Fair digital signature of contracts and agreements is an essential process in any electronic commerce scenario, and therefore also in data marketplaces, where the relationships and agreements among the different parties (consumers and providers) are more dynamic. In multi-party contract signing, N parties wish to sign a contract in a such a way that either all signatories obtain evidence of the signing or none obtains conflicting evidence regarding the honest signatories; the exchange must be fair. In this paper, we propose a blockchain-based multi-party contract signing protocol. This solution does not require the existence or potential intervention of a trusted third party (TTP), thus avoiding the difficulty of N signatories agreeing upon a TTP. In addition, this proposal meets the necessary requirements: fairness, timeliness, non-repudiation of origin, and non-repudiation of receipt. Furthermore, confidentiality can be easily achieved. To minimize costs associated with the use of blockchain, it should be invoked in the case of exception (analogous to optimistic solutions with a TTP) and by only one of the N signatories. However, when the use of blockchain is required, we show that its cost is within perfectly manageable margins.

This paper introduces a novel dynamic spectrum sharing (DSS) scheme designed for cell-free massive MIMO (CF-mMIMO) networks. The motivation behind this work arises from the urgent need to enhance spectrum utilization in modern wireless communication systems. Traditional spectrum allocation methods often struggle to meet the diverse spectrum demands of different wireless operators. The proposed approach addresses these challenges by enabling the spectrum provider (SP) to flexibly allocate and sell its spectrum resources, empowering wireless virtual operators to acquire bandwidth efficiently based on their specific needs. In our framework, each wireless operator is represented by a CF-mMIMO network, characterized by an average spectral efficiency metric that quantifies the potential value of additional spectrum acquisition. Leveraging a Stackelberg game formulation, our DSS policy achieves an equilibrium point that optimally allocates bandwidths to operators and determines corresponding prices. This approach not only enhances spectrum utilization but also fosters fair competition among operators. A key innovation of our work lies in the utilization of blockchain technology, where all spectrum transactions are managed through a smart contract. This ensures transparency, integrity, and auditability throughout the spectrum trading process. The novel protocol facilitates seamless information exchange, orchestrates the Stackelberg game dynamics, and delivers conclusive outcomes to both the SP and the CF-mMIMO operators.

In this article, we present the first solution for multi-party certified email that achieves fairness and confidentiality without the intervention of a TTP but with blockchain support when problems arise. Our solution preserves the confidentiality of the mail exchanged between the sender and the recipients even when the blockchain is involved. We provide a proof of concept implementation based on Ethereum smart contracts that shows it is feasible from a practical point of view. Because the functions of a smart contract are only executed on the blockchain in the case of conflict, costs are minimised compared to solutions that execute the functions on the blockchain in all cases (on-chain solutions). Moreover, the solution can be integrated into the existing email infrastructure.

The certified mail is a value-added service that is widely used in the paper world. However, the scientific community has not yet provided a solution for certified e-mail that has achieved widespread acceptance. This lack of a certified e-mail solution is not due to a lack of proposed approaches; because over the past 40 years, more than 100 protocols have been reported in journals and at conferences. The vast majority of these proposed protocols use a trusted third party (TTP) to achieve fairness. The few solutions without a TTP have not been successful due to their high computational and/or communication cost. Blockchain provides a new approach to develop the protocols without a TTP but without the prior drawbacks of the previous solutions without a TTP. Here, we present a new protocol for certified e-mail based on a blockchain without a conventional TTP that is integrated with the conventional e-mail infrastructure. The protocol is secure, efficient, and viable from a practical perspective.

In this paper, we present two proposals for multi-two-party contract signing with blockchain support. In the scenario in this paper, a customer wants to purchase products/services from multiple vendors with the requirement that she wants all or none of the products. Our proposals satisfy the security requirements for contract signing: fairness, timeliness, nonrepudiation, effectiveness and confidentiality. Furthermore, one of the two proposals requires the execution of blockchain functions only in the case of dishonest behaviour (bc-optimistic). The only previous solution found in the literature for the multi-two-party contract signing scenario requires the use of a trusted third party (TTP); however, the approach is highly inefficient, requiring the exchange of many messages. Our proposals are much more efficient than the TTP-based proposal, as our proposal reduces the number of exchanged messages required to complete multi-two-party contract signing. As the use of blockchain incurs a fee, we prove the technological and economic viability of our proposals on different blockchains.