Feature request: Offline send

[casey]

I'd like to be able to send transactions from a machine without network access, however this doesn't seem to be currently supported. Correct me if I'm wrong, but the finalize command expects to be able to broadcast the finalized transaction immediately, so if the sending wallet is offline, it won't be able to complete the send, receive, finalize sequence.

I think this is valuable for security, since it allows sending funds from secure machine that isn't connected to the internet.

The simplest implementation would be to add an option to finalize to dump the final transaction, which could then be moved to an online machine for broadcast. Or, if #2473 allowed an online-invoice, offline-send, online-finalize flow, that would work too, and would potentially mean one less transfer between machines. (As opposed to an offline-send, online-receive, offline-finalize-dump, online-broadcast flow.)

[casey]

I propose the following implementation:

• Add grin wallet finalize --output FILE to save a finalized transaction to FILE instead of broadcasting it to the network
• Add a grin server broadcast --input FILE command to broadcast a finalized transaction to the network. Since no wallet is necessary to broadcast a transaction, we can avoid involving the wallet at all and make this a grin server command.

If this sounds good, I can get cracking on a PR.

[ignopeverell]

Sounds very reasonable to me.

[Onefox]

You would also need to check on the send option since at the moment there is a node needed for that.

Take a look at the discussion here:
mimblewimble/grin#2227

[yeastplume]

This functionality is in place now via #255