Digital certificates, based on X.509 PKI standard, are located at the core of many security mechanisms implemented in services and applications. However, the usage of certificates has revealed flaws in the certificate validation process (e.g., possibility of unavailable or non-updated data). This fact implies security risks that are not assessed. In order to address these issues that such flaws entail, we propose a novel probabilistic approach for quantitative risk assessment in X.509 PKI, together with trust management when there is uncertainty. We have evaluated our risk assessment approach and demonstrated its usage, considering as a use case the secure installation of mobile applications. The results show that our approach provides more granularity, appropriate values according to the impact, and relevant information in the risk calculation than other approaches.

Electronic multicoupons are the digital version of paper-based booklet of coupons, which allow a customer to obtain goods or services from a merchant typically with discounts or gifts. In this paper, we propose a multicoupon scheme for multi-merchant environments that enhances the security and the efficiency as regards the previous solution that deals with this kind of scenario. On one hand, our scheme meets basic security requirements (unforgeability, reuse detection), but also unsplittability. This fact will help to increase the trust on the part of merchants. On the other hand, we focus on achieving a high degree of privacy for customers (anonymity, unlinkability, confidentiality) that will contribute to gaining the trust of the involved customers. Moreover, we provide measures to protect honest parties from dishonest participants, keeping in mind the scalability and efficiency of the multicoupon scheme, to attract customers and merchants. In addition, we show by implementation that our proposal outperforms previous solutions of this kind.

The advances in the digital world (e.g. the Internet, Communications, etc.) are closing the gap between consumers and providers, in both Business to Consumer and Business to Business environments. Through the Internet, providers can offer their products directly to consumers, which increase their choices and allow them to contract a set of different services/products from different providers. But sometimes the consumer needs an all-or-nothing exchange with these providers: an atomic exchange. The consumer has negotiated/obtained a set of conditions that are only met if she acquires all the services/products. Thus, we need an Atomic Multi-Two-Party contract signing protocol. Even though digital signature of contracts is a topic that has been widely studied, it lacks a solution when applied to Atomic Multi-Two-Party scenarios. Here, we propose the first solution to solve this problem.

A lot of new applications in the field of electronic commerce require customers to pay small amounts of money to purchase low-value services, goods or pieces of information. These kinds of payments, called micropayments, provide a trade-off between efficiency and security requirements. In a previous work, we presented an efficient and secure micropayment scheme which fulfills both the security properties that guarantee no financial risk and the desired privacy for customers. In this paper we present a successful implementation of the scheme on the Android Platform and its performance evaluation to prove that the proposal is viable using current mobile devices. Several tests validate that our scheme can execute very fast payments with our simple but secure spending protocol, even when we need to use multiple tokens to pay for more expensive products. Moreover, we have proved that data storage requirements are low.

The advances in electronic commerce are replacing many of the paper processes involved in commercial transactions, from the contracting process to the electronic payment process. When we are dealing with payments of high value, bank transfers are preferred. But this kind of payments does not allow the anonymity of the parties involved in the commercial transaction. Electronic checks is a useful payment system for this kind of scenario, but the proposed solutions do not allow an anonymous transference as in paper-based checks. In this paper, we propose a new electronic bearer bank check scheme. The system allows the e-check to be transferred to any person by its owner without depositing it, and remaining anonymous in front of the bank.

The development of new applications of electronic commerce (e-commerce) that require the payment of small amounts of money to purchase services or goods opens new challenges in the security and privacy fields. This kind of payments are called micropayments and they have to provide a tradeoff between efficiency and security requirements to pay low-value items. In this paper we present a new efficient and secure micropayment scheme which fulfils the security properties that guarantee no financial risk for merchants and the privacy of the customers. In addition, the proposed system defines a fair exchange between the micropayment and the desired good or service. In this fair exchange, the anonymity and untraceability of the customers are assured. Finally, customers can request a refund whether they are no more interested on the services offered by merchants.

Electronic multicoupons are the equivalent of paper-based coupons booklets, offered to customers in order to achieve discounts or gifts from one or more merchants for goods or services. To date, most of the previous proposals have focused in scenarios where multiple customers can interact with a single merchant. But this restriction is a serious drawback for the expansion of the use of electronic coupons, since each customer needs a preestablished relationship with each merchant. In this paper, we propose a new multicoupon scheme for multi-merchant scenarios, preserving security properties of previous single merchant schemes, but enhancing the efficiency and the security of the few solutions that deal with multi-merchant scenarios.

The brand new mobile devices are capable to access data networks and they are also able to execute complex applications. Furthermore, these devices implement location systems like GPS. The combination of both capabilities can be useful to build new ubiquitous applications based on user location. These applications provide to service customers valuable information related to their location. We present a new scheme to access Location-Based Services (LBS) which joins both the anonymity of its users and the privacy of their data with the capacity of providers to charge them. The protocol we describe here is based on micropayment schemes and it also adds for the first time a fair exchange between the service provider and the user, which is a micropayment for a location-based service.

A certified electronic mail protocol must avoid the selective receipt of messages in notification operations. No author based selective receipt is a desired feature for certified electronic mail protocols because the receiver must accept or reject the message before the identity of the sender is revealed. We present a certified e-mail protocol, based in our previous protocol presented in the International Security Workshop 2000, which avoids selective receipt. The modified protocol does not use the signature of the sender in the first step of the protocol and avoids temporally the identification of the sender. However, even without this signature, the protocol presents a dispute resolution system that avoids unfair situations. The result is a fair, asynchronous and efficient protocol. Moreover we have evaluated the role of the TTP in the fair exchange protocol, showing that the incorrect behavior of the TTP can be demonstrated in all cases, so the TTP is verifiable.

In ACISP 04, the authors of (F. Bao et al., 2004) suggested a fair exchange protocol for electronic contract signing between two parts. The protocol is an improvement of a previous one (S. Micali, 2003) and solves the vulnerabilities of that protocol. In (F. Bao et al., 2004) one of the signers can contact a trusted third party to resolve the exchange if he can't conclude it in the usual way. However, the other signer cannot contact the trusted third party, and for this reason, this signer can't know the final state of the exchange until, if it is the case, the trusted third party contact him or her. In this paper we have improved the protocol so that both signers can contact the TTP and solve the exchange whenever they want. We propose an asynchronous and fair protocol that improves the protocol presented in (F. Bao et al., 2004) maintaining the efficiency of the original protocol.

We present the design of a protocol for certified e-mail. Some proposed certified e-mail protocols involve a third party in order to guarantee a fair exchange. Users, therefore, have to deposit a great amount of trust in a remote third party. In addition to that, the third party can become a communication bottleneck. We propose a protocol that involves an organisation of third parties, but only in case of exception. It reduces the amount of trust deposited in the third parties, because a minority of malicious third parties cannot compromise the fairness of the exchange, since all the third parties make a decision voted on by members.