Online Cash Checks

Abstract

Savings of roughly an order of magnitude in space, storage, and bandwidth over previously published online electronic cash protocols are achieved by the techniques introduced in this paper. In addition, these techniques can increase convenience, make more efficient use of funds, and improve privacy. Three online schemes are presented. Each relies on the same techniques for encoding denominations in signatures and for 'devaluing' signatures to the exact amount chosen at the time of payment. They differ in how the unspent value is returned to the payer. In the first, all change is accumulated by the payer in a single 'cookie jar', which might be deposited at the bank during the next withdrawal transaction. The second and third schemes allow change to be distributed among unspent notes, which can themselves later be spent. The second scheme reveals to the shop and bank the maximum amount for which a note can be spent; the third does not disclose this information.