1. 1 Security Weakness in a Three-Party Password-Based Key Exchange Protocol Using Weil Pairing From ： ePrint (August 2005) Author ： Junghyun Nam, Seungjoo Kim, Dongho Won. Presented by Shih Yu Chen

2. 2 Outline Introduction Review of the Wen-Lee-Hwang Protocol Definitions and Notations Protocol Description Attack on the Wen-Lee-Hwang Protocol Conclusion

3. 3 Introduction  Wen-Lee-Hwang proposed protocol *Weil pairing. *Provable secure in the random oracle model.  But, it is not secure under active attack.  The security proof of the protocol was flawed.

4. 4 Review of the Wen-Lee-Hwang Protocol(1/6) Definitions

5. 5 Review of the Wen-Lee-Hwang Protocol(2/6) Notations S ： authentication server A 、 B ： users ID S 、 ID A 、 ID B ： the identities of S 、 A 、 B P ： generator of G 1 s ： secret key of S P s ： public key of S and P s =sP PW A 、 PW B ： password of A and B share with S G ： map-to-point function H ： one way hash function

6. 6 Review of the Wen-Lee-Hwang Protocol(3/6) Protocol Description

7. 7 Review of the Wen-Lee-Hwang Protocol(4/6)

8. 8 Review of the Wen-Lee-Hwang Protocol(5/6)

9. 9 Review of the Wen-Lee-Hwang Protocol(6/6) =ê(bP, aU)

10. 10 Attack on the Wen-Lee-Hwang Protocol(1/6) Attacker

11. 11 Attack on the Wen-Lee-Hwang Protocol(2/6)

12. 12 Attack on the Wen-Lee-Hwang Protocol(3/6)

13. 13 Attack on the Wen-Lee-Hwang Protocol(4/6)

14. 14 Attack on the Wen-Lee-Hwang Protocol(5/6) mP,  m,  m,  a bP,  b,  b,  m

15. 15 Attack on the Wen-Lee-Hwang Protocol(6/6)  a,  a --------------------------   m,  m =H(ID A ||K)

16. 16 Conclusion Through the attack, the authentication mechanism of the protocol is completely compromised. The effect of author’s attack is the same as that of a man-in-the-middle attack.