Xinwen Fu Anonymous Communication & Computer Forensics

1. 91.580.203 Computer & Network Forensics Xinwen Fu Anonymous Communication & Computer Forensics

2. Outline • Background • Onion routing • Attacks against anonymity • Tor

3. Motivation Protect the identity of participants in a distributed application, such as E-voting, E-shopping, E-cash, and military applications I know what’s going on!!! Eavesdropping

4. IP Packet Header Structure Sender Address Receiver Address Current Network Status • Commercial routers not under government control • Unencrypted data is completely open • Encrypted data still exposes communicating parties

5. Public Network Receiver Sender 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.

6. 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,...

7. 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

8. Goals of Anonymity: Sender Untraceability Evil Bob Alice Example: Wireless routers using NAT Senders unobservable….

9. Goals of Anonymity: Sender/Receiver Unlinkability Alice Evil Bob Senders and Receivers are observable, but not clear who is talking to whom

10. Outline • Background • Onion routing • Attacks against anonymity • Tor

11. 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)

12. 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)

13. 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)

14. 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)

15. 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

16. Receiver Sender B to R B S to A A to B A Anonymity Network Onion Routing Based on Mix Networks • 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 Traditional Spy Network

17. Bob Alice (eA, dA) (eB, dB) eB(message) dB(eB(message))=message Review of Public Key Cryptography • PrivateKeyBob(PublicKeyBob(Message))=Message • PublicKeyBob(PrivateKeyBob(Message))=Message

18. B to R M S to A S to R B A to B √ R R M M Onion-Like Encryption Receiver Sender B A

19. Why Buffering and Reordering Packets? • Disrupt the timing correlation between packets into and out of a mix mix

20. Crowds Blender Sender 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

21. 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

22. 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

23. Outline • Background • Onion routing • Attacks against anonymity • Tor

24. Connectivity Analysis Attacks x x C to R B to C S to A A to B B to C & C to R S to A & A to B Adversary HQ Attacks against Mix Networks B Sender Receiver C A The adversary knows that Sender communicates with Receiver

25. Outline • Background • Onion routing • Attacks against anonymity • Tor

26. Tor: A Practical Anonymous Protocol • Some combination of Chaum’s Mix and Crowds • Encrypt data packets by symmetric keys • Implement forward and backward anonymity • Has P2P functions • Easy to use • Open source

27. First Sight • A web server knows your ip: http://www.proxyway.com/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

29. IE

30. tor Privoxy Vidalia Tor Components Internet WWW Server

31. Directory Server 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

32. 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 Fu’s 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