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From: Jonathan Underwood <junderwood@bitcoinbank.co.jp>
Date: Fri, 28 Jun 2019 11:44:15 +0900
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To: Peter Gray <peter@coinkite.com>
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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)
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Hi Peter,

tl;dr The problem this solves is "How can a signer verify an address with
HD changing the address every time?"

As an aside: (This is sort of explaining the current PR for the 0x01 global
field (separate from mine))
The problem is more easily understood with change addresses: If someone can
alter my PSBT before signing, they could replace my change address with
their address, and my signer would not know unless the signer just guesses
all the path sets it knows, then derives thousands of change addresses and
searches (most likely a signer is offline, so gap limit doesn't work since
we can't tell which change addresses have tx history. So the 0x01 global
tag will tell the signer "here's how you get from your master private key
to the xpub used in the change output's output BIP32_DERIVATION tag... you
can then derive the same key and check it is yours before signing."

Back to my proposal, this problem extends across wallets, since,
for example, if I want to send from my cold wallet to my warm wallet, I
don't want to give my cold signer my warm master key just so it can derive
and check the key. That's what signatures are for. So this proposal says "A
signer can be built to only sign if it sees a signature that itself has
signed, then from that signed xpub(s) derives the BIP32_DERIVATION in the
outputs, and if the output doesn't match it will reject and not sign"

This creates a sort of "chain of trust" for the wallet.

Currently the best way to prevent this (hacker swapping the send to
address) without using signatures is to reuse the same address every time
you want to send to the warm wallet, since after a few times, the signers
(people) will be able to remember the address.

This is a huge HD drawback for high security requirement environments.
Having this data in the PSBT standard will allow Trezor etc. to create an
enforceable whitelist feature.

Let me know if you have feedback on the details.

Thanks,
Jon

2019=E5=B9=B46=E6=9C=8828=E6=97=A5(=E9=87=91) 0:07 Peter D. Gray <peter@coi=
nkite.com>:

> I haven't studied the new proposal in depth, but my first impression is:
>
> Wouldn't it just be easier and better to just sign the entire "outputs"
> section of the PSBT?
>
> The signature would cover every byte, and therefore would cover any
> future BIP additions to the outputs area, and also help non-multisig
> cases today.
>
> ---
> Peter D. Gray  ||  Founder, Coinkite  ||  Twitter: @dochex  ||  GPG:
> A3A31BAD 5A2A5B10
>
>

--=20
-----------------
Jonathan Underwood
=E3=83=93=E3=83=83=E3=83=88=E3=83=90=E3=83=B3=E3=82=AF=E7=A4=BE =E3=83=81=
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=82=AA=E3=83=95=E3=82=A3=E3=82=B5=E3=83=BC
-----------------

=E6=9A=97=E5=8F=B7=E5=8C=96=E3=81=97=E3=81=9F=E3=83=A1=E3=83=83=E3=82=BB=E3=
=83=BC=E3=82=B8=E3=82=92=E3=81=8A=E9=80=81=E3=82=8A=E3=81=AE=E6=96=B9=E3=81=
=AF=E4=B8=8B=E8=A8=98=E3=81=AE=E5=85=AC=E9=96=8B=E9=8D=B5=E3=82=92=E3=81=94=
=E5=88=A9=E7=94=A8=E4=B8=8B=E3=81=95=E3=81=84=E3=80=82

=E6=8C=87=E7=B4=8B: 0xCE5EA9476DE7D3E45EBC3FDAD998682F3590FEA3

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<div dir=3D"ltr">Hi Peter,<div><br></div><div>tl;dr The problem this solves=
 is &quot;How can a signer verify an address with HD changing the address e=
very time?&quot;</div><div><br></div><div>As an aside: (This is sort of exp=
laining the current PR for the 0x01 global field (separate from mine))</div=
><div>The problem is more easily understood with change addresses: If someo=
ne can alter my PSBT before signing, they could replace my change address w=
ith their address, and my signer would not know unless the signer just gues=
ses all the path sets it knows, then derives thousands of change addresses =
and searches (most likely a signer is offline, so gap limit doesn&#39;t wor=
k since we can&#39;t tell which change addresses have tx history. So the 0x=
01 global tag will tell the signer &quot;here&#39;s how you get from your m=
aster private key to the xpub used in the change output&#39;s output BIP32_=
DERIVATION tag... you can then derive the same key and check it is yours be=
fore signing.&quot;<br><br>Back to my proposal, this problem extends across=
 wallets, since, for=C2=A0example, if I want to send from my cold wallet to=
 my warm wallet, I don&#39;t want to give my cold signer my warm master key=
 just so it can derive and check the key. That&#39;s what signatures are fo=
r. So this proposal says &quot;A signer can be built to only sign if it see=
s a signature that itself has signed, then from that signed xpub(s) derives=
 the BIP32_DERIVATION in the outputs, and if the output doesn&#39;t match i=
t will reject and not sign&quot;</div><div><br></div><div>This creates a so=
rt of &quot;chain of trust&quot; for the wallet.</div><div><br></div><div>C=
urrently the best way to prevent this (hacker swapping the send to address)=
 without using signatures is to reuse the same address every time you want =
to send to the warm wallet, since after a few times, the signers (people) w=
ill be able to remember the address.</div><div><br></div><div>This is a hug=
e HD drawback for high security requirement environments. Having this data =
in the PSBT standard will allow Trezor etc. to create an enforceable whitel=
ist feature.</div><div><br></div><div>Let me know if you have feedback on t=
he details.</div><div><br></div><div>Thanks,</div><div>Jon</div></div><br><=
div class=3D"gmail_quote"><div dir=3D"ltr" class=3D"gmail_attr">2019=E5=B9=
=B46=E6=9C=8828=E6=97=A5(=E9=87=91) 0:07 Peter D. Gray &lt;<a href=3D"mailt=
o:peter@coinkite.com">peter@coinkite.com</a>&gt;:<br></div><blockquote clas=
s=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px solid r=
gb(204,204,204);padding-left:1ex">I haven&#39;t studied the new proposal in=
 depth, but my first impression is:<br>
<br>
Wouldn&#39;t it just be easier and better to just sign the entire &quot;out=
puts&quot; section of the PSBT?<br>
<br>
The signature would cover every byte, and therefore would cover any<br>
future BIP additions to the outputs area, and also help non-multisig<br>
cases today.<br>
<br>
---<br>
Peter D. Gray=C2=A0 ||=C2=A0 Founder, Coinkite=C2=A0 ||=C2=A0 Twitter: @doc=
hex=C2=A0 ||=C2=A0 GPG: A3A31BAD 5A2A5B10<br>
<br>
</blockquote></div><br clear=3D"all"><div><br></div>-- <br><div dir=3D"ltr"=
 class=3D"gmail_signature"><div dir=3D"ltr"><div><div dir=3D"ltr"><div dir=
=3D"ltr"><div>-----------------<br></div><div>Jonathan Underwood</div><div>=
=E3=83=93=E3=83=83=E3=83=88=E3=83=90=E3=83=B3=E3=82=AF=E7=A4=BE=E3=80=80=E3=
=83=81=E3=83=BC=E3=83=95=E3=83=93=E3=83=83=E3=83=88=E3=82=B3=E3=82=A4=E3=83=
=B3=E3=82=AA=E3=83=95=E3=82=A3=E3=82=B5=E3=83=BC</div><div>----------------=
-</div><div><br></div><div>=E6=9A=97=E5=8F=B7=E5=8C=96=E3=81=97=E3=81=9F=E3=
=83=A1=E3=83=83=E3=82=BB=E3=83=BC=E3=82=B8=E3=82=92=E3=81=8A=E9=80=81=E3=82=
=8A=E3=81=AE=E6=96=B9=E3=81=AF=E4=B8=8B=E8=A8=98=E3=81=AE=E5=85=AC=E9=96=8B=
=E9=8D=B5=E3=82=92=E3=81=94=E5=88=A9=E7=94=A8=E4=B8=8B=E3=81=95=E3=81=84=E3=
=80=82</div><div><br></div><div>=E6=8C=87=E7=B4=8B: 0xCE5EA9476DE7D3E45EBC3=
FDAD998682F3590FEA3</div></div></div></div></div></div>

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