一個有效率的電子郵件協定-基於橢圓曲線密碼系統

An Efficient E-mail Protocol Using Elliptic Curve Cryptography

徐尚堉(Shang-Yu Hsu)

指導教授 ： 廖冠捷

亞洲大學/資訊學院/資訊工程學系碩士班/碩士(2010年)

摘要

在各式各樣的網路服務中又屬電子郵件(e-mail)應用最為普遍，它幾乎已成為每個人生活中的一部分。在 2006年，Kim 等學者提出了兩個具備perfect forward secrecy的電子郵件協定。但可惜的是，Yoon及Yoo兩位學者於2007年指出Kim等人的方法並無法抵抗仿冒攻擊的威脅，即惡意的攻擊者有能力在郵件寄送階段成功假冒寄件者或郵件伺服器等身分。
然而，面對仿冒攻擊的威脅衝擊，Yoon與Yoo兩位學者並未提出有效的解決辦法。為了改進這項缺點，本論文第一部分首先提出了一個安全的郵件協定，不僅成功地解決可能的仿冒攻擊威脅，更進一步在方法中增加了時間戳記服務。如此一來，無論是傳送方或接收方皆能精確地掌握電子郵件傳送時間，因此能有效避免郵件遭到重送或竄改等攻擊。
此外，隨著資通科技的快速發展，由於行動通訊設備的普及。許多主要的電子郵件服務廠商(如：Gmail與Yahoo等)紛紛開始提供行動電子郵件服務，讓使用者可以隨時隨地透過手機收發e-mail。然而，考慮到多數行動通訊設備之整體資源受到限制，無法支援較複雜的運算工作(如：公開金鑰基礎建設相關運算)。再加上許多網路應用程式亦逐漸傾向在行動設備上以統一的瀏覽器介面執行。由此可見，針對行動商務相關應用設計一套安全且便利的電子郵件系統是絕對必要的。有鑑於此，本論文的第二項目的則是植基於橢圓曲線密碼系統設計一套更有效率的電子郵件系統以適用於資源有限的行動設備。
在安全性方面，本論文所提出的兩套電子郵件系統不但皆具備Perfect forward secrecy之特性，更能有效抵抗多種已知的攻擊方法。此外亦保有「不可否認性」、「訊息機密性」、「訊息完整性」及「身分驗證性」等四項電子件之基本安全需求。該研究結果不僅將會大幅提升行動電子郵件服務的功能面及應用面，亦可以做為其它密碼學研究及應用之基礎。

關鍵字

電子郵件 ； 橢圓曲線密碼學 ； Perfect forward secrecy

並列摘要

Electronic mail (e-mail) is one of the most important and widely used network applications. It has been used in communications between individuals, business organizations and governmental agencies around the world. In 2006, Kim et al. proposed two e-mail protocols with perfect forward secrecy. Unfortunately, Yoon and Yoo recently demonstrated that Kim et al.’s second e-mail protocol is insecure under two impersonation attacks, where an attacker can easily impersonate a legal sender in order to send a forged e-mail to a recipient, or and attacker can easily impersonate a legal e-mail server to get all e-mail content sent from senders to recipients.
However, Yoon and Yoo didn’t propose the improved version for the impersonation attacks. For this reason, the first goal of the thesis is to propose a secure e-mail protocol, which not only can remedy the weakness came from the impersonation attacks, but also can provide the additional functionality by attaching the time-stamp issued by the Time-Stamping Service (TSS). Hence, some time-sensitive e-mail content, typically for e-commerce, intellectual property protection, and records integrity, all benefit from the time-stamping service. In additions, some possible attacks, i.e. delay or replay, can also be eliminated.
With the rapid development of communication technologies, mobile communication environments are pervasive. Major e-mail providers, such as Gmail, Yahoo, currently provide mobile e-mail services. However, mobile devices are not equipped with sufficient resources to handle the expensive computations required for exploiting PKI. Furthermore, there has been growing interest in executing web-applications on resource-constrained mobile devices via browser-based interface. Therefore, it is necessary to design an efficient and secure e-mail system for mobile applications. According to the point, the second goal of the thesis is to design an efficient e-mail protocol with elliptic curve cryptography (ECC).
From the viewpoint of the security, these two e-mail systems proposed in this thesis not only can provide perfect forward secrecy but also can withstand variety of well-known attacks. Besides, some basic security requirements, such as non-repudiation, confidentiality, integrity, and authentication, can also be guaranteed. In this way, this model can be put on many deployed popular mobile e-mail services directly. Thus, we thought that this project is important and ought to be examined.

並列關鍵字

Electronic mail (e-mail) ； elliptic curve cryptography (ECC) ； Perfect forward secrecy

參考文獻

[1] C. Adms, P. Cain, D. Pinkas, R. Zuccherato, “Internet X.509, public key infrastructure time-stamp protocol (TSP),” RFC, 3161, 2001.

[2] G. Ateniese and C. Nita-Rotaru, “Stateless-recipient certified e-mail system based on verifiable encryption,” Proceedings of the Topics in Cryptology, The Cryptographers’ Track at the RSA Conference 2002, Vol. 2271 of Lecture Notes in Computer Science, 2002, pp. 182-199.

[3] A. Bacard, The Computer Privacy Handbook: A Practical guide to E-Mail Encryption, Data Protection, and PGP Privacy Software, Peachpit Press, 1995.

[4] A. Bahreman and J. D. Tygar, “Certified dlectronic mail, Proceedings of Symposium on Network and Distributed Systems Security,” 1994, pp. 3-19.

[5] F. Bao, R. H. Deng, and W. Mao, “Efficient and Practical Fair Exchange Protocols with Off-line TTP,” IEEE Symposium on Security and Privacy Proceedings, pp. 77-85, 1998.