As advances of smartphones and near field communication (NFC) technologies, sellers can deliver e-invoices to buyers' smartphones via NFC to reduce paper consumption. Therefore, buyers can use the received e-invoices for further warranty service as well as for returning and exchanging products. To identify buyers of e-invoices, current e-invoicing systems usually embed identities of buyers in e-invoices. Therefore, a merchant can make sure that a person is the buyer of a transaction based on the buyer identity in an e-invoice associated to the transaction. However, from the privacy perspective, one may not wish his identity to be recorded in an e-invoice. Even the identity in an e-invoice contains no personal identifiable information, people may still worry that retailers track their purchasing behaviors based on the buyer identities in e-invoices. Therefore, people may prefer to paying cash and using traditional paper-based invoices. To address this issue, this work proposes a privacy-preserving scheme for customers to generate secrets for different transactions and use the secrets to generate identities for e-invoicing. Therefore, a customer can claim that he is the recipient of an e-invoice by proving that he owns the secret to generate the identity in the e-invoice without revealing the secrets.

Moreover, this study implements the proposed scheme with commercially available devices. Based on the experimental results, a customer can generate secrets for further verification, transmit the secrets to a merchant, and received a generated e-invoice from the merchant via Simple NDEF Exchange Protocol (SNEP), a NFC P2P communication standard, in seconds. While the proposed scheme enhancing the convenience and privacy of current e-invoicing systems, more customers may wish to accept e-invoices.