Off-line fair payment protocols using convertible signatures

Abstract

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 signi cant advantage over the other entity. Fairness is an important property for electronic commerce. This paper identi es a design framework based on existing fair protocols which use o ine 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 speci c protocol is detailed based on RSA undeniable signatures which is more e cient than other similar fair payment schemes. Furthermore, in this protocol the nal signature obtained is always an ordinary RSA signature.