Martin Habovštiak [ARCHIVE] on Nostr: 📅 Original date posted:2021-07-15 📝 Original message: What would happen in 2) ...

📅 Original date posted:2021-07-15
📝 Original message:
What would happen in 2) if the node has data but the peer returned an
incorrect state?

On Wed, Jul 14, 2021, 20:13 Christian Decker <decker.christian at gmail.com>
wrote:

> Not quite sure if this issue is unique to eltoo tbh. While in LN-penalty
> loss-of-state equates to loss-of-funds, in eltoo this is reduced to
> impact only funds that are in a PTLC at the time of the loss-of-state.
>
> We have a couple of options here, that don't touch the blockchain, and
> are therefore rather lightweight:
>
> 1) Do nothing and keep the incentive to keep up to date backups. It
> still is a reduction in risk w.r.t. LN-penalty, since this is just an
> append only log of secrets, and old secrets don't harm you like
> attempting to close with an old commitment would.
> 2) Use the peer-storage idea, where we deposit an encrypted bundle with
> our peers, and which we expect the peers to return. by hiding the fact
> that we forgot some state, until the data has been exchanged we can
> ensure that peers always return the latest snapshot of whatever we gave
> them.
>
> The latter is the encrypted-blob idea that Rusty has been proposing for
> a while now.
>
> Cheers,
> Christian
>
> Anthony Towns <aj at erisian.com.au> writes:
> > Hello world,
> >
> > Suppose you have some payments going from Alice to Bob to Carol with
> > eltoo channels. Bob's lightning node crashes, and he recovers from an
> > old backup, and Alice and Carol end up dropping newer channel states
> > onto the blockchain.
> >
> > Suppose the timeout for the payments is a few hours away, while the
> > channels have specified a week long CSV delay to rectify any problems
> > on-chain.
> >
> > Then I think that that means that:
> >
> > 1) Carol will reveal the point preimages on-chain via adaptor
> > signatures, but Bob won't be able to decode those adaptor signatures
> > because those signatures will need to change for each state
> >
> > 2) Even if Bob knows the point preimages, he won't be able to
> > claim the PTLC payments on-chain, for the same reason: he needs
> > newer adaptor signatures that he'll have lost with the state update
> >
> > 3) For any payments that timeout, Carol doesn't have any particular
> > incentive to make it easy for Bob to claim the refund, and Bob won't
> > have the adaptor signatures for the latest state to do so
> >
> > 4) But Alice will be able to claim refunds easily. This is working how
> > it's meant to, at least!
> >
> > I think you could fix (3) by giving Carol (who does have all the adaptor
> > signatures for the latest state) the ability to steal funds that are
> > meant to have been refunded, provided she gives Bob the option of
> claiming
> > them first.
> >
> > However fixing (1) and (2) aren't really going against Alice or Carol's
> > interests, so maybe you can just ask: Carol loses nothing by allowing
> > Bob to claim funds from Alice; and Alice has already indicated that
> > knowing P is worth more to her than the PTLC's funds -- otherwise she
> > wouldn't have forwarded the PTLC to Bob in the first place.
> >
> > Likewise, everyone's probably incentivised to negotiate cooperative
> > closes instead of going on-chain -- better privacy, less fees, and less
> > delay before the funds can be used elsewhere.
> >
> > FWIW, I think a similar flaw exists even in the original eltoo spec --
> > Alice could simply decline to publish the settlement transaction until
> > the timeout has been reached, preventing Bob from revealing the HTLC
> > preimage before Alice can claim the refund.
> >
> > So I think that adds up to:
> >
> > a) Nodes should share state on reconnection; if you find a node that
> > doesn't do this, close the channel and put the node on your enemies
> > list. If you disagree on what the current state is, share your most
> > recent state, and if the other guy's state is more recent, and all
> > the signatures verify, update your state to match theirs.
> >
> > b) Always negotiate a mutual/cooperative close if possible, to avoid
> > actually using the eltoo protocol on-chain.
> >
> > c) If you want to allow continuing the channel after restoring an old
> > state from backup, set the channel state index based on the real
> time,
> > eg (real_time-start_time)*(max_updates_per_second). That way your
> > first update after a restore from backup will ensure that any old
> > states that your channel partner may not have told you about are
> > invalidated.
> >
> > d) Accept that if you lose connectivity to a channel partner, you will
> > have to pay any PTLCs that were going to them, and won't be able
> > to claim the PTLCs that were funding them. Perhaps limit the total
> > value of inbound PTLCs for forwarding that you're willing to accept
> > at any one itme?
> >
> > Also, layered commitments seem like they make channel factories
> > complicated too. Nobody came up with a way to avoid layered commitments
> > while I wasn't watching did they?
> >
> > Cheers,
> > aj
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