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.