[elihu]

The legality of encryption on ham isn't dubious, it's explicitly not allowed.

I don't know if there's any legal precedent or official policy regarding digital signatures; I would guess that they're probably okay because they don't obscure the meaning of the communication and anyone can verify them against the sender's public key (assuming that the public keys are published somewhere).

Communication with no privacy but with cryptographically secure signatures might be acceptable for emergency situations. It's unfortunate that ham rules are sufficiently restrictive that most of the tools we use on a day-to-day basis wouldn't be legal to use without substantial modification. But then again, we wouldn't want people trying to log into Facebook/Youtube/Reddit etc.. when the network runs at like 1200 baud (if it's packet radio on the 2 meter or 70 cm bands) or maybe in the low mbps (if it's over some kind of 802.11 b/g/whatever mesh network operating under part 97 rules).

Fortunately, while part 15 rules are pretty restrictive about power, they're less strict about antenna gain, so it's at least theoretically possible to make multi-mile connections without having to operate under ham rules. Building a large network out of point-to-point links with directional antennas, though, would be pretty difficult and laborious even in a non-disaster situation, so realistically I think the best local disaster communications option at the moment is to just use APRS and analog voice over 2 meters and accept that 1980's technology that sort of works is better than a modern internet experience that requires a lot of infrastructure that isn't working or available.

[nbadg]

My understanding is that it's a little bit more grey than that. Modification of signals with an intent to obscure the content of the transmission is explicitly forbidden. But obfuscation for other reasons is not expressly forbidden. So there's a question of "is this incidentally encrypted, or intentionally encrypted", which is (again, from my understanding, which is very limited) why I'm calling it dubious and not simply "forbidden".