Anonymous Credit Cards and its Collusion Analysis

Communications networks are traditionally used to bring information together. In this paper, we use them to keep information apart in order to protect personal privacy. We show how to implement anonymous credit cards using simple building blocks and standard cryptographic techniques. We propose a systematic method to determine the set of parties that must collude to bring information together. We show that, for the credit card protocol, all parties involved must collude against the customer in order to link her identity with the purchase, unless the store eavesdrops on the customer at the time of purchase, in which case, all but the store must collude.