The merchant presents the bill to your phone. You approve the charge by entering a code on your phone. This requires a) having a mobile phone, and b) having a

The merchant presents the bill to your phone. You approve the charge by entering a code on your phone. This requires a) having a mobile phone, and b) having a properly-working mobile phone (battery charged, able to receive a signal, etc). That's a lot to ask for a lot of people in a lot of locations. If a device representing a third party is requires, we'd be better off with an RSA-type key fob.