A Simple Off-line E-Cash System with Observers
A simple and transparent mobile e-cash system with observers is presented. Observers are cryptographic bank chips inserted in customer’s payment device, e.g. smart phone. System satisfies e-cash divisibility, anonymity, off-line payments, transferability and double-spending prevention requirements. Proposed e-cash system is based on two well-known cryptographic primitives, namely ElGamal encryption and Schnorr identification protocols, therefore it can be named as simple and transparent in the sense of security considerations. In Schnorr identification scheme Brand’s double spending prevention protocol is implemented during payment protocol. The use of observers allows to avoid e-cash data growing in size as it takes place in all divisible, anonymous, off-line and transferable e-cash systems. To increase observer’s security and avoid their clonability the use of physical unclonable function in the chip of observer is proposed.