  The most prevalent smart card-based payment method, EMV, currently offers no
privacy to its users. Transaction details and the card number are sent in
cleartext, enabling the profiling and tracking of cardholders. Since public
awareness of privacy issues is growing and legislation, such as GDPR, is
emerging, we believe it is necessary to investigate the possibility of making
payments anonymous and unlinkable without compromising essential security
guarantees and functional properties of EMV. This paper draws attention to
trade-offs between functional and privacy requirements in the design of such a
protocol. We present the UTX protocol - an enhanced payment protocol satisfying
such requirements, and we formally certify key security and privacy properties
using techniques based on the applied pi-calculus.
