I've done some research into the serval mesh project and Briar and similar; I do wish most cell phones had some mesh networking installed by default. Something to enable granny to send a text message to me via piggybacking off other people who happen to be near enough to have P2P communication.
This would have to be a protocol designed for offline-tolerance I suppose, so something like auto-exchanging private keys while the network is good and/or in person if network is down for message security, then when in no signal mode, regularly transmit and receive small messages encrypted with the same key-pairs as the real ones, then if they can decrypt the short test message (short enough to be easy to try / won't eat too much battery life), establish a dedicated connection long enough to transmit full messages back and forth before disconnecting.
IDK. Probably serval mesh, briar, gnutella or similar already have this figured out. I'd like it to be end-user easy for us to prep for that though so if we are in a venue like a theme park or stadium our phones can transparently be P2P over Blue-Fi-Drop
Have you heard about the GoTenna mesh project, https://gotennamesh.com/ ? I haven't got one yet, but it looks like the sort of thing you're describing (though its a device that works with existing smartphones, rather than being a protocol our phones could run. An advantage here is that with sufficient GoTenna mesh users in an area, one's anonymity could be increased, as typically (at least in the USA and most of western Europe) one can't buy SIM cards to access telephone networks without one's ID.
This would have to be a protocol designed for offline-tolerance I suppose, so something like auto-exchanging private keys while the network is good and/or in person if network is down for message security, then when in no signal mode, regularly transmit and receive small messages encrypted with the same key-pairs as the real ones, then if they can decrypt the short test message (short enough to be easy to try / won't eat too much battery life), establish a dedicated connection long enough to transmit full messages back and forth before disconnecting.
IDK. Probably serval mesh, briar, gnutella or similar already have this figured out. I'd like it to be end-user easy for us to prep for that though so if we are in a venue like a theme park or stadium our phones can transparently be P2P over Blue-Fi-Drop