Nostr has state of the art encryption and even better, has MLS in the works. Yes its initial focus was social media but there is not really a limit to what can and is being built with regards to the "other stuff" 

 I made a mistake. I should have double checked. Secrecy and privacy are two different things. Secrecy can be added on top of any communication. Privacy cannot, at least no generalizable method is known yet. 

 Privacy is fucking hard and keeps getting harder