Abstract Signed information is difficult to manage and this difficulty is even bigger when pieces of signed digital information have to be exchanged in an atomic procedure. Multiparty contract signing is an application of fair exchange of values, where the information to be exchanged are signed versions of a contract. Traditionally, this trust service requires the involvement of a trusted third party (TTP). This is a point of weakness for the generalization of the use of electronic contract signing protocols. Another usual problem for multiparty contract signing is that the protocols for multiparty exchanges are far more complex that those for two-party exchanges. Additionally, it is common that the signers demand protocols that assure the confidentiality of the contract. In this proposal blockchain-based technologies and smart contracts are used to obtain a protocol for electronic contract signing that does not need a TTP in any stage of the procedure. The protocol satisfies the ideal requirements for the contract signing service, even allowing the signature of confidential contracts. Moreover, the proposed solution is far more efficient than previous solutions for fair exchange protocols using blockchain presented up to the date thanks to the use of elliptic curve cryptography.

Abstract Touristic City Cards are managed by some city councils in order to make a step toward the SmartCity concept. Touristic City Cards enable the integration of different touristic services so as to facilitate the access to transport, cultural sites and touristic attractions to visitors. In this paper, we introduce a secure mobile system called mCityPASS that allows the smart management and secure access to this kind of service. We have implemented the system in a smartphone platform in such a way that the mobile device contains the purchased access element that holds a set of e-tickets that enables the visitors of a city to make use of the specified touristic services. The paper presents the details of the whole implementation of the system and an evaluation of the performance and the effectiveness of it. Also, an analysis of the privacy and the security properties of the presented scheme has been conducted. As a result, we can assure that the system preserves the privacy of the tourists and fulfills the more challenging security properties: fairness, non-overspending, exculpability, unsplittability and selective reusability.

Big cities and touristic destinations offer Touristic City Cards in order to make an step towards the SmartCity concept enabling the integration of different touristic services so as to facilitate the access to transport, cultural sites and touristic attractions to visitors. To preserve the privacy of the users and avoid the generation of users' profiles the integrated e-ticketing solution must achieve the property of anonymity. For this reason, a privacy-preserving solution to access, in a secure way, integrated services requires both a secure access protocol to purchase and validate the e-tickets and an anonymous payment system. This paper presents the integration of an anonymous payment system together with an anonymous e-ticketing system for secure access to touristic services, achieving other important properties like fairness, non-overspendig, exculpability and selective reusability.

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.

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.