Subject: Re: [bitcoin-dev] Zero-knowledge proofs e.g. Schnorr are incompatible with address signing without compromise

From: Pieter Wuille <[email-masked]>
Date: Thu, 28 Jul 2022 15:27:05 +0000

------- Original Message -------
On Thursday, July 28th, 2022 at 3:27 AM, Ali Sherief via bitcoin-dev <[email-masked]> wrote:

> Essentially, zero-knowledge proofs such as Schnorr are not compatible with address message signing - the public key cannot be retrieved from the address or the signature, so the address does not actually prove the authenticity of a Schnorr signature. That's why the public key is required as an input in the first place.

Yes, that's an intentional design choice in BIP340, see note 5: https://github.com/bitcoin/bips/blob/master/bip-0340.mediawiki#cite_ref-5-0. The choice is either batch verifiability or public key recovery.

I regret ever using public key recovery when introducing the old legacy message signing scheme. It should just have used script signatures like BIP322 proposes.

> In order to make it compatible with the address signing mechanism, the zero-knowledge part would have to be sacrificed in my BIP, or else a completely separate message signing format just for Taproot would be required

You can avoid relying on public key recovery, and include the public key + BIP340 signature in the encoded signature.

> (which, in my view, is redundant - there is already the draft BIP322 which can verify anything and everything, but nobody is implementing that

I think it would be much better if people would cooperate to get BIP322 to move forward than to keep inventing other formats. It's the obvious solution in my opinion: not restricted to single-key policies, compatible with every script type, and trivially extensible to future schemes.

> , just like BIP340).

How so? Every taproot compatible wallet has a BIP340 implementation.

Cheers,

--
Pieter