Proof of Payment

Proving that you paid someone is trivial on a transparent blockchain. Just point to the transaction, which lives on the chain, and which anyone can read. But on a privacy-preserving blockchain like Neptune Cash, the best you can do is point to some string of cryptographic data that lives there. And since it is privacy-preserving, that data is by design equivalent to gibberish without the right context. So it can't possibly be used as evidence to resolve a payment dispute. What we need is a way to generate a zero knowledge proof saying "I initiated the transaction correctly; if you didn't receive the funds it's because you gave me the wrong address or screwed up in some other way".

In the abstract, proof-of-payment is an important feature of money and the reason why we have receipts. The thing it resolves is payment disputes. I guess disputes might be resolved through some other technology, but a proof-of-payment seems pretty surgical.

One notes that both Monero 1 and ZCash 2 have proofs-of-payment. ZCash's version does not seem to be zero-knowledge.