For a more unix-y take on an encrypted store-and-forward network (so no persistence necessary when communicating, not even simplex), take a look at http://www.nncpgo.org/index.html (HTTPS: https://www.complete.org/nncp/). Key exchange happens completely out-of-band and it can also use pretty much any link you throw at it. On mobile, I've used it on Android using Termux but it's not the best experience for sure.
Reticulum is the cryptography-based networking protocol for both local and wide-area networks built on readily available hardware. Reticulum can operate even with very high latency and extremely low bandwidth.
The vision of Reticulum is to allow anyone to operate their own sovereign communication networks, and to make it cheap and easy to cover vast areas with a myriad of independent, interconnectable and autonomous networks.
The good part would be that it's really easy to build good UX on nostr. So if reticulum is having hard time getting good UX, then Nostr may actually be a reasonable strategy.
@fiatjaf, any interest in this?
the Sovereign Net.The Sovereign Trinity