The exchange of non-fungible tokens (NFTs) traditionally occurs on centralized NFT marketplaces, which often wield significant control over transactions, as the imposition of restrictions and fees. Additionally, the current NFT standard (ERC-721) lacks certain functionalities that hinder the seamless direct exchange of NFTs between owners. This article introduces a novel protocol that eliminates the need for an intermediary marketplace, as well as their intermediary smart contract or escrow contract, facilitating a fair exchange of NFTs as collectibles. The protocol is represented by an extension of the ERC-721 called Exchangeable NFTs (ExNFTs), which introduces the necessary functionalities to enable direct NFT exchanges. We analyze the limitations of the ERC-721 standard and build upon our previously proposed protocol of Rejectable NFTs (RejNFTs), which introduces a rejectable functionality empowering the receiver to reject the transfer of an NFT. Additionally, we discuss the reasons why achieving a fair direct exchange of NFTs is currently unattainable with the prevailing standards. We present the key functionalities of the ExNFT approach, along with a Solidity implementation of the ExNFTs. Furthermore, we evaluate the performance of the protocol, considering costs and security.

Abstract The conservation of ambient-sensitive pharmaceutical drugs is critical for maintaining their efficacy and safety, yet proper storage often becomes challenging once drugs reach patients’ homes. This paper presents a patient-centric drug quality conservation monitoring system that integrates IoT and blockchain technologies to address this gap. Sensor-equipped devices continuously track environmental parameters such as temperature, humidity, light, and movement, providing real-time alerts for any deviations that may compromise drug integrity. Patients are empowered to take corrective actions regarding conservation when necessary, while blockchain technology certifies their acknowledgment of non-consumable drugs, ensuring transparency and accountability. By leveraging cryptographic sensors and Ethereum-based smart contracts, the proposed platform offers an end-to-end solution to enhance patient engagement, ensure proper drug conservation, and support healthcare providers in remote monitoring. The protocol’s performance is evaluated in terms of cost, demonstrating its feasibility as an efficient and reliable solution for drug conservation monitoring.

Low Emission Zones (LEZ)s are areas where access restrictions to polluting vehicles are enforced. These infrastructures have become a main mechanism in large cities to deal with urban traffic and environmental pollution. A main problem of practical LEZs is that they generally depend on a camera network that identifies users and jeopardizes their privacy. In the literature, there are some privacy-preserving works that rely on camera-free approaches; however, they still suffer from a major issue: they depend on centralized entities to manage the vehicles’ accesses/departures and their corresponding fee payment. Those centralized entities represent a critical single point of failure in the system, endangering its security and availability. In order to address this situation, this paper proposes a new scheme that decentralizes the LEZ management, dealing with vehicle accesses as blockchain transactions, and pricing and charging them using smart contracts. In order to validate the deployability of the new scheme in real scenarios, it has been implemented and tested in both a controlled environment and a low-traffic street. The evaluation of the smart contracts’ costs in terms of gas has been included in the performed tests. The results obtained are satisfactory and show the feasibility of the new proposal.

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.

Contract signing is a trust service that requires a fair exchange of elements: each signer's signature on the contract. This kind of service usually relies heavily on the use of trusted third parties. These entities are an obstacle to extend the use of this service. In this paper we use blockchain-based technologies to provide a contract signing protocol that removes completely the involvement of trusted third parties while satisfying the requirements of this kind of service. We present a protocol that can be used for the signature of public contracts but also for the signature of private and confidential contracts. The protocol has been designed taking into account its possible extension to multiparty contracts.

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.

The traffic control system proposed in this article has as its main objectives the security, the reliability and the anonymity of the vehicle access to traffic-restricted urban zones. The aim of these zones is to reduce atmospheric pollution in urban centers. The presented approach makes use of smartphones as a communication device between the user and the virtual barrier, which acts as electronic validator and controls the access to the restricted zone. The access is fast (it does not require the complete stop of the vehicle) and is less invasive when compared to existing systems that are based on video camera recording. This is a versatile and cost-efficient system that can be adapted to other access-controlled urban settings: parking lots, sport event access zones, festivals, etc.

Great cities try to prevent from high levels of pollution and traffic jams by restricting the access of vehicles to centric zones, also known as Low-Emission Zones (LEZ). Depending on the topology of the city and also on the traffic patterns and contamination levels, the LEZ can be divided in multiple zones with different prices, obtaining a Multifare Low Emission Zone. Multifare LEZs must manage not only entrances and departures but also changes of zone. Some of the most important issues of LEZs are the risk of losing privacy of the citizen who drive through the LEZ and a significant error percentage on detection of fraudulent drivers. In this article, an Electronic Road Pricing (ERP) system designed specifically for cities with multifare Low-Emission Zones is proposed. Its aim is to detect fraud and to preserve driver's privacy. Using revocable anonymity only fraudulent drivers lose their privacy.

Automatic Fare Collection (AFC) systems calculate the fare that the users must pay depending on the time of service (time-based) or the points of entrance and exit of the system (distance-based). The progressive introduction of Information and Communication Technologies (ICT) allows the use of electronic tickets, which helps to reduce costs and improve the control of the infrastructures. Nevertheless, these systems must be secure against possible fraud and they must also preserve users’ privacy. Therefore, we have studied the security requirements for the timebased and distance-based systems and we have proposed a protocol for each of the AFC systems. The protocols offer strong privacy for honest users, i.e., the service provider is not able to disclose the identity of its users and, moreover, different journeys of the same user are not linkable between them. However, anonymity for users could be revoked if they misbehave. The protocols have been implemented in the Android mobile platform and its performance has been evaluated in two Android smartphones. The results remark that protocols are suitable to be used on AFC system with a medium class mobile device although they offer a better experience with a high-class smartphone. The appearance in the market of more powerful mobile devices suggests a better usability of our proposal in a near future.

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 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.

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.

Automatic Fare Collection (AFC) systems calculate the fare that the user must pay which depends on the points of entrance and exit of the system. These systems were paper-based, but the progressive introduction of Information and Communication Technologies (ICT) allows the use of electronic tickets which helps to reduce costs and it improves the control of the infrastructures. Nevertheless, these systems must be secure against possible fraud, and they must also preserve users' privacy. We propose an AFC system that offers strong privacy for honest users. The service provider can not disclose the identity of its users and, moreover, different journeys of the same user are unlinkable. However, anonymity for users could be revoked if they misbehave. This system has been designed in order to use personal mobile devices.

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.

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.