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.

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.

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.