An optimistic fair exchange protocol for proxy signatures

In this paper we present an optimistic fair exchange (OFE) protocol for proxy signatures. This protocol is useful in scenarios that a signer cannot participate in an OFE protocol himself and he needs to delegate his signing power to another one. We give a new security model for this type of protocols because the best and the most complete model until now does not consider all security aspects of this protocol. We construct a generic scheme and give a paradigm for it. Our scheme is setup-free and multi-user secure in the random oracle and chosen-key model.