傳統的數位簽章可以讓簽章者對於文件簽名，而公佈給驗證者來驗證此簽章是否為原簽章者所簽名過。不可否認簽章是一種非自身可驗證的數位簽章，不可否認簽章的特性在於只能在原簽章者的同意下才能進行驗證；另一方面，原簽章者將不能否認之前所簽過的不可否認簽章，假使原簽章者想要否認將會被察覺。在可轉換的特性，如果簽章者洩露一些秘密資訊，不可否認簽章將轉換成一般任何人都可以自身驗證的普通簽章。不可否認簽章加上可轉換的特性是非常吸引人的原因是因為簽章者可以隨時將不再重要的不可否認簽章，也就是不需要再經由簽章者同意才能驗證的不可否認簽章，讓每一個人都可以自身驗證。許多被提出的方法嘗試將可轉換的特性加入一個已知的不可否認簽章方法中，使得不可否認簽章能轉換成普通簽章；或者是提出一個全新的方法結構。然而，有許多傳統數位簽章方法結構已被提出且廣泛被應用，因此，在本篇論文中我們嘗試設計一個可雙向變換的模型，利用一個已存在且基於離散對數的普通簽章結構來架構我們的模型，使得任何基於離散對數的普通簽章都可以被變換成可轉換的不可否認簽章。

The concept of traditional digital signature let signer sign a message with his/her private key and the verifier could use signer’s public key to verify the validity of the signature whether was signed by the signer. Undeniable signature is a non-self-certified signature. It is only verified by the consent of the original signer. On the other hand, the original signer can’t deny the undeniable signature what he signed. It would be detected if the signer wants to deny. For the convertible property, the undeniable signature can be verified by anyone if some secrets are released by the original signer. The undeniable signature with convertible property is very attracted because the signer can decide which unimportant undeniable signature, which needn’t the original signer’s consent, could be verified by anyone. There are many proposed schemes try to introduce the convertible property into a known undeniable signature or design a complete new undeniable signature scheme. However, there are many excellent traditional signature were proposed and popularly applied. So in this paper, we try to design a dual transformation model. Using an existed discrete logarithm based signature scheme to construct our model, so that any discrete logarithm based signature can be transformed into a convertible-undeniable signature.

誌謝
中文摘要
Abstract
Table of Contents
Chapter 1 Introduction
Chapter 2 Preliminary
Definition 1：(Ordinary signature)
Definition 2：(Undeniable signature)
Definition 3：(TF function)
Definition 4：(CF function)
Chapter 3 Our model
3.1 TF function
3.2 Convertible undeniable signature generation
3.3 CF function
3.4 Ordinary signature verification
Chapter 4 Security Analysis
4.1 TF function security
4.2 Requirement of transformation
4.3 Unforgeability
Chapter 5 Discussion
5.1 Generalization capability
5.2 Comparison
Chapter 6 Example
6.1 TF function
6.2 Convertible undeniable signature generation
6.3 CF function and verification
Chapter 7 Conclusion
References

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