Plasma XT: Plasma Cash with much less per-user data checking

[kfichter]

@tasd

I think there is a slight change in assumptions. With Plasma MVP, the user can delegate the task of watching the chain and initiating withdrawals to someone else, so the user does not actually have to be online regularly. The delegate could grief the user by initiating unnecessary withdrawals, or could fail to do their job, but could not steal the user’s coins.

I would agree that there is a change of assumptions w.r.t Plasma MVP. I don’t believe that this changes any assumptions w.r.t Plasma Cash, particularly because of the discussion here: Watchtowers may not work in Plasma (Cash) - #5 by danrobinson.

Although I think it’s probably fine for a first implementation if users have to be online, it would definitely be very useful for users to be able to outsource this. I haven’t thought about it a lot - one solution off the top of my head would be for users to make an on-chain transaction that somehow specifies a third party that can sign off on the checkpoint instead. This would basically look like a 2of2 multisig with a special key (for the third party) that can sign off alone. The third party could then be sure that they haven’t signed off and challenge whenever they see a false positive.

Of course, this means that the third party could temporarily grief the user by refusing to sign the signature. Maybe this isn’t the worst thing if users can easily submit another transaction changing their third party provider.