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To: Anthony Towns <aj@erisian.com.au>
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Subject: Re: [bitcoin-dev] [Lightning-dev] More thoughts on NOINPUT safety
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Good morning aj,
First off, I have little to no idea of the issues at the lower-level Bitcoi=
n.
In any case ---
> - alternatively, we could require every script to have a valid signatur=
e
> that commits to the input. In that case, you could do eltoo with a
> script like either:
>
> <A> CHECKSIGVERIFY <B> CHECKSIG
> or <P> CHECKSIGVERIFY <Q> CHECKSIG
>
>
> where A is Alice's key and B is Bob's key, P is muSig(A,B) and Q is
> a key they both know the private key for. In the first case, Alice
> would give Bob a NOINPUT sig for the tx, and when Bob wanted to publish
> Bob would just do a SIGHASH_ALL sig with his own key. In the second,
> Alice and Bob would share partial NOINPUT sigs of the tx with P, and
> finish that when they wanted to publish.
>
> This is a bit more costly than a key path spend: you have to reveal
> the taproot point to do a script (+33B) and you have two signatures
> instead of one (+65B) and you have to reveal two keys as well
> (+66B), plus some script overhead. If we did the <P,Q> variant,
> we could provide a "PUSH_TAPROOT_KEY" opcode that would just push
> the taproot key to stack, saving 33B from pushing P as a literal,
> but you can't do much better than that. All in all, it'd be about 25%
> overhead in order to prevent cheating. [0]
>
> I think that output tagging doesn't provide a workable defense against th=
e
> third party malleability via a deeper-than-the-CSV-delay reorg mentioned
> earlier; but requiring a non-NOINPUT sig does: you'd have to replace
> the non-NOINPUT sig to make state 5 spend state 3 instead of state 4,
> and only the holders of the appropriate private key can do that.
At my point of view, if a NONINPUT sig is restricted and cannot be used to =
spend an "ordinary" 2-of-2, this is output tagging regardless of exact mech=
anism.
So the restriction to add a non-NOINPUT sig in addition to a NOINPUT sig is=
still output tagging, as a cooperative close would still reveal that the o=
utput is not a 2-of-2.
Ideally, historical data of whether onchain coin was used in Lightning or n=
ot should be revealed as little as possible.
So in a cooperative close (which we hope, to be a common case), ideally the=
spend should look no different from an ordinary 2-of-2 spend.
Of course if the channel is published on Lightning, those who participated =
in Lightning at the time will learn of it, but at least the effort to remem=
ber this information is on those who want to remember this fact.
Now, this can be worked around by adding a "kickoff" transaction that spend=
s the eltoo setup transaction.
The eltoo setup transaction outputs to an ordinary 2-of-2.
The kickoff outputs to an output that allows NOINPUT.
Then the rest of the protocol anchors on top of the kickoff.
The kickoff is kept offchain, until a non-cooperative close is needed.
Of course, as it is not a NOINPUT itself, it must need onchain fees attache=
d to it.
This of course complicates fees, as we know.
Alternately maybe the kickoff can be signed with `SIGHASH_SINGLE | SIGHASH_=
ANYONECANPAY` so that it is possible to add a fee-paying UTXO to it.
Regards,
ZmnSCPxj
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