Abstract. One important requirement of electronic cash systems is the anonymity of customers. Unconditional anonymity is also very well suited to support criminals in blackmailing. Maitland and Boyd proposed at ICICS 2001 an offline electronic cash system based on a group signature scheme. Their scheme cannot be used to solve blackmailing and other anonymity problems such as money laundering and illegal purchases. Chen, Zhang and Wang suggested an offline electronic cash scheme to prevent blackmailing by using the group blind signature. In their payment system, they used a group signature scheme of Camenisch and Stadler for large groups which is not secure. In this paper we improve these electronic cash systems to prevent blackmailing, money laundering and illegal purchases by using a secure coalition-resistant group blind signature scheme. Key words: cryptography, electronic cash system, blackmailing, group blind signatures. 1
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