Date: Sat, 29 Jun 2019 09:46:23 +0500
From: Dmitry Petukhov <dp@simplexum.com>
To: Jonathan Underwood <junderwood@bitcoinbank.co.jp>
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Cc: Bitcoin development mailing list <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] BIP174 extension proposal (Global Type:
 PSBT_GLOBAL_XPUB_SIGNATURE)
Precedence: list

=D0=92 Sat, 29 Jun 2019 09:19:41 +0900
Jonathan Underwood <junderwood@bitcoinbank.co.jp> wrote:

> > Other note: you have 'unused' value of 1 for `m` in your scheme, why
> > not require m=3D1 for single-sig case, and use 0 as indicator that
> > there are a serlal number following it?
> > =20
>=20
> 0x00 is single sig, aka, OP_CHECKSIG
>=20
> 0x01 is multisig, aka, 1-of-3, 1-of-2 OP_CHECKMULTISIG

This informatin is available in per-output redeem/witness script,
signer will be able to distinguish between multisig/single-sig by
looking at this script. I think it only need to know the total number
of keys participating in the signing, and check that this number
matches the particulars of redeem/witness script.