基於授權的可攜式通訊系統認證協定提供有效率的離線認證、資料機密性、使用者隱私保護與不可否認性。然而在現有的認證協定中，使用者的不可否認性皆建構在一個不合理之假設，即主網域為完全可信任的。為了移除此不合理的假設，並強化使用者的不可否認性，本論文提出一套新的基於授權之認證協定。除此之外，在離線認證中，新協定也移除漫遊網域的疲憊搜尋問題與增加離線時的使用者不可鍊結性，藉此改善認證效率與強化使用者身分隱私保護。此外本論文也提出偽造攻擊，用以指出在Wang等學者的認證協定中，漫遊網域可偽造使用者的漫遊資訊，即使使用者並未使用任何漫遊服務的情形下，向主網域索取額外的使用者通訊費用。

For portable communication systems, the delegation-based authentication protocol provides efficient subsequent login authentication, data confidentiality, User privacy protection, and non-repudiation.  However, in all proposed protocols, the non-repudiation of mobile stations is based on an unreasonable assumption that home location registers are trusted.  To remove this assumption and enhance the non-repudiation of the mobile stations, a new delegation-based authentication protocol is proposed.  The new protocol also removes the exhaustive search problem in the subsequent login authentication to improve the subsequent login authentication performance.  Moreover, the User unlinkability in the subsequent login authentication is also provided in the new protocol to enhance the User identity privacy protection.  Besides, a forgery attack is proposed to show that, in Wang et al. delegation-based authentication protocol, the mobile station cannot deny the malicious visit location registers’ overcharge, even though the mobile station obtains no services.

Table of Content
Chapter 1 Introduction	1
1.1 Our Contribution	3
Chapter 2 Review	6
2.1	Hwang and Sung Confidential Deniable Authentication Protocol	6
2.2	Underlying Security Assumptions	7
Chapter 3 Our Delegation-Based Authentication Protocol for Portable Communication System with Unlinkability and User's Non-repudiation	8
3.1	The proposed protocol	8
3.2	Security Analysis and Proofs	14
3.3	Comparison and Discussions	31
Chapter 4 Our Comment on Wang et al. Delegation-Based Authentication Protocol	35
4.1	Review of Wang et al. Protocol	35
4.2	Forgery Attack on Wang et al. Protocol	37
Chapter 5 Conclusions	39
References	40
Appendix A	42

List of Tables
Table 1: Security Property Comparison among Youn and Lim, Chen et al., and Our Protocols	34

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