Date: Mon, 11 May 2020 22:12:33 +0000
To: 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-341: Committing to all scriptPubKeys in the
	signature message
Precedence: list

Hi all,

On Tuesday, May 5, 2020 3:20 AM, Jonas Nick via bitcoin-dev <bitcoin-dev@li=
sts.linuxfoundation.org> wrote:

> This is a reasonable suggestion. Committing to every spent scriptPubKey a=
nd
> therefore every element of the TxOut instead of just the amount makes sen=
se
> conceptually. And it would be a small diff (~4 lines + rationale) compare=
d to
> the current bip-taproot version.

I agree.

There have been several steps so far towards making it possible for signers=
 to determine whether they can safely sign with just O(1) information per i=
nput. This was initially attempted in BIP141 (by committing to spent input,=
 to thwart the ability to lie about fees to ofline signers), and is improve=
d in the current BIP341.

I think the CoinJoin + offline signer model indeed shows that is still inco=
mplete, as it is yet another example where a signer may need to be provided=
 with the entire creating transaction, which would be very unfortunate.

It's also counter to the model proposed by BIP147 (PSBT) workflows: the ass=
umption is effectively already that it is sufficient to provide signers wit=
h just amount + scriptPubKey of the spent outputs. It feels very natural th=
at signatures then indeed also need to commit to all that data; otherwise t=
here should be ways that this information can be undetectably wrong.

AJ's approach seems great. It means not increasing the per-signature hashin=
g, while retaining the ability to cache information across BIP141/BIP341.

As for coinbaseness and height: these are indeed also things currently kept=
 track of in the UTXO set, but I don't think any signer is using this infor=
mation to determine whether to sign or not (which I think is the minimum re=
quirement for it to be included in a signature hash, see above). Signing he=
ight would cripple the ability to spend unconfirmed outputs, or force signe=
rs to reveal they're doing so (if done through a separate sighash flag) - b=
oth of which would be undesirable. That leaves coinbaseness, but I think th=
e utility is very low.

The only downside is that this potentially slows down review, but I agree w=
ith earlier comments that it's hard to see how this would hurt. I also thin=
k it's important to get these things right from the start. Many things insi=
de BIP341/BIP342 are extensible with future softforks, but signature hashes=
 for key-path spends is not one of them (the set of potential signature has=
h semantics must be committed to directly by the output, so changing them r=
equires a new output type - which would be highly unfortunate for fungibili=
ty reasons).

Thus, unless there are objections, I'd like to go through with this and mak=
e the suggested changes.

Thoughts?

--
Pieter