Pairing-Based Fair Electronic Cash Systems

Untraceable electronic cash (e-cash) indicates a user made a payment, but no one knows who the payer was. It hence ensures user privacy and becomes an attractive payment tool. However, untraceability property, also called anonymity, is easily abused by criminals who use it for activities such as money laundering, bribing, or blackmailing. To address these problems, researchers introduced fair e-cash systems in which the anonymity of e-cash is maintained if the e-cash is used legally, but is revoked if misused. Except untraceability and anonymity revocation, e-cash should be publicly verifiable, unforgeable, double-spending resistance, and not to be spent by non-owners. In addition, the e-cash system preferably allows bank-offline payment and to give change. Untraceable e-cash is realized by blind signature schemes in which a signer signs a sealed document, and a receiver can unseal the signed result to obtain a signer’s signature to the original document. Furthermore, if letting the sealed document include some tracking information by some method, we can trace the sealer by some corresponding method. Such construction is referred to as fair blind signature schemes. For an e-cash system, the signer will be a bank and the sealer is a user. That the anonymity can be revoked through the tracking information infers fairness. In this dissertation, we present two pairing-based fair blind signature schemes and develop two corresponding fair e-cash systems, named IDB-Cash and LEB-Cash. IDB-Cash is an identity-based cryptographic application, using participants’ identity information such as name or email address as public keys, and thus has the advantages of simple key distribution, key management and key access. In addition, the protocols in IDB-Cash possess communication efficiency because each has built-in identity-authentication and key-agreement functions, and it therefore eliminates the cost of necessary pre-built secure channels. As for IDB-Cash, it employs Boneh, Lynn, and Shacham’s short signature primitive to design a license-embedded blind signature scheme (LEBSS) to properly embed a user license into e-cash while keeping the license anonymous to the bank. The LEBSS is provably secure in terms of blindness and unforgeability, providing more robust e-cash than IDB-Cash does. Compared to related pairing-based fair e-cash literature, IDB-Cash and LEB-Cash are efficient in e-cash size, issuing cost and paying cost. IDB-Cash is welcome to the practice while LEB-Cash possesses both efficiency and provable security.