離線電子現金系統的安全分析之研究

A Research on Security Analyses of Offline Electronic Cash Systems

劉曜慶(Yao-Ching Liu)

指導教授 ： 張雅芬

國立臺中科技大學/資訊與流通學院/資訊工程系碩士班/碩士(2014年)

https://doi.org/10.6826/NUTC.2014.00043

摘要

電子現金在現代電子商務中扮演著重要的角色。透過發展成熟的網際網路技術，電子現金提供人們一個具備便利性、安全性、身分隱匿性和不可偽造性的支付工具。其中，藉由離線電子現金系統，消費者與商家進行交易不需要銀行共同參與便可驗證電子現金的正確性。此優異的特性使得離線電子現金系統成為重要的研究主題，因其能有效地降低銀行方運算資源的需求、增加整體系統的效率，並進一步提高電子現金系統的實用價值。近年來，離線電子現金系統與其驗證機制相關研究陸續被提出，但在這些研究的驗證架構中，商家與銀行進行驗證時才可進行現金重複消費的檢測，因而導致離線電子現金系統可能會面對更高的安全性風險。
在2013年，Mohanty等學者提出一個不需要憑證的群簽密法，該方法結合簽章與加密，並以該簽密法為基礎提出一個離線電子現金系統，他們宣稱藉由群簽密法可確保使用者的匿名性，令群管理人在特定的情況下可以揭示電子現金的擁有者以防止電子現金遭到非法濫用。然而，我們發現Mohanty等學者的離線電子現金方法無法抵禦偽造攻擊及重複消費。在本研究中，我們將會在第二章節詳細說明在Mohanty等學者的離線電子現金系統中惡意的使用者如何偽冒其他顧客向銀行購買電子現金、如何偽造可通過系統驗證的電子現金和惡意的商家可重複消費顧客的電子現金等缺失。
同年，Baseri等學者以RSA加密演算法為基礎提出一個離線電子現金系統(e-cash)，並且宣稱該系統具有使用者匿名性、重複消費檢測、不可偽造性、日期附加性以及防止偽造。但是經過研究後，我們發現該離線電子現金系統仍然無法抵禦使用者身份偽造攻擊。在第三章節中，我們將介紹惡意攻擊者如何分別利用三種不同的方式，來偽造一個有效的假身份向銀行購買電子現金，並且使用假身份與商家進行交易。當惡意攻擊者重複消費電子現金時，銀行無法正確追蹤出攻擊者的真正身份。本研究將詳細分析所發現之安全缺失並解釋這些漏洞將會如何危害離線電子現金系統。

關鍵字

電子現金 ； 離線電子現金系統 ； RSA加密演算法 ； 匿名性 ； 偽造 ； 身份偽造 ； 重複消費

並列摘要

Electronic cash (e-cash) plays an important role in electronic commerce. With the rapid growth of network technologies, e-cash provides a way to pay with convenience, security, anonymity and unforgeability. Offline electronic cash systems possess a superior property because the bank is not involved when a merchant verifies a customer’s e-cash. This makes the computational resource needed by the bank reduced, the overall system efficiency increased, electronic cash systems practical, and offline electronic cash systems become an important research topic. Recently, several corresponding studies have been proposed. However, double spending only can be detected when a merchant contacts the bank. This may make offline electronic cash systems be threatened seriously.
In 2013, Mohanty et al. proposed a certificateless group signcryption scheme and presented an offline e-cash system based on their signcryption scheme. They claimed that their scheme could protect the anonymity of customers better. In addition, the group manager has the ability to reveal the identity of the e-cash’s owner for some special situation to prevent the problem of e-cash abuse. However, we find that Mohanty et al.’s e-cash system is vulnerable to forgery attack and double spending. In this study, we will review Mohanty et al.’s offline electronic cash system and show how a dishonest customer can apply for a coin with any customer’s identity, how to forge a valid e-cash, and how a malicious merchant spends an honest customer’s coin twice in Chapter 2.
In the same year, Baseri et al. proposed an offline electronic cash (e-cash) system using RSA cryptosystem and asserted that their system satisfied anonymity, double spending detection, unforgeability, and date attachability properties and prevented forging . However, we find that their e-cash system is insecure against identity forgery. In Chapter 3, we show that an adversary has three ways to forge a valid identity to withdraw electronic coin (e-coin) from his account at the bank and pay it to the merchant in payment phase. When double spending is detected, the bank cannot reveal the attacker’s real identity. The found security flaws and system vulnerability will be shown in detail, and what damage it causes to the e-cash system will be explained.

並列關鍵字

electronic cash ； offline e-cash system ； RSA cryptosystem ； anonymity ； forgery ； identity forgery ； double spending

參考文獻

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