Why is a Payment Protocol Acknowledgement message not signed? The Payment Protocol (BIP 70) describes three messages passed between a payer and payee. The last one is an acknowledgement message sent by the payee.

This discussion talks about the acknowledgement being used as a receipt. In the event of a dispute, the payer can show the payee this receipt to prove payment.

However, unlike the payment request message, the acknowledgement message isn't signed. I may be missing something, but this looks like a weak form of receipt without a signature.

Although the acknowledgement attaches the payment message, the payment message doesn't reference the original payment request, which is the only message with a signature.

It seems like a signed acknowledgement message would provide strong proof of purchase, and would be very valuable from the perspective of consumer protection. Why wasn't this included in BIP 70?
This discussion talks about the acknowledgement being used as a receipt. In the event of a dispute, the payer can show the payee this receipt to prove payment.


I don't think it says that. Quote:


  When a payment request is signed it's not just the addresses. The memo is also signed. By keeping around the signed payment request, and the transactions you created to satisfy it, you obtain a mathematical proof of payment. The merchant cannot dispute what was paid for because the memo field contains a description, nor can they dispute that payment was delivered (unless they claim they lost control of their private keys, of course).


So, when going to dispute resolution, the buyer shows the signed PaymentRequest, which includes one or more Outputs, and shows a transaction to those outputs. 


  It seems like a signed acknowledgement message would provide strong proof of purchase, and would be very valuable from the perspective of consumer protection. Why wasn't this included in BIP 70?


Well, the server isn't required to provide a signed acknowledgement. It could just terminate the connection after the Payment was received. At that point, you'd be in exactly the same boat.