1. APPLAUS: A Privacy-Preserving Location Proof Updating System for Location-based Services Zhichao Zhu and Guohong Cao Department of Computer Science and Engineering The Pennsylvania State University, University Park, PA 16802 {zzhu, gcao}@cse.psu.edu

2. outline Introduction Preliminaries The Location Proof Updating System Security Analysis and Countermeasures Performance Evaluation Conclusions

3. outline Introduction Preliminaries The Location Proof Updating System Security Analysis and Countermeasures Performance Evaluation Conclusions

4. Introduction Location-based Services User location information Provide mobile users with a unique style of resource and services

5. Target: collecting a set of location proofs for each peer node. protecting the location privacy of peer nodes from each other, from adversary.

6. outline Introduction Preliminaries The Location Proof Updating System Security Analysis and Countermeasures Performance Evaluation Conclusions

7. Preliminaries Pseudonym Online Certification Authority Mobile node i with a set of M public/private key pairs. K i Pub serve as the pseudonyms of node i. K i prv enable node i to digitally sign messages.

8. Threat Model Target: track the location of mobile nodes. The adversary is internal, passive and global. Internal Passive Global

9. outline Introduction Preliminaries The Location Proof Updating System Security Analysis and Countermeasures Performance Evaluation Conclusions

10. The Location Proof Updating System Architecture Protocol Scheduling Location Proof Updates

11. Architecture Prover Witness Server CA Verifier

12. Protocol

13. (1) The prover broadcasts a location proof request through Bluetooth interface. (2) The witness decides whether to accept the location proof request. (3) the prover is responsible for submitting this proof to the location proof server.

14. Protocol (4) An authorized verifier can query the CA for location proofs of a specific prover. (5) The location proof server only returns hashed location rather than the real location to the CA, who then forwards to the verifier.

16. Scheduling Location Proof Updates

17. outline Introduction Preliminaries The Location Proof Updating System Security Analysis and Countermeasures Performance Evaluation Conclusions

18. Source Location Privacy Definition 2: source location unobservability. Identity I, observation O ∨ O, P(I) = P(I|O), the I is called unobservable.

19. Source Location Privacy Obviously, a system satisfies source location unobservability if and only if it has the property of pseudonym unlinkability.

20. Source Location Privacy

21. outline Introduction Preliminaries The Location Proof Updating System Security Analysis and Countermeasures Performance Evaluation Conclusions

22. Performance Evaluation Prototype Implementation Simulation results

23. Prototype Implementation Client: JAVA, Android Developer Phone, Android 1.6 OS. Server: C++, T4300 2.1GHz 3GB RAM laptop.

24. Prototype Implementation

27. Simulation results 1000 mobile nodes 3km * 3km area Each node has M=10 pairs of 128-bit public/private keys Interval proof : the required interval between two location proof updates. Interval contact : the mean real contact interval.

28. Message overhead ratio: the ratio of dummy traffic and real proof traffic.

29. Simulation results Proof delivery ratio: the percentage of location proof message that successfully uploaded to the location proof server.

30. Simulation results Average delay: the time difference between the time when a location proof update is needed and when the location proof message has reach the location proof server.

31. outline Introduction Preliminaries The Location Proof Updating System Security Analysis and Countermeasures Performance Evaluation Conclusions

32. This paper proposed a privacy-preserving location proof updating system, called APPLAUS. It uses statistically changed pseudonyms for each device to protect source location privacy. Extensive experimental and simulation results show that APPLAUS can provide location proofs effectively while preserving the source location privacy at the same time.

33. Thank you!