Implementation and Analysis of a Synchronisation Protocol for Fair Exchange with Strong Fairness and Privacy

Abstract

In distributed information systems, it is essential to ensure fairness and privacy in the exchange of digital items between parties that do not fully trust each other. In such environments, multiple entities (users, organizations, services, devices, etc.) interact by exchanging data, documents, or digital assets. In the absence of trust between the parties involved, the risk of unfairness arises, namely the possibility that one party gains unfair advantage over the other. Fair exchange protocols have been developed to ensure that no participant obtains this advantage, thereby guaranteeing that exchanges occur fairly. Privacy, another fundamental property, prevents the leakage of confidential information. However, achieving both strong fairness and privacy simultaneously remains a challenge. Protocols widely used in information systems for online payments, e-commerce, and digital signatures ensure fairness but compromise privacy because they rely on mediating centralised third parties that directly access participants’ data, including sensitive information. In this paper, we propose the implementation and analysis of a protocol that ensures strong fairness in distributed information systems, which ensures that none of the participants is left with both items and grants each party the ability to cancel the exchange. The model uses a split trusted third party, composed of two trusted execution environments (one for each participant) and a public bulletin board used solely for synchronisation. This architecture obviates the need of centralised intermediaries and preserves essential properties such as strong fairness, immediate and unilateral cancelation, and privacy. This work is an extended version of the paper presented at SBSeg 2025; it includes new experimental results, formal verification with the SPIN model-checker, and implementation improvements; therefore it contributes to the advancement of interoperable solutions that protect data privacy in distributed information systems.