Selling an Idea or a Product

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91.580.203 Computer Network Forensics
Xinwen Fu Anonymous Communication Computer
Forensics
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Outline

• Background
• Onion routing
• Attacks against anonymity
• Tor

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Motivation
Protect the identity of participants in a
distributed application, such as E-voting,
E-shopping, E-cash, and military applications
I know whats going on!!!
Eavesdropping
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Current Network Status

• Commercial routers not under government control
• Unencrypted data is completely open
• Encrypted data still exposes communicating parties

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Traffic Analysis Attack

• Public networks are vulnerable to traffic
  analysis attack.
• In a public network
• Packet headers identify recipients
• Packet routes can be tracked
• Volume and timing signatures are exposed

Encryption does not hide identity information of
a sender and receiver.
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Traffic Analysis Attack (cont.)

• Traffic Analysis reveals identities.
• Who is talking to whom may be confidential or
  private
• Who is searching a public database?
• What web-sites are you surfing?
• Which agencies or companies are collaborating?
• Where are your e-mail correspondents?
• What supplies/quantities are you ordering from
  whom?
• Knowing traffic properties can help an adversary
  decide where to spend resources for decryption,
  penetration,...

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Goals of Anonymity Receiver Untraceability
Evil
Alice
Bob
Receivers are not observable i.e. the attacker
does not know if B received a message
Senders are observable i.e. the attacker knows
that A sent a message to someone
Example radio
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Goals of Anonymity Sender Untraceability
Evil
Bob
Alice
Example Wireless routers using NAT
Senders unobservable.
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Goals of Anonymity Sender/Receiver Unlinkability
Alice
Evil
Bob
Senders and Receivers are observable, but not
clear who is talking to whom
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Outline

• Background
• Onion routing
• Attacks against anonymity
• Tor

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Anonymous Communication Systems

• A number of Anonymous Communication Systems have
  been realized. Several well-known systems are
• Anonymizer (anonymizer.com)
• Onion-Routing (NRL)
• Crowds (Reiter and Rubin)
• Anonymous Remailer (MIT LCS)
• Tor (MIT and EFF)
• Freedom (Zero-Knowledge Systems)
• Hordes (Shields and Levine)
• PipeNet (Dai)
• SafeWeb (Symantec)

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Basic Approach Anonymizing Proxy
anonymizing proxy

• Channels appear to come from proxy, not true
  originator
• May also filter traffic for identifying
  information
• Examples Penet Remailer (shut down), The
  Anonymizer, SafeWeb (Symantec)

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Anonymizer for Web Browsing
anonymizing proxy anonymizer.com

• User connects to the proxy first and types the
  URL in a web form
• Channels appear to come from proxy, not true
  originator
• The proxy may also filter traffic to remove
  identifying information
• It offers encrypted link to the proxy (SSL or SSH)

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Problems of Anonymizer
Internet
Phone System
Proxy
ISP
Responders
Encrypted link user to proxy

• ISP knows user connection times/volumes Can
  easily eavesdrop on outgoing proxy connections
  and learn all
• Proxy knows everything about connections
• So, both are fully trusted (single points of
  failure)

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Chaum Mixes (David Chaum)

• Underlying Idea for Mixmaster remailer, Onion
  Routing, ZKS Freedom, Web Mixes
• Basic description A network of mix nodes
• Special Onion-like encryption Cell
  (message/packet) wrapped in multiple layers of
  public-key encryption by sender, one for each
  node in a route
• Decrypted layer tells mix next node in route
• Reordering Mixes hold different cells for a time
  and reorder before forwarding to respective
  destinations
• Rerouting use a few proxies

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Onion Routing Based on Mix Networks
Traditional Spy Network

• Sender selects a route through the mix network
• An intermediate mix only knows where the packet
  comes from, and what is the next stop of the
  packet

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Review of Public Key Cryptography

• PrivateKeyBob(PublicKeyBob(Message))Message
• PublicKeyBob(PrivateKeyBob(Message))Message

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Onion-Like Encryption
Receiver
Sender
B
A
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Why Buffering and Reordering Packets?

• Disrupt the timing correlation between packets
  into and out of a mix

mix
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Crowds
Sender
Blender
Web server

• User machines are the network
• "Blender" announces crowd members to all members
• Jondo" at machine flips weighted coin
• If Heads forwards to random crowd member
• If Tails connects to end Web address
• All Jondos on path know path key
• All connections from a source use same path for
  lifetime of that crowd

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Crowds Virtues

• Good on sender protections
• No single point of failure
• Peer-to-peer design means minimal long-term
  network services
• More lightweight crypto than mix-based systems

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Crowds Limitations

• All users must run Perl code
• Requires users to have longrunning high-speed
  Internet connections
• Entirely new network graph needed for new or
  reconnecting Crowd member
• Connection anonymity dependent on data anonymity
• Anonymity protection limited to Crowd size
• Rather weak on responder protections
• Lacks perfect forward anonymity
• The intermediate nodes knows the receiver

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Outline

• Background
• Onion routing
• Attacks against anonymity
• Tor

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Connectivity Analysis Attacks
Attacks against Mix Networks
B
Sender
Receiver
C
A
The adversary knows that Sender communicates with
Receiver
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Outline

• Background
• Onion routing
• Attacks against anonymity
• Tor

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Tor A Practical Anonymous Protocol

• Some combination of Chaums Mix and Crowds
• Encrypt data packets by symmetric keys
• Implement forward and backward anonymity
• Has P2P functions
• Easy to use
• Open source

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First Sight

• A web server knows your ip http//www.proxyway.co
  m/www/check-ip-address/whatis-my-ip-address.html
• Tor to hide your ip
• Tor downloading webpage
• http//tor.eff.org/download.html.en
• Manual for Windows setup
• http//tor.eff.org/docs/tor-doc-win32.html.en

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IE
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Tor Components
Internet
WWW Server
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Tor Network

• Onion router list C\Documents and
  Settings\fu\Application Data\Tor\cached-status

Application Server
Client
Tor Network
Legend
Client or Server or Onion Router
Onion Router
Directory Server
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References

• D. Chaum, (1981), Untraceable electronic mail,
  return addresses, and digital pseudonyms,
  Communications of the ACM, Vol. 24, No. 2,
  February, pp. 84--88.
• Andrei Serjantov, Roger Dingledine and Paul
  Syverson, From a Trickle to a Flood Active
  Attacks on Several Mix Types , In Proceedings of
  the Information Hiding Workshop, 2002
• Andreas Pfitzmann et al., Anonymity,
  Unobservability, and Pseudonymity A Proposal
  for Terminology, 2000,
• Xinwen Fu, welcome to Xinwen Fus homepage,
  http//www.homepages.dsu.edu/fux/, 2007
• Cisco Systems, Inc., Catalyst 2950 and Catalyst
  2955 Switch Software Configuration Guide,
  12.1(19)EA1, 2007
• Cisco Systems, Inc., Catalyst 2900 Series
  Configuration Guide and Command Ref, 2007