[Lightning-dev] Pay-to-Open and UX improvements

[Ethan Heilman]

>From where I'm sitting the fact that OP_CAT allows people to build
more powerful constructions in Bitcoin without introducing additional
complexity at the consensus layer is a positive not a negative. Using
OP_CAT or OP_SUBSTRING to enforce ECDSA nonce reuse is a very powerful
protocol tool for enforcing fairness in layer two protocols.

On Tue, Dec 17, 2019 at 11:27 AM ZmnSCPxj via Lightning-dev
<lightning-dev at lists.linuxfoundation.org> wrote:
>
> Good morning t-bast,
>
> Further, we can enforce that RBF is signalled for every spend of the output by:
>
>     <0> OP_CHECKSEQUENCEVERIFY OP_DROP <R> OP_SWAP OP_CAT <ACINQ> OP_CHECKSIG
>
> Requiring that RBF is signalled gives a little more assurance.
> Suppose ACINQ becomes evil and double-spends the output.
> The transaction that is posted in the mempool must be marked by RBF due to the `OP_CHECKSEQUENCEVERIFY` opcode, since `nSequence` also doubles as RBF opt-in.
> Then anyone who notices the double-spend can RBF the double-spending transaction to themselves rather than ACINQ.
> This also further publishes ACINQ private key, until the winning transaction has an `OP_RETURN` output that pays the entire value as fees and nobody can RBF it further.
>
> This is a minor increase in the assurability of the construction, by making any output that is double-spent directly revocable in favor of the miners.
> Again, it requires `OP_CAT`, which is a very dangerous opcode, allowing such powerful constructions.
>
> Regards,
> ZmnSCPxj
>
>
> > Thanks a lot David for the suggestion and pointers, that's a really interesting solution.
> > I will dive into that in-depth, it could be very useful for many layer-2 constructions.
> >
> > Thanks ZmnSCPxj as well for the quick feedback and the `OP_CAT` construction,
> > a lot of cool tricks coming up once (if?) we have such tools in the future ;)
> >
> > Le mar. 17 déc. 2019 à 16:14, ZmnSCPxj <ZmnSCPxj at protonmail.com> a écrit :
> >
> > > Good morning David, t-bast, and all,
> > >
> > > > I'm not aware of any way to currently force single-show signatures in
> > > > Bitcoin, so this is pretty theoretical. Also, single-show signatures
> > > > add a lot of fragility to any setup and make useful features like RBF
> > > > fee bumping unavailable.
> > >
> > > With `OP_CAT`, we can enforce that a particular `R` is used, which allows to implement single-show signatures.
> > >
> > >     # Assuming signatures are the concatenation of (R,s)
> > >     <R> OP_SWAP OP_CAT <ACINQ> OP_CHECKSIG
> > >
> > > The above would then feed `s` only on the witness stack.
> > >
> > > Regards,
> > > ZmnSCPxj
>
>
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