1 Security analysis of an enhanced authentication key exchange protocol Authors ： H.Y. Liu, G.B. Horng, F.Y. Hung Presented by F.Y. Hung Date ： 2005/5/20

2 Outline  Introduction  Review of Hwang et al.’s protocol  Attack  Conclusions

3 Introduction (1)  Diffie-Hellman key agreement protocol  Man in middle attack  Authentication key exchange protocol  Perfect forward secrecy

4 Introduction (2) 2004 Hwang-Chang- Lin-Tsai 2005 This paper 1998 Harn and Lin Based on Diffie-Hellman 2001 Harn and Lin Multiple common session key 2003 Hwang, Shiau and Lai Using XOR Perfect forward secrecy High computation cost Forgery signature attack

5 Review of Hwang et al.’s protocol  Notations  Step 1 -- Alice computes  Step 2 -- Bob verifies Alice and computes  Step 3 -- Alice verifies Bob  Multiple common session keys

6 Notations p ： a large prime chosen by system g ： a primitive root in GF(p) ： long-term private key chosen by Alice and Bob respectively, and associative public keys and certificates are

7 Step 1 AliceBob

8 Step 2 AliceBob

9 Step 3 Alice

10 Multiple common session keys

11 Attack Eve does the following steps to impersonate Alice and agree session key with Bob Step 1 Step 2 Step 3

12 Step 1  The adversary Eve randomly chooses t and computes

13 Step 2  When Bob receives the message, he verifies the message by the following equation ：

14 Step 3  The adversary can compute two of the session key Computed by Eve

15 Conclusions  In this paper, we show that Hwang- Chang-Lin-Tsai protocol is vulnerable to forgery signature attack.  To find an efficient and secure authentication key exchange protocol deserves future work.

16 Thanks!