中文摘要
現今，隨著通訊技術的快速發展，許多廣受歡迎的服務皆植基於這些先進技術的優勢被開發出來，其中一個受歡迎的服務就是無線通訊。無所不在的無線網路允許分散在各處的行動使用者們無論在任何時間和地點都能有效率地互相通訊，即使是在移動的狀態之下也能進行通訊。此外，小型且精密的行動電話也大幅提升了行動裝置的可攜性。基於快速移動及高可攜性的特色，使得無線通訊在個人通訊的活動裡扮演著極為重要的角色。
在過去的文獻裡，有許多關於行動通訊安全的協定被提出。在這些協定當中，驗證是非常重要的項目，因為它是抵禦攻擊者的第一道防線，確保每個成員在通訊過程的正確性。因此，一些高效率的安全協定也被相繼提出。然而，這些協定並不適合目前行動通訊系統的環境。而且，部分的協定存在相同的缺點，例如行動使用者與系統之間通訊時的高運算量及高傳輸成本。為考量行動使用者僅具備低運算及低通訊能力的條件下，其認證協定需要更進一步改善。
在本論文中，我們植基於Hwang和Chang的協定提出了一個改進方法，解決VLRs與HLR進行認證時，需要時間同步的基本假設。另外，本改進方法不僅達到雙向認證，相較於現存的協定，例如Hwang和Chang的協定[2],及Fan等人的協定[1]，我們的方法也大幅減少行動使用者的運算量及通訊成本。
關鍵字: 認證，全球行動通訊系統，行動通訊，安全性

Abstract
Abstract- Nowadays, according to the fast progress of communication technologies, there are so many services have been developed in literature to take the advantages of these communication technologies. Among these services, wireless communication is the most important one. Wireless communication is playing an important role in human communication activities by holding the features of mobility and usability. There were many security mechanisms for mobile communications that had been proposed in the past. In these mechanisms, the authentication is very important task in mobile network system. It performs the first defense to prevent attackers before any other communication activities to make sure that all the entities are correctness. Some proposed protocols have been introduced in the literature, which are quite efficient. However they are based on the techniques which are not suitable in current mobile communication system. On the other hand, some schemes have the same drawback such as high bandwidth between mobile user and the system. Therefore, in order to satisfy the problem that the mobile users have low computation and communication capability, authentication schemes need more upgrading.
In this paper, an improvement of Hwang and Chang’s protocol is proposed to resolve some drawbacks: the authentication must be under the assumption of clock synchronization between visited location register (VLRs) and home location register (HLR). Moreover, the improved protocol not only does the proposed solution achieve mutual authentication, but also greatly reduces the computation and communication cost of the mobile users as compared with the existing authentication schemes, for instance, Hwang and Chang’s protocol [2], Fan et al.’s protocol [1].

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