In modern era, remote user can access the multiple-services from anywhere in the world at any time through Internet. So, to provide the legitimacy among the users, various remote user authentication schemes have been designed. Recently, Chuang and Chang has proposed a scheme under multi-server architecture based on three security factors namely, smart card, password and biometric and claimed that, their scheme can resist several kind of attacks and can be successful to provide more security properties than that of existing schemes. In this paper, we have reviewed their schemes and proved that Chuang and Chang's scheme cannot resist server spoofing or user impersonate attack, password guessing attack and also fails to achieve forward key secrecy. To overcome their weaknesses and fulfill such important security requirements, we have proposed an improved remote user authentication scheme under multi-server environment.

Rapid development of computer networks laid a trend to share the information worldwide. However, one of the important criterions is to provide the confidentiality to the shared data over the insecure communication channel. Due to the growth of computer technology; assuring the security of such shared information becomes challenging and complicated task among the researchers. In this consequence, the dynamic key approach is most desirable as compared to static key. In this respect, the AVK is one of the best techniques to achieve the perfect security as per the literature. In this paper, we have proposed a dynamic mechanism of AVK to enhance the security by increasing the randomness among the successive keys.