Fast Privacy-Preserving Punch Cards
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Loyalty programs in the form of punch cards that can be redeemed for benefits have long been a ubiquitous element of the consumer landscape. However, their increasingly popular digital equivalents, while providing more convenience and better bookkeeping, pose a considerable risk to consumer privacy. This paper introduces a privacy-preserving punch card protocol that allows firms to digitize their loyalty programs without forcing customers to submit to corporate surveillance. We also present a number of extensions that allow our scheme to provide other privacy-preserving customer loyalty features. Compared to the best prior work, we achieve a 14x reduction in the computation and a 25x reduction in communication required to perform a "hole punch," a 62x reduction in the communication required to redeem a punch card, and a 394x reduction in the computation time required to redeem a card. Much of our performance improvement can be attributed to removing the reliance on pairings present in prior work, which has only addressed this problem in the context of more general loyalty systems. By tailoring our scheme to punch cards and related loyalty systems, we demonstrate that we can reduce communication and computation costs by orders of magnitude.
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