Second, I'm going to try to do tests with Elliptic keys and base64. Thanks for the advice @eapl
Finally, I'd like to give my opinion. Secure direct messages are a feature that ActivityPub and Mastodon don't have, to give an example. By including it as an extension, we're already taking a significant leap forward from the competition. Does it make sense to include it in a public feed? In fact, we're already doing that. When we reply to a user, mentioning them at the beginning of the message, it's already a direct message. The message is within a thread, perhaps breaking the conversation. Direct messages would help isolate conversations between 2 users, as well as keeping a thread cleaner and maintaining privacy. I insist, it's optional, it doesn't break compatibility with any client and implementing it isn't complex. If you don't like it, you're free to not use it. If you don't have a public key, no one can send you direct messages.
Bush fire sunsets are the nicest
Some people send "get better" messages, but someone sent me some flowers, so I have something to look at during recovery. This picture shows a white and a pink flower from that bunch.