I’ll preface by saying there is work on a new DM NIP I believe that tries to address the metadata to begin with. This is a better solution as it prevents the relay operator from snooping as well.

NIP-42 is an auth message from the relay to the client that can occur at anytime during the websocket connection. For DMs, it would work like this:

- You (via your client) make a REQ for kind 4 events (DMs) for your pubkey

- The relay sends back an AUTH challenge

- You sign the auth challenge and send it back to the relay who validates that you are the one requesting your own DMs.

If you signed the auth message with a different pubkey than you requested DMs for, the relay would reject your AUTH.