Background: https://gist.github.com/callebtc/a6cc0bd2b6f70e081e478147c40fc578

Proof of concept:

## Very short summary of the problem

In Lightning, you don't know the fee in advance. But in ecash, you need to pay for everything upfront (ecash isn't divisible, the mint can't simply return the change back to the user when they overpay Lightning fees).

Solution: The wallet provides a set of "blank outputs" (blinded secrets) when asking a mint to make a Lightning payment.

The blank outputs don't have an amount but the mint can "imprint" amounts in them and return blind signatures (= ecash) to the wallet if they overpaid fees.

This is beautiful. We sat down in a virtual circle (thank you phyro, #[0] ) and thought hard about possible solutions and discovered this.

Unpredictable LN fees and backups were the two big issues in Cashu's protocol design and now we fixed both! 🎉