Further improvement on the modified authenticated key agreement scheme

Abstract

In 2003, Hsu et al. showed that the Ku-Wang modified authentication key agreement scheme is vulnerable to the modification attack and further proposed an improvement of the Ku-Wang scheme. However, we find that the Hsu et al. scheme still suffers from the same modification attack. This paper will first show the security flaw in the Hsu et al. scheme, and then propose an improved scheme to repair the security flaw.

Introduction

In 1976, Diffie and Hellman [1] first proposed an asymmetric key agreement scheme to solve the key distribution problem in an open network. However, the Diffie–Hellman scheme is obviously vulnerable to the man-in-middle attack. It is because that the communicating parties have no way to verify each other. In order to defeat the attack, Seo and Sweeney [2], in 1999, proposed an authenticated key agreement scheme, called SAKA, which uses a pre-shared password to provide user authentication. However, in 2000, Tseng [3] showed that the SAKA cannot resist the replay attack in which an attacker can cheat the honest party into believing a wrong session key. Tseng also proposed an improved scheme to prevent the replay attack.

In 2000, Ku and Wang [4] pointed out that the Tseng scheme suffers from two kinds of attacks: the backward replay attack without modification and the modification attack. In the first attack, an attacker can masquerade as one communicating party and replay the exchanged messages to cheat the other one. In the second attack, an attacker can alter the exchanged messages to cheat one party into believing a wrong session key. Ku and Wang [4] also proposed a modified authenticated key agreement scheme to defeat these two attacks. Unfortunately, in 2003, Hsu et al. [5] showed that the Ku–Wang scheme is still vulnerable to the modification attack and gave an improvement to enhance the security of the Ku–Wang scheme. Moreover, the Hsu et al. scheme is more efficient than the previous schemes [2], [3], [4].

However, we find that the Hsu et al. scheme is still vulnerable to the modification attack. An attacker can cheat the participants to accept a wrong session key by altering the exchanged messages. This paper will first review the Hsu et al. scheme, and then show the security flaw in the Hsu et al. scheme in the Section 2. Then, we will further propose an improved scheme and analyze the security in the Section 3. Finally, conclusions will be given in the Section 4.