A Formal IT-Security Model for a Weak Fair-Exchange Cooperation with Non-repudiation Proofs

This article presents a formal IT-security model for the step-by-step exchange of digital items. Following the taxonomy of Asokan the model presented here addresses the security requirements for a so-called ldquoweakrdquo fair exchange. ldquoWeakrdquo refers to the fact, that third parties are used to dissolve disputes. In this model, non-repudiation proofs are used in an external dispute to establish weak fairness. It shows how many unproved steps can be tolerated by one party without loss of fairness. The model is based on the idea of a ldquocontinuous balance of obligations and their proofsrdquo. This idea was proposed 1993 by Grimm, but never since formalized properly.