Few topics in retail banking have generated as much attention as digital payments—transactions that don’t involve physical cards or cash. The emergence of payment systems that are designed to function seamlessly with mobile devices, in-app methods, or browsers has prompted wide-ranging innovation from banks, digital giants, and fintechs. The volume of digital payments is soaring and by 2020 is likely to approach $5 trillion worldwide and $800 billion in the United States.  That growth b...