Subject: Re: [bitcoin-dev] Statechain implementations

From: Ruben Somsen <[email-masked]>
Date: Thu, 26 Mar 2020 19:53:13 +0100

Hey Christian,

Thanks for chiming in :)

>It might be worth adopting the late fee binding we have in eltoo

That is where my thinking originally went as well, but then I remembered
that this alters the txid, causing the settlement tx to become invalid.
What I am suggesting should be functionally the same (albeit less
space-efficient): a secondary output that can be spent by anyone, which can
be used to fee bump the kickoff tx with CPFP. I believe this same idea was
considered for Lightning as well at some point. Do you happen to recall if
there was some kind of non-standardness issue with it?

>Wouldn't that result in a changing pubkey at each update, and thus require
an onchain move to be committed?

I have yet to take a closer look at the math, but my understanding is that
the same key (x) gets redistributed. First x = s1 + o1 and after the
transfer x = s2 + o2 (not the actual math, but it demonstrates how the
transitory key can change from o1 to o2). Assuming s1 is then thrown away
(trust assumption), o1 becomes harmless information.

Cheers,
Ruben

On Thu, Mar 26, 2020 at 6:17 PM Greg Sanders <[email-masked]> wrote:

> > Wouldn't that result in a changing pubkey at each update, and thus
> require an onchain move to be committed?
>
> Suggestion was in line with original proposal where no keys are changing
> ever, just not presupposing existence of MuSig.
>
> On Thu, Mar 26, 2020 at 1:15 PM Christian Decker via bitcoin-dev <
> [email-masked]> wrote:
>
>> Ruben Somsen via bitcoin-dev <[email-masked]>
>> writes:
>> > Regarding modification 1, I agree with ZmnSCPxj that
>> > Decker-Wattenhofer is your next best option, given that eltoo is not
>> > yet available. But if you are going to use a kickoff transaction, keep
>> > in mind that every previous owner will have a copy of it. Because of
>> > this, you can't include a fee, and will instead need to have a second
>> > output for CPFP. This way a previous owner will at least have to pay
>> > the fee if they want to publish it. Note that it's still an
>> > improvement, because even if the kickoff transaction gets posted, it
>> > basically becomes no different than what it would have been, had you
>> > not used a kickoff transaction at all.
>>
>> It might be worth adopting the late fee binding we have in eltoo by
>> having the kickoff transaction input spending the funding tx signed with
>> sighash_single. This works because we only have 1 input and 1 output
>> that we really care about, and can allow others to attach fees at
>> will. That'd at least remove the need to guess the feerate days or
>> months in advance and thus having to overestimate.
>>
>> > Regarding modification 2, I like it a lot conceptually. It hadn't
>> > occurred to me before, and it's a clear security improvement. The only
>> > question is something Greg Sanders mentioned: whether it's enough to
>> > justify the added complexity of using 2P ECDSA. The alternative would
>> > be to simply use a regular 2-of-2 multisig (until Schnorr arrives,
>> > possibly).
>>
>> Wouldn't that result in a changing pubkey at each update, and thus
>> require an onchain move to be committed?
>>
>> > I'm looking forward to seeing statechains become a reality.
>>
>> That'd indeed be great :-)
>>
>> Cheers,
>> Christian
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