Fairness in Non-Repudiation Protocols

We indicate two problems with the specifications of fairness that are currently used for the verification of non-repudiation and other fair-exchange protocols. The first of these problems is the implicit assumption of perfect information. The second problem is the possible lack of effectiveness. We solve both problems in isolation by giving new definitions of fairness, but leave the combined solution for further work. Moreover, we establish a hierarchy of various definitions of fairness, and indicate the consequences for existing work.