1. LEXP: Preserving User Privacy and Certifying the Location Information Author ： Ken Nakanishi, Jin Nakazawa, and Hideyuki Tokuda Presented by Yi Cheng Lin

2. Outline Introduction Architecture  Detector Registration  Address Notification  Address Resolution  Ticket Publication and Verification Security Analysis Conclusion

3. Introduction Many kind of location sensing technologies already exist, and more are under development Location information Exchange Protocol (LEXP) was designed for preserving user privacy and certifying users location information

4. Architecture

5. Parameter Each client, detector, service provider, and ticket have its own unique identifier CID, DID, SID TID h(x),h 2 (x) denote the MD5 hash value of x {x} key denote the encrypted data of x by key

6. Detector Registration h( DID )h( password )address password header{h(DID) h(password) h(detector_publickey) secretkey} resolver_publickey header{h(detector_publickey)} resolver_privatekey Register the secret key information

7. Address Notification h(RFID)h(CID)addressra update

8. Address Resolution h(RFID)h(CID)addressra rb header{h(DID) h(RFID) rb} secretkey header{ra address rb} secretkey h(RFID)addressra

9. Ticket Publication(1/2) 1. Generate a pair of Diffie-Hellman key 2. D-public key 3. C-public key D-private key C-private key DES session key 4. {h(RFID) ra} sessionkey h(RFID)addressra Check h(RFID)

10. Ticket Publication(2/2) random number R ticket = {TID timestamp} detector-privatekey {DID TID R ticket} sessionkey DIDdector’s addressTIDrt(=at this point’s ra)Rticket TIDrth N (R)n(at first n = N, n is the consumable time of ticket)

11. Ticket Verification {validity r} detector_privatekey Check h N-n (h n (R)) = h N (R)

12. Security Analysis Address Notification  CID is secret Address Resolution  No attacker can get secret key (in a secret channel) Ticket Publication  Only detector and client have ra Ticket Verification  h n (R) is one-time password  The password required next time is h n-1 (R), which no one can generate unless he knows R

13. Conclusion LEXP guarantees users to keep anonymity, guarantees applications that a user cannot forge his location information Applying the modules of ‘chain of confidence’ and one-time password