An efficient fair exchange protocol that enforces the merchant to be honest

A new optimistic fair exchange protocol between a customer (C) and a merchant (M) is presented in this paper. It makes use of trusted third party (TTP) but its use is kept to minimum when disputes arise. The protocol is for exchanging a payment with a digital product such as software. A new idea, in which if the parties are willing to exchange then they are enforced to be honest, is originated in this protocol. The protocol has the following features: (1) it only comprises three messages to be exchanged between C and M in the exchange phase; (2) it guarantees strong fairness for both C and M; (3) it allows both parties to be sure of the item that they will receive from the other party before they receive it; and (4) it resolves disputes automatically online.