Abstract
When buying and selling digital products online both the customer and merchant need to trust each other. However, not all parties are honest and hence a class of exchange protocols has been developed that are called Optimistic Fair Exchange Protocols. This paper surveys the field of Fair Exchange Protocols and then presents a special type of protocol between a customer (C) and a merchant (M) that enforces one of them to be honest. It makes minimal use of a Trusted Third Party (TTP). The protocol has the features that it: (1) only comprises three messages to be exchanged between C and M; (2) guarantees strong fairness for both C and M; (3) allows both parties to be sure of the item that they will receive from the other party; and (4) resolves disputes automatically online.