In 2009, Xu et al. found that Lee et al.'s [3] scheme is vulnerable to offline password guessing attack. Xu et al. also demonstrated that Lee and Chiu's [4] scheme is vulnerable to forgery attack. Furthermore, Lee and Chiu's scheme does not achieve mutual authentication and thus can not resist malicious server attack. Therefore, Xu et al. proposed an improved scheme that inherits the merits of Lee et al.'s and Lee and Chiu's schemes and resists different possible attacks. However, we found that Xu et al.'s scheme is vulnerable to forgery attack. This paper presents an improved scheme to resolve the aforementioned problem, while keeping the merits of Xu et al.'s scheme.
Today's real-time systems (RTS) are characterized by managing large volumes of dispersed data making real-time distributed data processing a reality. Large business houses need to do distributed processing for many reasons, and they often must do it in order to stay competitive. So, efficient database management algorithms and protocols for accessing and manipulating data are required to satisfy timing constraints of supported applications. Therefore, new research in distributed real-time database systems (DRTDBS) is needed to investigate possible ways of applying database systems technology to real-time systems. This paper first discusses the performance issues that are important to DRTDBS, and then surveys the research that has been done so far on the issues like priority assignment policy, commit protocols and optimizing the use of memory in non-replicated/replicated environment pertaining to distributed real time transaction processing. In fact, this study provides a foundation for addressing performance issues important for the management of very large real time data and pointer to other publications in journals and conference proceedings for further investigation of unanswered research questions.
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