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.

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.

Message exchange for acknowledgment of receipt (e.g., for services that provide recipients with certain documents, such as sanctions, summonses, or requirements, which are often sent by public administrations) has traditionally been provided through protocols based on the use of trusted third parties (TTPs). More recently, solutions have been proposed that totally or partially replace the TTP with a blockchain. However, an analysis of these proposals reveals that they have one or more of the following drawbacks: failure to meet confidentiality requirements, the use of TTPs, and susceptibility to attacks that make them unfair. In this paper, we present a method of providing fair delivery for receipts based on a blockchain that does not suffer from the above disadvantages: it is fair and confidential and does not use a TTP. In addition, we demonstrate the feasibility of the protocol through a cost analysis and a proof-of-concept implementation, showing that our solution is more economical than existing solutions, and can be considered cost-effective even under low time requirements.

This work addresses the problem of blockchain-based contract signing. We introduce the notion of a protocol step, which is a key concept that facilitates uniform and fair comparisons among existing protocols (especially in light of a claim regarding the minimal step count of a previous proposal that is not substantiated upon closer analysis). We conduct a comprehensive evaluation of all prior protocols with respect to both mandatory and optional security requirements. On the basis of this analysis, we propose a novel contract signing protocol that, under optimistic execution, does not invoke any on-chain functions and requires only three protocol steps to be completed, thus identifying it as the most efficient solution. A security analysis is presented, thus demonstrating that the proposed protocol satisfies all the mandatory security requirements. We also discuss how the generated signatures and evidence can be aligned with legal and regulatory frameworks. These results confirm the feasibility and efficiency of our approach.