A mobile phone is a phone that can make and receive telephone calls over a radio link, e.g., by connected to a tower of a cellular network. In addition to making calls, conventional mobile phones now support a variety of other electronic services, e.g., text messaging, email, web surfing, photography, and short-range wireless communications. Mobile phones and other mobile devices, e.g., laptops and tablet computers, are increasingly being carried everywhere by mobile device users. Since users are almost always carrying these mobile devices, the mobile devices can be used for various purposes in addition to communications purposes. For example, mobile devices that include electronic wallet application programs are gaining in popularity as an alternative to traditional wallets and physical payment cards.