Optimistic Fair Exchange Protocol for E-Commerce

For e-commerce payments, fair exchange is one of the essential problems. The optimistic fair exchange protocol allows two parties to efficiently exchange items so that either each party gets the other’s item or neither does. We propose a new optimistic fair exchange protocol that is efficient and applicable to any digital signature scheme such as RSA or DSA. In our protocol, we introduce pre-signature, post-signature and notarized signature by prescribing the form of the digital signatures. Furthermore, we introduce a parameter that represents the expiration date of the pre-signature to realize the timely termination of the protocol.