P4R: Privacy-Preserving Pre-Payments with Refunds for Transportation Systems

We propose a new lightweight payment scheme for transit systems called P4R: Privacy-Preserving Pre-Payments with Refunds. In P4R a user deposits money to obtain a bundle of credentials, where each credential allows to make an arbitrary ride. The actual fare of a trip is determined on-the-fly when exiting. Overpayments are refunded where all trip refunds of a user are aggregated in a single token thereby saving memory and increasing privacy. We build on Brands’ e-cash scheme to realize the pre-payment system and a new variant of blind Boneh-LynnShacham signatures to implement the refund capabilities. Our construction is secure against malicious users and guarantees user privacy. We also provide an efficient implementation that shows the suitability of our scheme as future transit payment system.