Due to ubiquity of internet, the development of electronic commerce is growing up rapidly. Many payment mechanisms of electronic transactions are proposed, such as electronic cash, credit cards, and electronic wallets. Electronic cash (e-cash) is like the digital version of paper cash, which possesses some key features such as anonymity and untraceability, and it can be used on the Internet for various electronic transactions. In general, electronic cash is classified into two types which are on-line electronic cash and off-line electronic cash. Each of them is suitable for different applications and environments. All of the proposed electronic cash schemes only focus on one of two types. In these schemes, users must decide which type of electronic cash they will use later before withdrawing. However, in this thesis we propose a novel electronic cash scheme which supports a user to withdraw a generic e-cash and then decide to spend it as an on-line e-cash or an off-line e-cash. Our proposed scheme is more convenient for users and more flexible for the bank and shops. It not only satisfies the necessary properties of electronic cash but also provides flexibility.

Due to resource-saving and efficiency consideration, electronic voting (e-voting) gradually replaces traditional paper-based voting in some developed countries. An anonymous e-voting system that can be used in elections with large electorates must meet various security requirements, such as anonymity, uncoercibility, tally correctness, unrecastability, verifiability, and so on. Especially, the uncoercibility property is an essential property which can greatly reduce the possibility of coercion and bribe. Since each voter can obtain one and only one voting receipt in an electronic voting system, coercers or bribers can enforce legal voters to show their voting receipts to identify whether the enforced voters follow their will or not. It turns out that the coercion and bribe will succeed more easily in digital environments than that in traditional paper-based voting. In this dissertation, we analyze four possible scenarios leading to coercion and discover that the randomization property is necessary to blind-signature-based e-voting systems against coercion. Based on this result, we extend our research and come up with two provably secure randomized blind signature schemes from different cryptographic primitives, which can be adopted as key techniques for an electronic voting system against coercion and bribery.

Due to the growing interest in electronic commerce, more and more transactions now happen online. Thus, fair transactions between customers and merchants are getting important. To gain the fairness of the exchange of digital items, fair exchange protocols have been proposed and well studied. Most of the traditional fair exchange protocols are concerned about the exchange of digital items, such as digital signatures, contracts, and documents. Recently, researchers pay attention to the exchange of digital goods along with electronic cash, and have proposed some fair transaction protocols based on electronic cash. To buy digital contents via electronic cash through network, the anonymity property as traditional cash possesses must be guaranteed. It means that the payment information of the customer cannot be revealed to anyone else including the trusted third party (TTP) who helps the customer and the merchant with resolving possible disputes in the protocol. Since the customer and the merchant may not trust each other in an electronic transaction. In a fair exchange protocol, a TTP is employed to achieve true fairness. An on-line TTP has to take part in all transactions while they are proceeding. Despite it can gain true fairness, it is inefficient due to on-line interaction with the TTP. On the other hand, an off-line TTP does not need to join in the transaction protocol in normal cases. Instead, it participates in the protocol only when disputes happen. It is efficient and fair, and more feasible in practical situations. In this thesis, we propose a fair transaction protocol based on electronic cash. With the extended research on electronic cash, we have designed a fair transaction protocol that is suitable for any electronic cash system. By using an off-line TTP, the protocol is more efficient and practical. Furthermore, payment information of the customer cannot be known to anyone else including the TTP, and thus, the anonymity of the customer is protected completely in our protocol.

The technology of electronic cash makes it possible to transmit digital money over communication networks during electronic transactions. Owing to the untraceability and unforgeability properties, electronic cash can protect the privacy of customers and guarantee the security of payments in the transactions. This manuscript introduces an efficient electronic cash protocol where it only requires minimal storage for each customer to withdraw w dollars from the bank and spend the w dollars in a sequence of transactions. Compared with traditional electronic cash protocols, the proposed method greatly reduces not only the storage required for the customers but the communication traffic in the transactions as well. Furthermore, the computation cost of the entire protocol is lower than the traditional ones and it also achieves the customer efficiency property. It turns out that the proposed protocol is much more suitable for the storage-limited or hardware-limited environments, such as smart card computing or mobile commerce, than the traditional electronic cash protocols in a sequence of payments. In addition, we examine the security of the proposed electronic cash protocol from the customer¡¦s, the shop¡¦s, and the bank¡¦s points of view, respectively. Since the proposed protocol is based on a generic partially blind signature scheme, it can be implemented by any partially blind signature scheme as long as it is secure and user efficient.

京都大学 / 0048 / 新制・課程博士 / 博士(情報学) / 甲第20511号 / 情博第639号 / 新制||情||111(附属図書館) / 京都大学大学院情報学研究科社会情報学専攻 / (主査)教授 石田 亨, 教授 中村 佳正, 教授 岡部 寿男, 岡本 龍明 / 学位規則第4条第1項該当 / Doctor of Informatics / Kyoto University / DFAM

Sistemas de Gerenciamento de Identidade Centrados no Usuário têm sido utilizados na Internet como meio de evitar o gerenciamento de múltiplas contas em sites e serviços na Web. Embora o uso de tais sistemas apresente benefícios, usuários podem ter sua privacidade prejudicada, uma vez que suas identidades tendem a ser conhecidas e controladas por uma entidade central. Dessa maneira, os acessos a serviços e o comportamento dos usuários tendem a ser facilmente rastreáveis em toda a rede. Por outro lado, do ponto de vista dos serviços, existem casos onde o conhecimento e a comprovação de informações do usuário é uma necessidade para o controle de acesso e provimento do serviço. Assim, o objetivo deste trabalho é propor uma solução de gerenciamento de identidades que proteja a privacidade e, ao mesmo tempo, possibilite a comprovação de atributos de identidade do usuário para um provedor de serviços no ambiente Web atual. Esta proposta inova dentre os trabalhos relacionados encontrados na literatura devido à sua adequação às necessidades e limitações existentes no ambiente típico da interação entre usuários e sites na Internet. A verificação do cumprimento dos objetivos de autenticação de atributos de identidade e privacidade do usuário é realizada por meio da análise formal do protocolo da solução. Ainda, com a aplicação de uma métrica, são analisados as condições e níveis de anonimato de um usuário no uso do sistema. / User-centric Identity Management Systems have been used on the Internet for avoiding the management of multiple users accounts in different sites and services on the Web. Although those systems can bring some benefits for its users, their privacy may be jeopardized since their identities are likely to be known and controlled by a central entity. This way, users behavior and their accesses to services are likely to be easily tracked on the network. On the other side, from the services point of view, there are occasions where the knowledge and verification of some users aspects and attributes are necessary for access control and service providing. Thereby, the goal of this work is to propose a solution for identity management that provides enhanced privacy for user and, at the same time, allows them to prove attributes of their identity to a service provider on the current Web environment on the Internet. This proposal innovates when compared to related works due to its suitability to the environment and its interactions between clients and sites on the Internet. The objectives related to the verification of identitys attributes and privacy concerns in this proposal are analyzed by formal methods. This work also presents an analysis on the conditions and levels of anonymity when users interact with the system based on a metric.

Providing a security proof is always an important issue in the process of designing a cryptographic scheme or protocol. We often show the security of a cryptosystem via ¡§problem reduction.¡¨ In this thesis, lots of emphasis was put on the review of techniques for proving the security of cryptosystems. These techniques consist of Random Oracle Model and Forking Lemma. We also introduced some well-known cryptographic schemes which can be proved secure using these techniques. Then we offered a security proof of a blind signature scheme based on the one proposed by Fan. In the end, we made a comparison between our proof and the proof of another blind signature scheme provided by David Pointcheval and Jacques Stern. Some arguments and discussions about using the Random Oracle Model to prove the security of a cryptosystem were also included.

Due to the mature of networks and communication technologies, electronic commerce is growing up rapidly and many advanced applications in electronic commerce are developed recently, such as on-line shopping, on-line bidding, and on-line gambling. There are numerous types of gambling like typical lottery, sport lottery, and poker gambling where our research will focus on the lottery games. Because of some special characteristics of the lottery games such as fairness and anonymity, it is hard to design a perfect electronic lottery protocol. The transaction mechanism in an electronic lottery protocol is an important issue since it will affect the benefits of customers if it is not fair or secure. The generating of random winning numbers in a lottery game has been discussed in many papers, but the fairness and anonymity for purchasing tickets and claiming the prizes are only discussed in few papers where these previous results cannot completely cope with the problems of fairness and anonymity. In the thesis, we will propose two electronic lottery protocols that can achieve the fairness and perfect anonymity simultaneously.

Due to the flush development of academic research, a great deal research results have been published in conference proceedings and journals. However, these articles need to be inspected by some professionals in specific fields. It is the most important that fairness must be guaranteed during the entire process of reviewing. Nevertheless, the privacy of reviewers may be leaked out because that the reviewers must sign their comments on the reviewed papers. The leakage of the reviewers' privacy may affect the judgement of the reviewers on the papers. In addition, the authors of a paper have to show their names to the editor of a conference proceedings or a journal when submitting the paper, so that it may also affect the decision of the editor on this paper. The major reason of the above problems is that the privacy or anonymity of the reviewers and the authors is not protected well, such that the reviewers and the editor cannot perform the reviewing processes without disturbance. In order to cope with the problems, we deeply analyze the privacy issue in the paper review system and then propose a generic idea, which is independent of the underlying cryptographic components, to achieve the anonymity property and other key requirements in a secure paper review scheme.

Sistemas de Gerenciamento de Identidade Centrados no Usuário têm sido utilizados na Internet como meio de evitar o gerenciamento de múltiplas contas em sites e serviços na Web. Embora o uso de tais sistemas apresente benefícios, usuários podem ter sua privacidade prejudicada, uma vez que suas identidades tendem a ser conhecidas e controladas por uma entidade central. Dessa maneira, os acessos a serviços e o comportamento dos usuários tendem a ser facilmente rastreáveis em toda a rede. Por outro lado, do ponto de vista dos serviços, existem casos onde o conhecimento e a comprovação de informações do usuário é uma necessidade para o controle de acesso e provimento do serviço. Assim, o objetivo deste trabalho é propor uma solução de gerenciamento de identidades que proteja a privacidade e, ao mesmo tempo, possibilite a comprovação de atributos de identidade do usuário para um provedor de serviços no ambiente Web atual. Esta proposta inova dentre os trabalhos relacionados encontrados na literatura devido à sua adequação às necessidades e limitações existentes no ambiente típico da interação entre usuários e sites na Internet. A verificação do cumprimento dos objetivos de autenticação de atributos de identidade e privacidade do usuário é realizada por meio da análise formal do protocolo da solução. Ainda, com a aplicação de uma métrica, são analisados as condições e níveis de anonimato de um usuário no uso do sistema. / User-centric Identity Management Systems have been used on the Internet for avoiding the management of multiple users accounts in different sites and services on the Web. Although those systems can bring some benefits for its users, their privacy may be jeopardized since their identities are likely to be known and controlled by a central entity. This way, users behavior and their accesses to services are likely to be easily tracked on the network. On the other side, from the services point of view, there are occasions where the knowledge and verification of some users aspects and attributes are necessary for access control and service providing. Thereby, the goal of this work is to propose a solution for identity management that provides enhanced privacy for user and, at the same time, allows them to prove attributes of their identity to a service provider on the current Web environment on the Internet. This proposal innovates when compared to related works due to its suitability to the environment and its interactions between clients and sites on the Internet. The objectives related to the verification of identitys attributes and privacy concerns in this proposal are analyzed by formal methods. This work also presents an analysis on the conditions and levels of anonymity when users interact with the system based on a metric.