Mobile wallets promise to allow people to easily manage their accounts and
to carry less cards. However, the slow adoption of contactless point of
sales (POS) terminals by merchants limits the potential of Near-Field Communication (NFC)
based payment devices. In this paper, we discuss Wild Card, a secure and backward
compatible way to make mobile payment through conventional magnetic stripe
based POS terminals. The device resembles a traditional credit card in its physical
dimensions and stays in the phone case. It can be programmatically set by a NFC-enabled mobile phone to
any card number that the user owns. The key technologies that enables Wild Cards are a fully
programmable magnetic stripe, an energy harvesting system that allow the card to be
charged and programmed by the phone through NFC, and a security mechanism that makes card
information resilient to attacks on mobile devices. With a prototype, we
evaluate the feasibility of Wild Cards in terms of functionality and energy budget.