interesting idea. I'm not personally interested on having DM conversations on twtxt (for now), although I see the community could be interested in.

I'd suggest to enable the Discussion section in your Github repo to receive comments, as we did for timeline https://github.com/sorenpeter/timeline/discussions

my first thought is that encrypting messages with Elliptic keys is not as easy as with RSA, although I tried doing something similar a few months ago with ECIES https://github.com/eapl-gemugami/owl/blob/main/src/app/controller/ecies_demo.php

another one would be to allow changing public keys over time (as it may be a good practice [0]). A syntax like the following could help to know what public key you used to encrypt the message, and which private key the client should use to decrypt it:

!<nick url> <encrypted_message> <public_key_hash_7_chars>

Also I'd remove support for storing the message as hex, only allowing base64 (more compact, aiming for a minimalistic spec, etc.)

[0] https://www.brandonchecketts.com/archives/its-2023-you-should-be-using-an-ed25519-ssh-key-and-other-current-best-practices