Efficient E-Cash in Practice: NFC-Based Payments for Public Transportation Systems

Near field communication (NFC) is a recent popular technology that will facilitate many aspects of payments with mobile tokens. In the domain of public transportation payment systems electronic payments have many benefits, including improved throughput, new capabilities (congestion-based pricing etc.) and user convenience. A common concern when using electronic payments is that a user’s privacy is sacrificed. However, cryptographic e-cash schemes provide provable guarantees for both security and user privacy. Even though e-cash protocols have been proposed three decades ago, there are relatively few actual implementations, since their computation complexity makes an execution on lightweight devices rather difficult. This paper presents an efficient implementation of Brands [11] and ACL [4] e-cash schemes on an NFC smartphone: the BlackBerry Bold 9900. Due to their efficiency during the spending phase, when compared to other schemes, and the fact that payments can be verified offline, these schemes are especially suited for, but not limited to, use in public transport. Additionally, the encoding of validated attributes (e.g. a user’s age range, zip code etc.) is possible in the coins being withdrawn, which allows for additional features such as variable pricing (e.g. reduced fare for senior customers) and privacy-preserving data collection. We present a subtle technique to make use of the ECDHKeyAgreement class that is available in the BlackBerry API (and in the API of other systems) and show how the schemes can be implemented efficiently to satisfy the tight timing imposed by the transportation setting.