Re: How can you challenge/response with a QR code?
Or a mag stripe? Maybe it could work with sonic payments, though I have no idea how they work (never heard of them until now) so who knows.
I suppose that the POS could ask for a pin, although the idea here is that you wave the phone at the terminal and go, rather than interacting with it. I suppose it's better than asking you to video yourself doing the chicken dance in the store and uploading that as a response.