Fast Privacy-Preserving Punch Cards

TL;DR

This paper presents a highly efficient privacy-preserving digital punch card protocol that significantly reduces computational and communication costs, enabling secure and private loyalty programs without corporate surveillance.

Contribution

It introduces a novel, optimized protocol for digital punch cards that outperforms prior work by removing complex cryptographic dependencies, achieving substantial efficiency gains.

Findings

14x reduction in punch operation computation

11x reduction in punch operation communication

128x reduction in card redemption computation

Abstract

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 privacy risk. 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 $14\times$ reduction in the computation and a $11\times$ reduction in the communication required to perform a "hole punch," a $55\times$ reduction in the communication required to redeem a punch card, and a $128\times$ reduction in the computation time required to redeem a card. Much of our performance improvement can be attributed to removing the reliance on pairings or range proofs 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.