"Implement a NIP-17 messaging app"
https://nostr-messages.surge.sh/
FWIW and still a WIP, I am implementing a backend app communication based on NIP-17
https://github.com/trbouma/safebox/blob/acquire_lock/docs/NAUTH-PROTOCOL.md
https://github.com/trbouma/safebox/blob/acquire_lock/docs/NAUTH-PROTOCOL.md