Distributed multi-party fair exchange

Over the years, researchers focused on either two-party cases or multi-party computation securely, fairly and optimistically. Most of the studies in the literature require a trusted third party (TTP) to be present for fairness either at every step of the protocol or only optimistically intervene in case of malicious actions. However, having a single TTP creates a single point of failure (in terms of security) in the protocol. To distribute this responsibility, literature has different solutions. Blockchain is one of the most used solutions for decentralizing exchange protocols and multi-party computations. Secret sharing is another procedure used for distributing responsibility, however, it is mostly used for 2-party settings. As far as we know, there is no optimistic multi-party fair exchange solution using multiple TTPs in the literature. In this work, we present a Distributed Multi-Party Fair Exchange Protocol, building on top of a previous research [Alper and Küpçü, 2021], by distributing the responsibility of a single TTP to multiple (m) TTPs. We are using secret sharing to distribute the decryption share used for fairness to TTPs. Even with a malicious subset of TTPs, the protocol provides a fair result, as long as there are threshold-many honest TTPs. Even though the performance of the protocol is worse (slower, as expected) than the previous study, it carries the importance of being the first optimistic multi-party fair exchange protocol with multiple trusted third parties in the literature.