Attribute-based optimistic fair exchange: How to restrict brokers with policies

Optimistic fair exchange (OFE) is a kind of protocols for solving the fair exchange problem between two participants with the help of an arbitrator that only needs to be involved when dispute occurs. As far as we are concerned, all previous work on OFE does not take into account user's attributes such as nationality and age. We identify that in some applications, the attributes could play an important role in the exchange to take place, and OFE may not be suitable to these scenarios. We introduce a new notion named attribute-based optimistic fair exchange (ABOFE) to solve the fair exchange problem in the attribute-based setting. We formalise the notion of ABOFE and present a security model in the multi-user setting under the chosen-key attack. We also present a generic construction of ABOFE from existing cryptographic primitives and prove that our proposal is secure with respect to our definition in the standard model. An instantiation in the standard model is discussed.