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From: Hugo Nguyen <hugo@nunchuk.io>
Date: Thu, 11 Feb 2021 11:11:11 -0800
Message-ID: <CAPKmR9udiK+2gC2qUueAfKm7VBf8csM-3WOz1+-bMNR0iVeGew@mail.gmail.com>
To: Dmitry Petukhov <dp@simplexum.com>, 
 Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] Proposal: Bitcoin Secure Multisig Setup
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Hi Pavol,

On Thu, Feb 11, 2021 at 8:25 AM Dmitry Petukhov via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> =D0=92 Thu, 11 Feb 2021 05:45:33 -0800
> Hugo Nguyen via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
> wrote:
>
> > > > ENCRYPTION_KEY =3D SHA256(SHA256(TOKEN))
> > >
> > > This scheme might be vulnerable to rainbow table attack.
> > >
> >
> > Thank you for pointing this out! Incidentally, Dmitry Petukhov also
> > told me the same privately.
>
> My thought was that if TOKEN has the characteristics of a password
> (short ASCII string), then it would be better to use key derivation
> function designed for passwords, like PBKDF2.
>
> The counter-argument to this is that this adds another code dependency
> for vendors, if the device firmware does not already have the required
> key derivation function.
>
> Maybe this could be solved by going into opposite direction - make the
> "token" even longer, use the mnemoic.
>
> The issue is that entering long data of the shared key into the device
> manually is difficult UX-wise.
>
> Hww vendors that allow to enter custom keys into their device already
> have to face this issue, and those who allow to enter custom keys via
> mnemonic probably tackled this somehow.
>
> Maybe the shared key for multisig setup can be entered in the same way
> ? (with maybe additional visual check via some fingerprint).
>

You just gave me a great idea! We can reuse the BIP32 seed words list!
Perhaps the encryption key can just be 6 words, but it'll be derived the
same way. BIP39 also uses PBKDF2 as a key derivation function, so it
matches with what you described here.

And all HWW should have this functionality already.

Best,
Hugo


>
> Although we would then have another issue of potential confusion
> between two procedures (entering the main key and entering the shared
> key for multisig setup), and the measures has to be taken to prevent
> such confusion.
>
> The approaches can be combined - specify a key derivation function
> suitable for passwords; via secure channel, share a password and/or the
> derived key. If hww supports derivation function, it can derive the key
> from password. If hww supports only keys, the key can be entered raw or
> via mnemonic.
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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<div dir=3D"ltr"><div>Hi Pavol,</div><br><div class=3D"gmail_quote"><div di=
r=3D"ltr" class=3D"gmail_attr">On Thu, Feb 11, 2021 at 8:25 AM Dmitry Petuk=
hov via bitcoin-dev &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation=
.org">bitcoin-dev@lists.linuxfoundation.org</a>&gt; wrote:<br></div><blockq=
uote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1p=
x solid rgb(204,204,204);padding-left:1ex">=D0=92 Thu, 11 Feb 2021 05:45:33=
 -0800<br>
Hugo Nguyen via bitcoin-dev &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfo=
undation.org" target=3D"_blank">bitcoin-dev@lists.linuxfoundation.org</a>&g=
t;<br>
wrote:<br>
<br>
&gt; &gt; &gt; ENCRYPTION_KEY =3D SHA256(SHA256(TOKEN))=C2=A0 <br>
&gt; &gt;<br>
&gt; &gt; This scheme might be vulnerable to rainbow table attack.<br>
&gt; &gt;=C2=A0 <br>
&gt; <br>
&gt; Thank you for pointing this out! Incidentally, Dmitry Petukhov also<br=
>
&gt; told me the same privately.<br>
<br>
My thought was that if TOKEN has the characteristics of a password<br>
(short ASCII string), then it would be better to use key derivation<br>
function designed for passwords, like PBKDF2.<br>
<br>
The counter-argument to this is that this adds another code dependency<br>
for vendors, if the device firmware does not already have the required<br>
key derivation function.<br>
<br>
Maybe this could be solved by going into opposite direction - make the<br>
&quot;token&quot; even longer, use the mnemoic.<br>
<br>
The issue is that entering long data of the shared key into the device<br>
manually is difficult UX-wise.<br>
<br>
Hww vendors that allow to enter custom keys into their device already<br>
have to face this issue, and those who allow to enter custom keys via<br>
mnemonic probably tackled this somehow.<br>
<br>
Maybe the shared key for multisig setup can be entered in the same way<br>
? (with maybe additional visual check via some fingerprint).<br></blockquot=
e><div><br>You just gave me a great idea! We can reuse the BIP32 seed words=
 list! Perhaps the encryption key can just be 6 words, but it&#39;ll be der=
ived the same way. BIP39 also uses=C2=A0PBKDF2 as a key derivation function=
, so it matches with what you described here.<br><br>And all HWW should hav=
e this functionality already.<br><br>Best,<br>Hugo<br>=C2=A0</div><blockquo=
te class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px =
solid rgb(204,204,204);padding-left:1ex">
<br>
Although we would then have another issue of potential confusion<br>
between two procedures (entering the main key and entering the shared<br>
key for multisig setup), and the measures has to be taken to prevent<br>
such confusion.<br>
<br>
The approaches can be combined - specify a key derivation function<br>
suitable for passwords; via secure channel, share a password and/or the<br>
derived key. If hww supports derivation function, it can derive the key<br>
from password. If hww supports only keys, the key can be entered raw or<br>
via mnemonic.<br>
_______________________________________________<br>
bitcoin-dev mailing list<br>
<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_blank">=
bitcoin-dev@lists.linuxfoundation.org</a><br>
<a href=3D"https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev" =
rel=3D"noreferrer" target=3D"_blank">https://lists.linuxfoundation.org/mail=
man/listinfo/bitcoin-dev</a><br>
</blockquote></div></div>

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