to all the participants
Limitations of crowds

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7. Tor
- The Onion Routing
 Tor is the most advanced implementation
of Onion Routing in use today
 Provides perfect forward secrecy
 Tor uses Diffie-Hellman key exchange

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 Tor provides Directory Servers
- protection against compromised nodes
 Uses Leaky –pipe circuit topology
 Currently 60+ Tor nodes online

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8.Conclusions and
future work
 No router will ever know the full path
that is traveled by the onion.
 So the communication is completely
anonymous.

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Future improvements
 Adding a time delay to traffic at proxy could
complicate timing attacks against the local –
COR configuration to determine the first
COR.
 Using partial route padding on individual
connections besides link padding.
 Hopping short lived OR connections for
long lived application connections.

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The second generation Onion
Routing design generally resists
traffic analysis more effectively than
any other published and deployed
mechanisms for Internet
communication .

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References
 The Onion Routing Home Page. http://www.onion-router.net/
 https://torproject.org/
 Paul Syverson _ Gene Tsudiky,Michael Reed _ Carl Landwehr,
Towards analysis of onion routing security -2000
 D. Goldschlag, M. Reed, P. Syverson. “Onion Routing for
Anonymous and Private Internet Connections," Communications
of the ACM, vol. 42, num. 2, February 1999
 D. Goldschlag, M. Reed, P. Syverson. “Hiding Routing
Information, in Information Hiding, R. Anderson, ed., LNCS vol.
1174, Springer-Verlag, 1996, pp. 137–150.

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25. Thank you
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