anarkio on Nostr: Monero TXs can be decrypted 1) via the private view key or 2) via the TX key. ...

Monero TXs can be decrypted 1) via the private view key or 2) via the TX key. Providing the TX key is 100% proof that x XMR was sent to address x with TXID x (the relevant ed25519 calculations can be seen here: https://codeberg.org/anarkio/pages/src/branch/main/nostril/js/verify-tx-key.js). Normally this would be bad for privacy and you would keep the TX key secret, but it is OK if the tips are intended to be publicly viewable and verifiable (and it is more privacy-preserving than the alternative, which is providing the private view key).