Method for retrieving past payment preimages

[wbobeirne]

Maintaining lightning purchases in a decentralized way has been a big use case for WebLN for me, but going the node identity route poses some obvious privacy issues (that I appreciate everyone raising concerns about!) However, with preimages, we can provide proof that we paid for something without revealing our node. Distilling this process into a WebLN method could allow for accessing things we've paid for while maintaining our anonymity.

This poses a few problems though:

• What assures the server that this isn't other user's preimage that's been shared?
• What does the server provide WebLN to give it enough information to look up the past payment? An exact match of the memo? Can something be standardized around this?
• What tools do node APIs provide to "search" for payments? Once someone's node has gotten into the many thousands, is this feasible anymore?

I'd be interested to hear if anyone has particular suggestions around these issues, or of there's anything coming up around the protocol to make this use-case easier.

[torkelrogstad]

Here's a use case for retrieving payment preimages:

At Suredbits, we are working on APIs that require payment over LN, but send the data before payment is made. The data is sent encrypted, and the decryption key is the payment preimage. We're working on including this in our online API explorer. It'd be very cool to be able to receive an encrypted blob of information + an invoice, and then decrypt the information locally after the invoice is paid.

One simple thing that would help with this use case is simply returning the preimage in the response when paying an invoice. I don't really see any downsides to this.

For looking up past preimages it would make sense to be able to query by payment hash/invoice, IMO.

[torkelrogstad]

I see now that Lightning Joule returns the preimage when paying an invoice:

webln.sendPayment("lnbc100n1pw5q727pp594697u97esh5uau7vj42z7u4z4vdk2lzm4yj3k2u5y0fzt3cfl3sd8y0v3xgatjv96xjmmwygarydfsxqczcgnrdpskumn9ds3r5gn5d93kketjwv3zcgn9wejkuapz8g38xatzwd3hy6tzv53zcgn4w45kgg36yf3rwdfnxcexgdedvscnqc3dxsmxzcfd893kyvfdx4jkxwp4xdnrzcf4xsmzytpzv4uxx6rpdenk2g36yf3xjardv4uzytpzwduk6cn0ds3r5gj923y9256yyf7sxqrrsswzcgdts9fpr8v0l4ac7rk7v9n65ghk5yxgkemh5z85fn3q6j63s9udrnfe8h78lf2ywl9248ampuk9c3zyus6v3ukemk85ewtmvngrcq76nges").then(res => console.log("send payment result: %O", res))

send payment result:  
Object { preimage: "4fcd6da9c87220276626bb7c9b347b583333103ed2857bdc14e7891d483cf670" }

This does not seem to line up with what's on webln.dev: https://webln.dev/#/api/send-payment

Am I misunderstanding something here?

[wbobeirne]

Looks like that's a misdocumentation, sorry about that. The typescript types show the real truth of it: https://github.com/joule-labs/webln/blob/master/src/provider.ts#L15-L17