Good morning Joao,

C-lightning already does this.
If it is given an invoice for a payment hash it already knows, it silently 
succeeds and returns the preimage without sending out any money.

This should extend to any wallets built on top of C-lightning.

I am uncertain what the other implementations do.

Regards,
ZmnSCPxj


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‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
On Wednesday, February 13, 2019 7:53 PM, Joao Joyce <joao.jo...@netcabo.pt> 
wrote:

> Hi,
>
> Recently I've asked about the possibility of user identification across 
> multiple payments which now I get that it is a bad idea.
>
> In that previous exchange ZmnSCPxj suggested that for purposes of proving 
> payments (for instance for a user to redownload an already bought document) 
> the service could just ask for the preimage of an invoice that was already 
> paid.
>
> I was wondering if there's interest in standardising a way for payment 
> invoices to contain information to signal wallets that they can skip the 
> payment process if they are able to provide the service with a preimage for a 
> previous payment for that particular item.
>
> Use cases could extend beyond simple download of an item. For instance paying 
> for 30 days of a service and every time you want to access that service you 
> just scan the LN invoice.
>
> I'm aware that intermediate nodes know about this preimage but I'd rather not 
> focus on that for now.
>
> Thank you,
> João Joyce


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