Does anyone have a doc with the specifics of how tap-to-pay works? Specifically, what happens when mulitple individuals are in proximity? I imagine there's a payment request broadcast followed by a payment allow response, followed by the broadcaster sending a subsequent request message to a selected payment response? I'm curious about the specifics and what kind of clever things the protocol has to maximize security and minimize DoS, and what kind of information is transmitted on each leg of the protocol