Date: Mon, 21 Dec 2020 22:57:15 +0000
To: Karl-Johan Alm <karljohan-alm@garage.co.jp>,
 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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 <CALJw2w4bFp8qcxNzz7hn_ZeGG8WWTPu-mRv6A4cPt66EHimyeg@mail.gmail.com>
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Subject: Re: [bitcoin-dev] BIP-0322 (generic signmessage) improvements
Precedence: list

On Sunday, December 20, 2020 9:37 PM, Karl-Johan Alm via bitcoin-dev <bitco=
in-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 inconcl=
usive as well. That doesn't really reduce the functionality (given that "in=
conclusive" was already a potential result), and it obviously makes it much=
 more accessible to a variety of software.

This suggestion breaks the original use of inconclusive though: the ability=
 to detect that future features are used in the signature. The idea was to =
use divergence between "consensus valid" and "standardness valid" as a prox=
y for future extensions to be detected (e.g. OP_NOPn, future witness versio=
ns, ...). I think it's undesirable that these things now become uncondition=
ally invalid (until the BIP is updated, but once that happens old validator=
s will give a different result than new ones).

Since the BIP no longer relies on a nebulous concept of standardness, and i=
nstead specifically defines which standardness features are to be considere=
d, this seems easy to fix: whenever validation fails due to any of those, r=
equire reporting inconclusive instead of invalid (unless of course somethin=
g actually invalid also happens). In practice I guess you'd implement that =
(in capable validators) by still doing validation twice, once with all feat=
ures enabled that distinguish between valid/invalid, and if valid, again bu=
t now with the features enabled that distinguish between valid and (invalid=
 or inconclusive).

Cheers,

--
Pieter