Fair Exchange of Digital Signatures Using Conditional Signature

To implement fair exchange of digital signatures, we need an efficient scheme to commit a digital signature safely to a specific receiver. To satisfy this security requirement in an efficient manner, we introduce a new variant of digital signature called conditional signature which is a specially interpreted signature on a message and a condition together. By imposing a signer-chosen condition which describes expected action of a specified receiver, conditional signature can be used as a private negotiation statement in two-party communication. We model negotiation problem using conditional signature and then construct a fair exchange protocol. We show that matching negotiation and real exchange give a fair exchange of digital signatures.