Paper 2023/585

Secret Swapping: Two Party Fair Exchange

Abstract

Consider two participants who wish to each reveal some secret information to each other, but both will only reveal their secret if there is a guarantee that the other will do so too. Conventionally, whoever sends their secret first makes themselves vulnerable to the possibility that the other party will cheat and won’t send their secret in reply. Fair Exchange (FE) protocols are a class of cryptographic construction which allow an exchange like this to occur without either party making themselves vulnerable. It is widely believed that a trusted third party arbitrator is required to intervene in the case of a dispute. We present a FE protocol that allows two parties to exchange secrets, safe in the knowledge that either both secrets will be exchanged, or neither will; without the involvement of a third party. This construction requires that the parties run online interactive processes, with reasonable timeliness requirements on the messages. In this work we provide a scheme for swapping arbitrarily sized secrets with security that reduces to the strength of an underlying symmetric authenticated encryption scheme.