Exclusions and related trust relationships in multi-party fair exchange protocols

Some electronic commerce transactions are inherently performed between more than two parties. In this context, it is thus important to determine whether the underlying fair exchange protocols allowing the secure implementation of such transactions enable participants to exclude other entities from a protocol execution. This is an important point that has not been sufficiently addressed when analysing such kind of protocols, and that may be crucial for the successful accomplishment of multi-party electronic transactions. In this paper we define the properties related to exchange protocols and exclusions, study exclusion scenarios on two wellknown multi-party fair exchange protocols and point out the implications that exclusions may have on the trust relationships between participants, and, more generally, on electronic commerce. Two new protocols more robust than existing multi-party fair exchange protocols are therefore proposed.