Off-Line Fair Payment Protocols Using Convertible Signatures

An exchange or payment protocol is considered fair if neither of the two parties exchanging items or payment at any time during the protocol has a signiicant advantage over the other entity. Fairness is an important property for electronic commerce. This paper identiies a design framework based on existing fair protocols which use ooine trusted third parties, but with convertible signatures as the underlying mechanism. We show that in principle any convertible signature scheme can be used to design a fair payment protocol. A speciic protocol is detailed based on RSA undeniable signatures which is more eecient than other similar fair payment schemes. Furthermore, in this protocol the nal signature obtained is always an ordinary RSA signature.