Date: Tue, 22 Dec 2020 00:22:37 +0000
To: Pieter Wuille <bitcoin-dev@wuille.net>,
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
From: Pieter Wuille <bitcoin-dev@wuille.net>
Reply-To: Pieter Wuille <bitcoin-dev@wuille.net>
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Subject: Re: [bitcoin-dev] BIP-0322 (generic signmessage) improvements
Precedence: list

On Monday, December 21, 2020 2:57 PM, Pieter Wuille via bitcoin-dev <bitcoi=
n-dev@lists.linuxfoundation.org> wrote:

> On Sunday, December 20, 2020 9:37 PM, Karl-Johan Alm via bitcoin-dev bitc=
oin-dev@lists.linuxfoundation.org wrote:
>
> > Thanks a lot for taking the time to brush up the BIP. For what it's
> > worth, I am all for these changes, and I see them as clear
> > improvements all around.
> > IIRC Pieter was the one who originally suggested the two-checks
> > approach (invalid, inconclusive, valid) which is being modified here,
> > so would be good if you chimed in (or not -- which I'll assume means
> > you don't mind).
>
> I agree with the idea of permitting incomplete validators to return incon=
clusive as well. That doesn't really reduce the functionality (given that "=
inconclusive" was already a potential result), and it obviously makes it mu=
ch more accessible to a variety of software.
>
> This suggestion breaks the original use of inconclusive though: the abili=
ty to detect that future features are used in the signature. The idea was t=
o use divergence between "consensus valid" and "standardness valid" as a pr=
oxy for future extensions to be detected (e.g. OP_NOPn, future witness vers=
ions, ...). I think it's undesirable that these things now become unconditi=
onally invalid (until the BIP is updated, but once that happens old validat=
ors will give a different result than new ones).
>
> Since the BIP no longer relies on a nebulous concept of standardness, and=
 instead specifically defines which standardness features are to be conside=
red, this seems easy to fix: whenever validation fails due to any of those,=
 require reporting inconclusive instead of invalid (unless of course someth=
ing actually invalid also happens). In practice I guess you'd implement tha=
t (in capable validators) by still doing validation twice, once with all fe=
atures enabled that distinguish between valid/invalid, and if valid, again =
but now with the features enabled that distinguish between valid and (inval=
id or inconclusive).

Re-reading your proposed text, I'm wondering if the "consensus-only validat=
ion" extension is intended to replace the inconclusive-due-to-consensus-and=
-standardness-differ state. If so, I don't think it does, and regardless it=
 doesn't seem very useful.

What I'm suggestion could be specified this way:
* If validator understands the script:
  * If signature is consensus valid (as far as the validator knows):
    * If signature is not known to trigger standardness rules intended for =
future extension (well-defined set of rules listed in BIP, and revisable): =
return valid
    * Otherwise: return inconclusive
  * Otherwise: return invalid
* Otherwise: return inconclusive

Or in other words: every signature has a well-defined result (valid, invali=
d, inconclusive) + validators may choose to report inconclusive for anythin=
g they don't understand.

This has the property that as long as new consensus rules only change thing=
s that were covered under for-future-extension standardness rules, no two v=
alidators will ever claim valid and invalid for the same signature. Only va=
lid+inconclusive or invalid+inconclusive.

Cheers,

--
Pieter