Paper 2023/585

Two Party Fair Exchange

Abstract

Fair Exchange (FE) protocols are a class of cryptographic protocol in which two parties, X and Y , exchange some secret data, where the ability of each party to receive secret data is contingent on having sent secret data of their own. When exchanging secret data without the support of FE protocols, whoever sends their secret first makes themselves vulnerable to the possibility that the other participant will cheat and won’t send their secret in return. It is widely believed that FE protocols satisfying the soundness notion of Strong Fairness require a third party arbitrator trusted by both X and Y , however in this paper we construct an FE scheme satisfying a notion of Strong Fairness with no third party involvement. This is achieved by embracing nondeterminism and slightly relaxing the definitions of security from the perfect to the computational domain. The protocol presented in this paper allows two parties running interactive processes to exchange secrets, safe in the knowledge that either both secrets will change hands, or neither will, without the involvement of a trusted third party. The security of the protocol presented in this work reduces to the strength of an underlying symmetric authenticated encryption scheme.