A New Replay Attack Against Anonymous Communication Networks

1. A New Replay Attack Against AnonymousCommunication Networks Xinwen Fu October 9, 2014

2. Outline • Introduction • Replay attack against anonymous communication systems - Tor • Evaluation • Summary Xinwen Fu

3. Internet Security • Internet has brought convenience to our everyday lives • However, it has also become a breeding ground for a variety of crimes • Malicious codes (worm and viruses) caused $13.2 billions in financial losses worldwide in 2001 • We need to understand these attacks and design corresponding countermeasures • We present our research on a new attack against anonymous communication systems Xinwen Fu

4. Traditional Spy Network Intelligence Center • Indirectly send secret to Intelligence headquarter through a number of intermediate agents. • Protect the intelligence agent (i.e., source of secret) from being identified. Xinwen Fu

5. Anonymous Communication Network - Tor • Client: the user of the Tor network • Server: the target TCP applications such as web servers • Tor router: the special proxy relays the application data • Directory server: servers holding Tor router information Xinwen Fu

6. Problem Definition of Attacks against Tor • Alice is sending messages to Bob through encrypted and anonymous channel, how can Evil confirm the communication relationship between Alice and Bob? Network Alice Bob Xinwen Fu

7. Outline • Introduction • Replay attack against anonymous communication systems - Tor • Evaluation • Summary Xinwen Fu

8. Basic Principle of the Replay Attack • A circuit is created by a sender and dedicated for a pair of users circuit A circuit C circuit B Xinwen Fu

9. Replay Attack against Tor • A packet comes from Alice through circuit A, and goes to Bob after circuit C • Replayed packet X causes a (special) decryption error at the end of circuit C at Eve 2 • An AES counter is synchronized through path and disturbed • The duplicated packet disrupt the counter • Therefore, Circuits A and C are created by Alice • Claim: Alice is communication with Bob Alice Eve 1 at Entry Eve 2 at Exit Bob circuit A circuit B circuit C t t t t Xinwen Fu KB Kn K1 KA KB Kn K1 KA … … … … … Bob, Message M Alice Bob, Message M Alice

10. Outline • Introduction • Replay attack against anonymous communication systems - Tor • Evaluation • Summary Xinwen Fu

11. Experiment Setup Xinwen Fu

12. Duplication Causing Path Teardown Xinwen Fu

13. Outline • Introduction • Replay attack against anonymous communication systems - Tor • Evaluation • Summary Xinwen Fu

14. Summary and Future Work • We identified a new replay attack against Tor and the attack can seriously degrade anonymity that Tor provides • Need only one packet!!! • Our experiments validate the feasibility and effectiveness of the replay attack • Future work: develop countermeasure against the replay attack Xinwen Fu

15. Thank you! Xinwen Fu

16. Traditional Spy Network • Indirectly send secret to Intelligence headquarter through a number of intermediate agents. • Protect the intelligence agent (i.e., source of secret) from being identified. Xinwen Fu

17. Replay Attack against Tor Xinwen Fu KB Kn K1 KA KB Kn K1 KA … … … … … Bob, Message M Alice Bob, Message M Alice