Prevent double spending through retry

crowgames commented 4 years ago

Reading the specification, I wonder how a payment handler would be able to prevent double spending when a user issues a Payment Request retry().

As of my understanding, a Payment Handler probbably triggers a payment after receiving a PaymentRequestEvent. Through the retry, a second PaymentRequestEvent is triggered, which is identical to the first one. For an unaware Payment Handler this would lead to double spending.

In case of switching Payment Method/Handler through a retry, the first Payment Handler stays unaware of the retry mechanism and the second one is unaware of this being a retry. Which would lead to double spending as well, wouldn't it?

Am I missing something in this interpretation of the specifications? Is there a way of the PaymentHandler to detect a retry or deal with this issue in general?

ianbjacobs commented 4 years ago

Hi @crowgames,

The paymentRequestId attribute should help. It is set to the (unique) id of the payment request [1]. So the payment handler should be able to know that this is the same transaction.

Let me know if this would work for your use case.

Ian

[1] https://www.w3.org/TR/payment-request/#id-attribute

crowgames commented 4 years ago

Thanks for your reply @ianbjacobs! What if a user changes the payment handler though? If I my first request is to Handler A and the second (through retry) is to Handler B, the request ID does not help, since both A and B only observe it once. Thanks in advance for the clarification!

marcoscaceres commented 4 years ago

A user cannot switch payment handlers during retry. One can only change the things being retried (eg. new credit card expiry date).

w3c / payment-request

Prevent double spending through retry #882