for cafe-society.news, I wish to have a user train a model, then encrypt it to the public keys of multiple subscribers. In order to subscribe, someone will send a lightning payment to the trainer and put their public encyption key in the memo. As most trainers will be hugely successful, it will not make sense to send a private nostr message to each and every subscriber. I would like to encrypt content to to a keybox https://www.php.net/manual/en/function.sodium-crypto-box-keypair.php and post one encrypted message to nostr. My question is does anyone know of a keybox library for nostr keys? It doesn't really break anything to use 25519 keys, but it would be nice to use the same keys.