The cryptography expert did not want to resign himself to this apparent conflict of privacy and security. Together with Gunnar Hartung and Matthias Nagel of KIT and Max Hoffmann of Ruhr-Universität Bochum, he has now presented the basics of an “electronic purse” that works anonymously, but prevents misuse at the same time. The “black-box accumulation plus” (BBA+) protocol developed by them transfers all necessary account data to the card used or the smartphone and guarantees their confidentiality with the help of cryptographic methods. At the same time, BBA+ offers security guarantees for the operator of the bonus or payment system: The protocol guarantees a correct account balance and is mathematically constructed such that the identity of the user is disclosed as soon as the attempt is made to pay with a manipulated account.
The new protocol is a further development of an anonymous bonus card system that was also designed by the KIT research group. For collecting and redeeming points, however, it required an internet connection to prevent misuse. “Our new protocol guarantees privacy and security for customers during offline operation as well,” Andy Rupp says. “This is needed for ensuring the payment system’s suitability for daily use. Think of a subway turnstile or a toll bridge. There you may have no internet connection at all or it is very slow.” Also its high efficiency makes the protocol suited for everyday use: During first test runs, researchers executed payments within about one second.