Off-line electronic cash based on secret-key certificates

An off-line electronic coin system is presented that offers multi-party security and unconditional privacy of payments. The system improves significantly on the efficiency of the previously most efficient such system known in the literature, due to application of a recently proposed technique called secret-key certificates. By definition of secret-key certificates, pairs consisting of a public key and a matching certificate can be simulated with indistinguishable probability distribution. This allows a variety of polynomial-time reductions from a well-known signature scheme to the cash system. In particular, the withdrawal protocol can be proved to be restrictive blind with respect to one account holder, relying only on a standard intractability assumption; no such result has been proved before in the literature. Another consequence of the application of the secret-key certificate technique is that the withdrawal protocol is not a blind signature issuing protocol. This falsifies the popular belief that efficient privacy-protecting off-line electronic cash systems must be based on withdrawal protocols that are blind signature issuing protocols.