Anonymous and Authenticated Key Exchange for Roaming Networks

1. Guomin Yang et al. IEEE Transactions on Wireless Communication Vol. 6 No. 9 September 2007 Anonymous and Authenticated Key Exchange for Roaming Networks

2. Agenda • Preliminaries • Introduction • Security requirements • Proposed Scheme : AAKE-R • Analysis • Conclusion

3. Eavesdropping C A B Preliminaries(1/9)Basic requirements of network security • Data Confidentiality – keeping information secret from all but who are authorized to it

4. Impersonation A B C Preliminaries(2/9) Basic requirements of network security • Authentication – corroboration of identity of entity

5. Preliminaries (3/9) Symmetric key cryptosystem • a.k.aSecret key cryptosystem Symmetric encryption for data confidentiality Message Authentication Code (MAC) for Authentication

6. Preliminaries (4/9) Asymmetric key cryptosystem • a.k.aPublic key cryptosystem Asymmetric encryption for data confidentiality Digital signature scheme for Authentication

7. Preliminaries (5/9) Symmetric vs Asymmetric Comparison table

8. Preliminaries (6/9) Authenticated Key Exchange (AKE) • Key distribution in symmetric setting is a problem • Two different types of key • Long-term key • Set up initial key for each entity • Key Pre-distribution System • Session (short-term) key • After long-term key set up, share secret information among 2 or multi entities • Key Establishment System • Authenticated key exchange is a solution to establish session key

9. Preliminaries (7/9) Authenticated key exchange (AKE) • In asymmetric setting, two entities authenticate each other and establish session key using digital signature scheme. • Key transport: one party creates and transfers it to the other(s) • Key exchange: a shared secret is derived by two or more parties as a function of information contributed by. No party can determine the resulting value.

10. Preliminaries (8/9) Diffie-Hellman key exchange To authenticate each other, these values should be signed using digital signature scheme

11. Preliminaries (9/9) Cryptographic hash function • A cryptographic hash function is a transformation that takes an input and returns a fixed-size string, which is called the hash value • One-wayness – calculating H(x) = y is easy, but given y, to find x is difficult • Collision free – Two different x1, x2 cannot have the same hash value y

12. Introduction (1/2) Roaming network • A technology lets a user originally subscribed to a network can travel to another network administrated by a different operator and access services provided by this network as a visiting user or a guest • User can enjoy a much broader coverage in terms of services or geographical areas without being limited by that of their own networks

13. Introduction (2/2) Roaming network Home service area Foreign service area Home server Foreign server Roaming user

14. Security requirements • Server Authentication – The user is sure about the identity of the foreign server • Subscription validation – The foreign server is sure about the identity of the home server of the user • Key Establishment – The user and the foreign server establish a random session key which is known only to them and is derived from contributions of both of them. In particular, the home server should not obtain the session key • User Anonymity – Besides the user and the home server, no one including the foreign serve can tell the identity of the user • User Untraceability – Besides the user and the home server, no one including the foreign server is able to identify any previous protocol runs which have the same user involved

15. Proposed AAKE-R(1/5)Notation Notation table

16. Proposed AAKE-R (2/5) Building blocks • AKE (Authenticated Key Exchange) • AAKE (Anonymous Authenticated Key Exchange) • AKT (Authenticated Key Transport)

17. Proposed AAKE-R (3/5) Assumption • There is a direct link between roaming user and foreign server and another direct link between home server and foreign server • Roaming user know the public key of foreign server • Each user knows its home server’s public key and each server knows the public keys of all its subscribers • All servers know the public keys of all other servers in roaming network

18. Proposed AAKE-R (4/5)Proposed scheme

19. Proposed AAKE-R (5/5) optimized version

20. Security analysis Key Establishment User Anonymity & User traceability Subscription validation Server Authentication

21. Comparison with other protocols

22. Conclusion • A secure and generic AAKE-R construction using AAKE and AKT as building blocks • It satisfies the security requirements of AAKE-R suggested by the authors

23. Weak points • User privacy violation – The home server can track roaming user • They do not suggest detailed performance evaluation. I think the overhead is big due to several asymmetric computation

24. Future work • Addressing user tracking problem by home server • Study of additional requirements such as supporting differentiated access • Try to find a way to reduce the number of asymmetric computation modifying AAKE-R or design novel AAKE-R that haslower computation overhead even though it satisfies same requirements