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From: Jonathan Underwood <junderwood@bitcoinbank.co.jp>
Date: Sat, 29 Jun 2019 17:11:56 +0900
Message-ID: <CAMpN3mLmVwKwMwjjPGV3Z1JjeLmejMLkTN+3+c0Hu3K0-0GjyA@mail.gmail.com>
To: Dmitry Petukhov <dp@simplexum.com>
Content-Type: multipart/alternative; boundary="0000000000006f5046058c71f395"
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

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Even if the difference is apparent outside the signed data (in the output).
Signing the data explicitly is more secure.

ie. if some sort of vulnerability / way to break this system for 1-of-1
multisig is found, someone who signed a single sig xpub whitelist will not
be exposed.

2019=E5=B9=B46=E6=9C=8829=E6=97=A5(=E5=9C=9F) 13:43 Dmitry Petukhov <dp@sim=
plexum.com>:

> =D0=92 Sat, 29 Jun 2019 09:19:41 +0900
> Jonathan Underwood <junderwood@bitcoinbank.co.jp> =D0=BF=D0=B8=D1=88=D0=
=B5=D1=82:
>
> > > 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?
> > >
> >
> > 0x00 is single sig, aka, OP_CHECKSIG
> >
> > 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.
>

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<div dir=3D"ltr"><div>Even if the difference is apparent outside the signed=
 data (in the output). Signing the data explicitly is more secure.<br><br>i=
e. if some sort of vulnerability / way to break this system for 1-of-1 mult=
isig is found, someone who signed a single sig xpub whitelist will not be e=
xposed.</div><br><div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail=
_attr">2019=E5=B9=B46=E6=9C=8829=E6=97=A5(=E5=9C=9F) 13:43 Dmitry Petukhov =
&lt;<a href=3D"mailto:dp@simplexum.com">dp@simplexum.com</a>&gt;:<br></div>=
<blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-=
left:1px solid rgb(204,204,204);padding-left:1ex">=D0=92 Sat, 29 Jun 2019 0=
9:19:41 +0900<br>
Jonathan Underwood &lt;<a href=3D"mailto:junderwood@bitcoinbank.co.jp" targ=
et=3D"_blank">junderwood@bitcoinbank.co.jp</a>&gt; =D0=BF=D0=B8=D1=88=D0=B5=
=D1=82:<br>
<br>
&gt; &gt; Other note: you have &#39;unused&#39; value of 1 for `m` in your =
scheme, why<br>
&gt; &gt; not require m=3D1 for single-sig case, and use 0 as indicator tha=
t<br>
&gt; &gt; there are a serlal number following it?<br>
&gt; &gt;=C2=A0 <br>
&gt; <br>
&gt; 0x00 is single sig, aka, OP_CHECKSIG<br>
&gt; <br>
&gt; 0x01 is multisig, aka, 1-of-3, 1-of-2 OP_CHECKMULTISIG<br>
<br>
This informatin is available in per-output redeem/witness script,<br>
signer will be able to distinguish between multisig/single-sig by<br>
looking at this script. I think it only need to know the total number<br>
of keys participating in the signing, and check that this number<br>
matches the particulars of redeem/witness script.<br>
</blockquote></div><br></div>

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