Hi Chuck,

Both Bitcoin Core and bitcoinj are about to ship with the protocol as-is, so any changes from this point on have to be backwards compatible.

On Thu, Jan 30, 2014 at 6:47 AM, Chuck <chuck+bitcoindev@borboggle.com> wrote:

I presume the receipt R=(PaymentRequest,[transactions]) would suffice.

That's all you need to prove payment, yes.

In the well-behaved case, I presume the point is to help the
merchant associate some arbitrary data with the purchase as well as
provide a refunding address for the customer.

That's right (+memo). And to provide an additional hook for future features, like recurring billing, ECDH key agreements etc.

In Step 3, it's critical the customer sign the message with the private
key of the refund address, so that the merchant can be confident the
refund address is correct.

Refund addresses as specced currently are optional. For instance bitcoinj currently doesn't use them and won't until HD wallets support is done.

Let's get some practical experience with what we've got so far. We can evolve PaymentRequest/Payment/PaymentACK in the right direction with backwards compatible upgrades, I am hoping.