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From: Rusty Russell <rusty@rustcorp.com.au>
To: Luke Dashjr <luke@dashjr.org>, bitcoin-dev@lists.linuxfoundation.org
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Subject: Re: [bitcoin-dev] Making OP_TRUE standard?
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Luke Dashjr <luke@dashjr.org> writes:
> An OP_TRUE-only script with a low value seems like a good example of where the
> weight doesn't reflect the true cost: it uses a UTXO forever, while only
> costing a weight of 4.
>
> I like Johnson's idea to have some template (perhaps OP_2-only, to preserve
> expected behaviour of OP_TRUE-only) that when combined with a 0-value is
> always valid only if spent in the same block.
>
> I wonder if it would make sense to actually tie it to a transaction version
> bit, such that when the bit is set, the transaction is serialised with +1 on
> the output count and 00000000000000000181 is simply injected into the
> transaction hashing... But for now, simply having a consensus rule that a bit
> MUST be set for the expected behaviour, and the bit may ONLY be set when the
> last output is exactly 00000000000000000181, would allow us to code the
> transaction serialisation up later. (Maybe it should be the first output
> instead of the last... Is there any legitimate reason one would have multiple
> such dummy outputs?)
Your zero-val-OP_TRUE-can't-be-spent-after-same-block SF is interesting,
but if we want a SF just give us SIGHASH_NOINPUT and we'll not need this
at all (though others still might). It's nicer than the previous
discussions on after-the-fact feebumping[1] though.
Meanwhile, our best mitigation against UTXO bloat is:
1. Make the fees as low as possible[2]
2. Put a CSV delay on the to-remote output (currently there's asymmetry)
3. Attach more value to the OP_TRUE output, say 1000 satoshi.
But turns out we probably don't want an OP_TRUE output nor P2SH, because
then the spending tx would be malleable. So P2WSH is is.
This brings us another theoretical problem: someone could spend our
OP_TRUE with a low-fee non-RBF tx, and we'd not be able to use it to
CPFP the tx. It'd be hard to do, but possible. I think the network
benefits from using OP_TRUE (anyone can clean, and size, vs some
only-known-to-me pubkey) outweighs the risk, but it'd be nice if OP_TRUE
P2WSH spends were always considered RBF.
Thanks,
Rusty.
[1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2018-April/015864.html
[2] Because bitcoin core use legacy measurements, this is actually 253
satoshi per kilosipa for us, see https://github.com/ElementsProject/lightning/commit/2e687b9b352c9092b5e8bd4a688916ac50b44af0
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