NIP-04

Encrypted direct messages

NIP-04 is an older direct-message format. It encrypts the text of a private message so only the sender and receiver should be able to read it, but it does not hide as much surrounding information as newer private-message designs.

You can think of it as putting a note inside a locked box, while still leaving the box shape and delivery route fairly visible. It was useful early on, but modern Nostr clients usually prefer newer private messaging NIPs when they can.

Read more in the official 04.md NIP document.