Magic Wormhole: get things from one computer to another, safely

Nope. It needs to contact the "mailbox server" to coordinate the rest of the protocol. Two machines with local connectivity (e.g. on the same LAN, but your WAN connection is broken) could still implement the second half of the protocol, where they use each other's IP addresses to make a direct connection, but without the first half they couldn't learn those addresses or exchange the key-negotiation messages.

We've sketched out some approaches to working in a disconnected environment like that, using local multicast and mDNS/ZeroConf/Bonjour to act as an alternate mailbox server (https://github.com/magic-wormhole/magic-wormhole/issues/48). There's still design work needed, though, and I fear it would degrade the experience for fully-connected nodes (extra timeouts), so it might want to be opt-in with a `--offline` flag on both sides.