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Date: Sat, 9 Sep 2017 19:38:21 +0530
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Subject: Re: [bitcoin-dev] Proposal: Extended serialization format for BIP-32
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Hi,

I understand the motivation of adding the birthdate field. However, not
very comfortable with having this in the public key serialization. There
are privacy implication of both the birthday field and having the complete
derivation path, which takes space.

I am fine with Thomas proposal of {x,y,z}. Having additional version byte
field looks modular but since since we already have the big enough version
field in bip32, better to use that instead of adding more bytes.

Thomas, can you please explain why we require different version for P2WPKH
or P2WSH versus (P2WPKH or P2WSH) nested in P2SH. It looked to me that they
would have the same output bitcoin address and under same account.

On Fri, Sep 8, 2017 at 2:09 AM, <
bitcoin-dev-request@lists.linuxfoundation.org> wrote:

> Send bitcoin-dev mailing list submissions to
>         bitcoin-dev@lists.linuxfoundation.org
>
> To subscribe or unsubscribe via the World Wide Web, visit
>         https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
> or, via email, send a message with subject or body 'help' to
>         bitcoin-dev-request@lists.linuxfoundation.org
>
> You can reach the person managing the list at
>         bitcoin-dev-owner@lists.linuxfoundation.org
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> When replying, please edit your Subject line so it is more specific
> than "Re: Contents of bitcoin-dev digest..."
>
>
> Today's Topics:
>
>    1. Re: Proposal: Extended serialization format for   BIP-32
>       wallets (Andreas Schildbach)
>    2. Re: Proposal: Extended serialization format for BIP-32
>       wallets (Pavol Rusnak)
>    3. Re: Fast Merkle Trees (Mark Friedenbach)
>    4. Re: Proposal: Extended serialization format for BIP-32
>       wallets (Thomas Voegtlin)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Thu, 7 Sep 2017 21:35:49 +0200
> From: Andreas Schildbach <andreas@schildbach.de>
> To: bitcoin-dev@lists.linuxfoundation.org
> Subject: Re: [bitcoin-dev] Proposal: Extended serialization format for
>         BIP-32 wallets
> Message-ID: <oos72e$rjp$1@blaine.gmane.org>
> Content-Type: text/plain; charset=utf-8
>
> On 09/07/2017 06:23 PM, Pavol Rusnak via bitcoin-dev wrote:
> > On 07/09/17 06:29, Thomas Voegtlin via bitcoin-dev wrote:
> >> A solution is still needed to wallets who do not wish to use BIP43
> >
> > What if we added another byte field OutputType for wallets that do not
> > follow BIP43?
> >
> > 0x00 - P2PKH output type
> > 0x01 - P2WPKH-in-P2SH output type
> > 0x02 - native Segwit output type
> >
> > Would that work for you?
>
> I think that would work.
>
> > The question is whether this field should be present only if depth==0x00
> > or at all times. What is your suggestion, Thomas?
>
> In case of Bitcoin Wallet, the depth is not null (m/0'/[0,1]) and still
> we need this field. I think it should always be present if a chain is
> limited to a certain script type.
>
> There is however the case where even on one chain, script types are
> mixed. In this case the field should be omitted and the wallet needs to
> scan for all (known) types. Afaik Bitcoin Core is taking this path.
>
>
>
> ------------------------------
>
> Message: 2
> Date: Thu, 7 Sep 2017 22:00:05 +0200
> From: Pavol Rusnak <stick@satoshilabs.com>
> To: Andreas Schildbach <andreas@schildbach.de>, Bitcoin Protocol
>         Discussion <bitcoin-dev@lists.linuxfoundation.org>
> Subject: Re: [bitcoin-dev] Proposal: Extended serialization format for
>         BIP-32 wallets
> Message-ID: <40ed03a1-915c-33b0-c4ac-e898c8c733ba@satoshilabs.com>
> Content-Type: text/plain; charset=windows-1252
>
> On 07/09/17 21:35, Andreas Schildbach via bitcoin-dev wrote:
> > In case of Bitcoin Wallet, the depth is not null (m/0'/[0,1]) and still
> > we need this field.
>
> But the depth of exported public key will be null. It does not make
> sense to export xpub for m or m/0' for your particular case.
>
> > I think it should always be present if a chain is
> > limited to a certain script type.
>
> I am fine with having the path there all the time.
>
> > There is however the case where even on one chain, script types are
> > mixed. In this case the field should be omitted and the wallet needs to
> > scan for all (known) types. Afaik Bitcoin Core is taking this path.
>
> Is that really the case? Why come up with a hierarchy and then don't use
> it?
>
> --
> Best Regards / S pozdravom,
>
> Pavol "stick" Rusnak
> CTO, SatoshiLabs
>
>
> ------------------------------
>
> Message: 3
> Date: Thu, 7 Sep 2017 13:04:30 -0700
> From: Mark Friedenbach <mark@friedenbach.org>
> To: Russell O'Connor <roconnor@blockstream.io>
> Cc: Bitcoin Protocol Discussion
>         <bitcoin-dev@lists.linuxfoundation.org>
> Subject: Re: [bitcoin-dev] Fast Merkle Trees
> Message-ID: <40D6F502-3380-4B64-BCD9-80D361EED35C@friedenbach.org>
> Content-Type: text/plain; charset="us-ascii"
>
> TL;DR I'll be updating the fast Merkle-tree spec to use a different
>       IV, using (for infrastructure compatability reasons) the scheme
>       provided by Peter Todd.
>
> This is a specific instance of a general problem where you cannot
> trust scripts given to you by another party. Notice that we run into
> the same sort of problem when doing key aggregation, in which you must
> require the other party to prove knowledge of the discrete log before
> using their public key, or else key cancellation can occur.
>
> With script it is a little bit more complicated as you might want
> zero-knowledge proofs of hash pre-images for HTLCs as well as proofs
> of DL knowledge (signatures), but the basic idea is the same. Multi-
> party wallet level protocols for jointly constructing scriptPubKeys
> should require a 'delinearization' step that proves knowledge of
> information necessary to complete each part of the script, as part of
> proving the safety of a construct.
>
> I think my hangup before in understanding the attack you describe was
> in actualizing it into a practical attack that actually escalates the
> attacker's capabilities. If the attacker can get you to agree to a
> MAST policy that is nothing more than a CHECKSIG over a key they
> presumably control, then they don't need to do any complicated
> grinding. The attacker in that scenario would just actually specify a
> key they control and take the funds that way.
>
> Where this presumably leads to an actual exploit is when you specify a
> script that a curious counter-party actually takes the time to
> investigate and believes to be secure. For example, a script that
> requires a signature or pre-image revelation from that counter-party.
> That would require grinding not a few bytes, but at minimum 20-33
> bytes for either a HASH160 image or the counter-party's key.
>
> If I understand the revised attack description correctly, then there
> is a small window in which the attacker can create a script less than
> 55 bytes in length, where nearly all of the first 32 bytes are
> selected by the attacker, yet nevertheless the script seems safe to
> the counter-party. The smallest such script I was able to construct
> was the following:
>
>     <fake-pubkey> CHECKSIGVERIFY HASH160 <preimage> EQUAL
>
> This is 56 bytes and requires only 7 bits of grinding in the fake
> pubkey. But 56 bytes is too large. Switching to secp256k1 serialized
> 32-byte pubkeys (in a script version upgrade, for example) would
> reduce this to the necessary 55 bytes with 0 bits of grinding. A
> smaller variant is possible:
>
>     DUP HASH160 <fake-pubkey-hash> EQUALVERIFY CHECKSIGVERIFY HASH160
> <preimage> EQUAL
>
> This is 46 bytes, but requires grinding 96 bits, which is a bit less
> plausible.
>
> Belts and suspenders are not so terrible together, however, and I
> think there is enough of a justification here to look into modifying
> the scheme to use a different IV for hash tree updates. This would
> prevent even the above implausible attacks.
>
>
> > On Sep 7, 2017, at 11:55 AM, Russell O'Connor <roconnor@blockstream.io>
> wrote:
> >
> >
> >
> > On Thu, Sep 7, 2017 at 1:42 PM, Mark Friedenbach <mark@friedenbach.org
> <mailto:mark@friedenbach.org>> wrote:
> > I've been puzzling over your email since receiving it. I'm not sure it
> > is possible to perform the attack you describe with the tree structure
> > specified in the BIP. If I may rephrase your attack, I believe you are
> > seeking a solution to the following:
> >
> > Want: An innocuous script and a malign script for which
> >
> >    double-SHA256(innocuous)
> >
> > is equal to either
> >
> >    fast-SHA256(double-SHA256(malign) || r) or
> >    fast-SHA256(r || double-SHA256(malign))
> >
> > or  fast-SHA256(fast-SHA256(double-SHA256(malign) || r1) || r0)
> > or  fast-SHA256(fast-SHA256(r1 || double-SHA256(malign)) || r0)
> > or ...
> >
> > where r is a freely chosen 32-byte nonce. This would allow the
> > attacker to reveal the innocuous script before funds are sent to the
> > MAST, then use the malign script to spend.
> >
> > Because of the double-SHA256 construction I do not see how this can be
> > accomplished without a full break of SHA256.
> >
> > The particular scenario I'm imagining is a collision between
> >
> >     double-SHA256(innocuous)
> >
> > and
> >
> >     fast-SHA256(fast-SHA256(fast-SHA256(double-SHA256(malign) || r2) ||
> r1) || r0).
> >
> > where innocuous is a Bitcoin Script that is between 32 and 55 bytes long.
> >
> > Observe that when data is less than 55 bytes then double-SHA256(data) =
> fast-SHA256(fast-SHA256(padding-SHA256(data)) || 0x8000...100) (which is
> really the crux of the matter).
> >
> > Therefore, to get our collision it suffices to find a collision between
> >
> >     padding-SHA256(innocuous)
> >
> > and
> >
> >     fast-SHA256(double-SHA256(malign) || r2) || r1
> >
> > r1 can freely be set to the second half of padding-SHA256(innocuous), so
> it suffices to find a collision between
> >
> >    fast-SHA256(double-SHA256(malign) || r2)
> >
> > and the first half of padding-SHA256(innocuous) which is equal to the
> first 32 bytes of innocuous.
> >
> > Imagine the first opcode of innocuous is the push of a value that the
> attacker claims to be his 33-byte public key.
> > So long as the attacker doesn't need to prove that they know the
> discrete log of this pubkey, they can grind r2 until the result of
> fast-SHA256(double-SHA256(malign) || r2) contains the correct first
> couple of bytes for the script header and the opcode for a 33-byte push.  I
> believe that is only about 3 or 4 bytes of they need to grind out.
> >
>
> -------------- next part --------------
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>
> ------------------------------
>
> Message: 4
> Date: Thu, 7 Sep 2017 22:39:17 +0200
> From: Thomas Voegtlin <thomasv@electrum.org>
> To: "bitcoin-dev@lists.linuxfoundation.org"
>         <bitcoin-dev@lists.linuxfoundation.org>
> Subject: Re: [bitcoin-dev] Proposal: Extended serialization format for
>         BIP-32 wallets
> Message-ID: <9e74dc17-105c-b43c-7780-4fa690043fe2@electrum.org>
> Content-Type: text/plain; charset=windows-1252
>
>
>
> On 07.09.2017 18:23, Pavol Rusnak wrote:
> > On 07/09/17 06:29, Thomas Voegtlin via bitcoin-dev wrote:
> >> A solution is still needed to wallets who do not wish to use BIP43
> >
> > What if we added another byte field OutputType for wallets that do not
> > follow BIP43?
> >
> > 0x00 - P2PKH output type
> > 0x01 - P2WPKH-in-P2SH output type
> > 0x02 - native Segwit output type
> >
> > Would that work for you?
> >
> > The question is whether this field should be present only if depth==0x00
> > or at all times. What is your suggestion, Thomas?
> >
>
>
> well, in my initial proposal, I wrote that this value should be user
> visible. That is why I used version bytes. If you create an extra byte
> field, and then use base58 or bech32 encoding, the value will not be
> user visible anymore.
>
> The initial implementation of segwit xpub/xprv in Electrum used a flag
> that was not user visible (I added 1 to the bip32 version bytes, which
> leaves the xpub/xprv prefix unchanged). I have experimented with that
> invisible flag for more than 6 months now, and I am now convinced that
> it is better to make that flag user visible.
>
> The reason is that when users create wallets with multisig scripts, they
> need to combine several master public keys. However, these master public
> keys should all be of the same type: it would not make sense to create a
> 2 of 3 multisig wallet with a one xpub, one ypub and one zpub. By
> imposing that all master keys are of the same type, we ensure that all
> cosigners agree on the script type that will be used to derive addresses.
>
> In other words, if users are exposed to master keys and need to
> manipulate them, it is better to let them see what they are doing.
>
> OTOH if you do not plan to expose your users to these keys, you probably
> do not need a serialization format.
>
>
> ------------------------------
>
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
>
> End of bitcoin-dev Digest, Vol 28, Issue 17
> *******************************************
>

--001a11438e8814d4c70558c23b7b
Content-Type: text/html; charset="UTF-8"
Content-Transfer-Encoding: quoted-printable

<div dir=3D"ltr"><div><div><div>Hi,<br><br></div>I understand the motivatio=
n of adding the birthdate field. However, not very comfortable with having =
this in the public key serialization. There are privacy implication of both=
 the birthday field and having the complete derivation path, which takes sp=
ace.<br><br></div>I am fine with Thomas proposal of {x,y,z}. Having additio=
nal version byte field looks modular but since since we already have the bi=
g enough version field in bip32, better to use that instead of adding more =
bytes.<br><br></div>Thomas, can you please explain why we require different=
 version for P2WPKH or P2WSH versus (P2WPKH or P2WSH) nested in P2SH. It lo=
oked to me that they would have the same output bitcoin address and under s=
ame account.<br><div><div><div><div><div><div><div><div><div class=3D"gmail=
_extra">=C2=A0<br><div class=3D"gmail_quote">On Fri, Sep 8, 2017 at 2:09 AM=
,  <span dir=3D"ltr">&lt;<a href=3D"mailto:bitcoin-dev-request@lists.linuxf=
oundation.org" target=3D"_blank">bitcoin-dev-request@lists.linuxfoundation.=
org</a>&gt;</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"mar=
gin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1=
ex">Send bitcoin-dev mailing list submissions to<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 <a href=3D"mailto:bitcoin-dev@lists.linuxfounda=
tion.org">bitcoin-dev@lists.<wbr>linuxfoundation.org</a><br>
<br>
To subscribe or unsubscribe via the World Wide Web, visit<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 <a href=3D"https://lists.linuxfoundation.org/ma=
ilman/listinfo/bitcoin-dev" rel=3D"noreferrer" target=3D"_blank">https://li=
sts.linuxfoundation.<wbr>org/mailman/listinfo/bitcoin-<wbr>dev</a><br>
or, via email, send a message with subject or body &#39;help&#39; to<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 <a href=3D"mailto:bitcoin-dev-request@lists.lin=
uxfoundation.org">bitcoin-dev-request@lists.<wbr>linuxfoundation.org</a><br=
>
<br>
You can reach the person managing the list at<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 <a href=3D"mailto:bitcoin-dev-owner@lists.linux=
foundation.org">bitcoin-dev-owner@lists.<wbr>linuxfoundation.org</a><br>
<br>
When replying, please edit your Subject line so it is more specific<br>
than &quot;Re: Contents of bitcoin-dev digest...&quot;<br>
<br>
<br>
Today&#39;s Topics:<br>
<br>
=C2=A0 =C2=A01. Re: Proposal: Extended serialization format for=C2=A0 =C2=
=A0BIP-32<br>
=C2=A0 =C2=A0 =C2=A0 wallets (Andreas Schildbach)<br>
=C2=A0 =C2=A02. Re: Proposal: Extended serialization format for BIP-32<br>
=C2=A0 =C2=A0 =C2=A0 wallets (Pavol Rusnak)<br>
=C2=A0 =C2=A03. Re: Fast Merkle Trees (Mark Friedenbach)<br>
=C2=A0 =C2=A04. Re: Proposal: Extended serialization format for BIP-32<br>
=C2=A0 =C2=A0 =C2=A0 wallets (Thomas Voegtlin)<br>
<br>
<br>
------------------------------<wbr>------------------------------<wbr>-----=
-----<br>
<br>
Message: 1<br>
Date: Thu, 7 Sep 2017 21:35:49 +0200<br>
From: Andreas Schildbach &lt;<a href=3D"mailto:andreas@schildbach.de">andre=
as@schildbach.de</a>&gt;<br>
To: <a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org">bitcoin-dev@li=
sts.<wbr>linuxfoundation.org</a><br>
Subject: Re: [bitcoin-dev] Proposal: Extended serialization format for<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 BIP-32 wallets<br>
Message-ID: &lt;oos72e$rjp$<a href=3D"mailto:1@blaine.gmane.org">1@blaine.g=
mane.org</a><wbr>&gt;<br>
Content-Type: text/plain; charset=3Dutf-8<br>
<br>
On 09/07/2017 06:23 PM, Pavol Rusnak via bitcoin-dev wrote:<br>
&gt; On 07/09/17 06:29, Thomas Voegtlin via bitcoin-dev wrote:<br>
&gt;&gt; A solution is still needed to wallets who do not wish to use BIP43=
<br>
&gt;<br>
&gt; What if we added another byte field OutputType for wallets that do not=
<br>
&gt; follow BIP43?<br>
&gt;<br>
&gt; 0x00 - P2PKH output type<br>
&gt; 0x01 - P2WPKH-in-P2SH output type<br>
&gt; 0x02 - native Segwit output type<br>
&gt;<br>
&gt; Would that work for you?<br>
<br>
I think that would work.<br>
<br>
&gt; The question is whether this field should be present only if depth=3D=
=3D0x00<br>
&gt; or at all times. What is your suggestion, Thomas?<br>
<br>
In case of Bitcoin Wallet, the depth is not null (m/0&#39;/[0,1]) and still=
<br>
we need this field. I think it should always be present if a chain is<br>
limited to a certain script type.<br>
<br>
There is however the case where even on one chain, script types are<br>
mixed. In this case the field should be omitted and the wallet needs to<br>
scan for all (known) types. Afaik Bitcoin Core is taking this path.<br>
<br>
<br>
<br>
------------------------------<br>
<br>
Message: 2<br>
Date: Thu, 7 Sep 2017 22:00:05 +0200<br>
From: Pavol Rusnak &lt;<a href=3D"mailto:stick@satoshilabs.com">stick@satos=
hilabs.com</a>&gt;<br>
To: Andreas Schildbach &lt;<a href=3D"mailto:andreas@schildbach.de">andreas=
@schildbach.de</a>&gt;, Bitcoin Protocol<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 Discussion &lt;<a href=3D"mailto:bitcoin-dev@li=
sts.linuxfoundation.org">bitcoin-dev@lists.<wbr>linuxfoundation.org</a>&gt;=
<br>
Subject: Re: [bitcoin-dev] Proposal: Extended serialization format for<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 BIP-32 wallets<br>
Message-ID: &lt;<a href=3D"mailto:40ed03a1-915c-33b0-c4ac-e898c8c733ba@sato=
shilabs.com">40ed03a1-915c-33b0-c4ac-<wbr>e898c8c733ba@satoshilabs.com</a>&=
gt;<br>
Content-Type: text/plain; charset=3Dwindows-1252<br>
<br>
On 07/09/17 21:35, Andreas Schildbach via bitcoin-dev wrote:<br>
&gt; In case of Bitcoin Wallet, the depth is not null (m/0&#39;/[0,1]) and =
still<br>
&gt; we need this field.<br>
<br>
But the depth of exported public key will be null. It does not make<br>
sense to export xpub for m or m/0&#39; for your particular case.<br>
<br>
&gt; I think it should always be present if a chain is<br>
&gt; limited to a certain script type.<br>
<br>
I am fine with having the path there all the time.<br>
<br>
&gt; There is however the case where even on one chain, script types are<br=
>
&gt; mixed. In this case the field should be omitted and the wallet needs t=
o<br>
&gt; scan for all (known) types. Afaik Bitcoin Core is taking this path.<br=
>
<br>
Is that really the case? Why come up with a hierarchy and then don&#39;t us=
e it?<br>
<br>
--<br>
Best Regards / S pozdravom,<br>
<br>
Pavol &quot;stick&quot; Rusnak<br>
CTO, SatoshiLabs<br>
<br>
<br>
------------------------------<br>
<br>
Message: 3<br>
Date: Thu, 7 Sep 2017 13:04:30 -0700<br>
From: Mark Friedenbach &lt;<a href=3D"mailto:mark@friedenbach.org">mark@fri=
edenbach.org</a>&gt;<br>
To: Russell O&#39;Connor &lt;<a href=3D"mailto:roconnor@blockstream.io">roc=
onnor@blockstream.io</a>&gt;<br>
Cc: Bitcoin Protocol Discussion<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfo=
undation.org">bitcoin-dev@lists.<wbr>linuxfoundation.org</a>&gt;<br>
Subject: Re: [bitcoin-dev] Fast Merkle Trees<br>
Message-ID: &lt;<a href=3D"mailto:40D6F502-3380-4B64-BCD9-80D361EED35C@frie=
denbach.org">40D6F502-3380-4B64-BCD9-<wbr>80D361EED35C@friedenbach.org</a>&=
gt;<br>
Content-Type: text/plain; charset=3D&quot;us-ascii&quot;<br>
<br>
TL;DR I&#39;ll be updating the fast Merkle-tree spec to use a different<br>
=C2=A0 =C2=A0 =C2=A0 IV, using (for infrastructure compatability reasons) t=
he scheme<br>
=C2=A0 =C2=A0 =C2=A0 provided by Peter Todd.<br>
<br>
This is a specific instance of a general problem where you cannot<br>
trust scripts given to you by another party. Notice that we run into<br>
the same sort of problem when doing key aggregation, in which you must<br>
require the other party to prove knowledge of the discrete log before<br>
using their public key, or else key cancellation can occur.<br>
<br>
With script it is a little bit more complicated as you might want<br>
zero-knowledge proofs of hash pre-images for HTLCs as well as proofs<br>
of DL knowledge (signatures), but the basic idea is the same. Multi-<br>
party wallet level protocols for jointly constructing scriptPubKeys<br>
should require a &#39;delinearization&#39; step that proves knowledge of<br=
>
information necessary to complete each part of the script, as part of<br>
proving the safety of a construct.<br>
<br>
I think my hangup before in understanding the attack you describe was<br>
in actualizing it into a practical attack that actually escalates the<br>
attacker&#39;s capabilities. If the attacker can get you to agree to a<br>
MAST policy that is nothing more than a CHECKSIG over a key they<br>
presumably control, then they don&#39;t need to do any complicated<br>
grinding. The attacker in that scenario would just actually specify a<br>
key they control and take the funds that way.<br>
<br>
Where this presumably leads to an actual exploit is when you specify a<br>
script that a curious counter-party actually takes the time to<br>
investigate and believes to be secure. For example, a script that<br>
requires a signature or pre-image revelation from that counter-party.<br>
That would require grinding not a few bytes, but at minimum 20-33<br>
bytes for either a HASH160 image or the counter-party&#39;s key.<br>
<br>
If I understand the revised attack description correctly, then there<br>
is a small window in which the attacker can create a script less than<br>
55 bytes in length, where nearly all of the first 32 bytes are<br>
selected by the attacker, yet nevertheless the script seems safe to<br>
the counter-party. The smallest such script I was able to construct<br>
was the following:<br>
<br>
=C2=A0 =C2=A0 &lt;fake-pubkey&gt; CHECKSIGVERIFY HASH160 &lt;preimage&gt; E=
QUAL<br>
<br>
This is 56 bytes and requires only 7 bits of grinding in the fake<br>
pubkey. But 56 bytes is too large. Switching to secp256k1 serialized<br>
32-byte pubkeys (in a script version upgrade, for example) would<br>
reduce this to the necessary 55 bytes with 0 bits of grinding. A<br>
smaller variant is possible:<br>
<br>
=C2=A0 =C2=A0 DUP HASH160 &lt;fake-pubkey-hash&gt; EQUALVERIFY CHECKSIGVERI=
FY HASH160 &lt;preimage&gt; EQUAL<br>
<br>
This is 46 bytes, but requires grinding 96 bits, which is a bit less<br>
plausible.<br>
<br>
Belts and suspenders are not so terrible together, however, and I<br>
think there is enough of a justification here to look into modifying<br>
the scheme to use a different IV for hash tree updates. This would<br>
prevent even the above implausible attacks.<br>
<br>
<br>
&gt; On Sep 7, 2017, at 11:55 AM, Russell O&#39;Connor &lt;<a href=3D"mailt=
o:roconnor@blockstream.io">roconnor@blockstream.io</a>&gt; wrote:<br>
&gt;<br>
&gt;<br>
&gt;<br>
&gt; On Thu, Sep 7, 2017 at 1:42 PM, Mark Friedenbach &lt;<a href=3D"mailto=
:mark@friedenbach.org">mark@friedenbach.org</a> &lt;mailto:<a href=3D"mailt=
o:mark@friedenbach.org">mark@friedenbach.org</a>&gt;&gt; wrote:<br>
&gt; I&#39;ve been puzzling over your email since receiving it. I&#39;m not=
 sure it<br>
&gt; is possible to perform the attack you describe with the tree structure=
<br>
&gt; specified in the BIP. If I may rephrase your attack, I believe you are=
<br>
&gt; seeking a solution to the following:<br>
&gt;<br>
&gt; Want: An innocuous script and a malign script for which<br>
&gt;<br>
&gt;=C2=A0 =C2=A0 double-SHA256(innocuous)<br>
&gt;<br>
&gt; is equal to either<br>
&gt;<br>
&gt;=C2=A0 =C2=A0 fast-SHA256(double-SHA256(<wbr>malign) || r) or<br>
&gt;=C2=A0 =C2=A0 fast-SHA256(r || double-SHA256(malign))<br>
&gt;<br>
&gt; or=C2=A0 fast-SHA256(fast-SHA256(<wbr>double-SHA256(malign) || r1) || =
r0)<br>
&gt; or=C2=A0 fast-SHA256(fast-SHA256(r1 || double-SHA256(malign)) || r0)<b=
r>
&gt; or ...<br>
&gt;<br>
&gt; where r is a freely chosen 32-byte nonce. This would allow the<br>
&gt; attacker to reveal the innocuous script before funds are sent to the<b=
r>
&gt; MAST, then use the malign script to spend.<br>
&gt;<br>
&gt; Because of the double-SHA256 construction I do not see how this can be=
<br>
&gt; accomplished without a full break of SHA256.<br>
&gt;<br>
&gt; The particular scenario I&#39;m imagining is a collision between<br>
&gt;<br>
&gt;=C2=A0 =C2=A0 =C2=A0double-SHA256(innocuous)<br>
&gt;<br>
&gt; and<br>
&gt;<br>
&gt;=C2=A0 =C2=A0 =C2=A0fast-SHA256(fast-SHA256(fast-<wbr>SHA256(double-SHA=
256(malign) || r2) || r1) || r0).<br>
&gt;<br>
&gt; where innocuous is a Bitcoin Script that is between 32 and 55 bytes lo=
ng.<br>
&gt;<br>
&gt; Observe that when data is less than 55 bytes then double-SHA256(data) =
=3D fast-SHA256(fast-SHA256(<wbr>padding-SHA256(data)) || 0x8000...100) (wh=
ich is really the crux of the matter).<br>
&gt;<br>
&gt; Therefore, to get our collision it suffices to find a collision betwee=
n<br>
&gt;<br>
&gt;=C2=A0 =C2=A0 =C2=A0padding-SHA256(innocuous)<br>
&gt;<br>
&gt; and<br>
&gt;<br>
&gt;=C2=A0 =C2=A0 =C2=A0fast-SHA256(double-SHA256(<wbr>malign) || r2) || r1=
<br>
&gt;<br>
&gt; r1 can freely be set to the second half of padding-SHA256(innocuous), =
so it suffices to find a collision between<br>
&gt;<br>
&gt;=C2=A0 =C2=A0 fast-SHA256(double-SHA256(<wbr>malign) || r2)<br>
&gt;<br>
&gt; and the first half of padding-SHA256(innocuous) which is equal to the =
first 32 bytes of innocuous.<br>
&gt;<br>
&gt; Imagine the first opcode of innocuous is the push of a value that the =
attacker claims to be his 33-byte public key.<br>
&gt; So long as the attacker doesn&#39;t need to prove that they know the d=
iscrete log of this pubkey, they can grind r2 until the result of fast-SHA2=
56(double-SHA256(<wbr>malign) || r2) contains the correct first couple of b=
ytes for the script header and the opcode for a 33-byte push.=C2=A0 I belie=
ve that is only about 3 or 4 bytes of they need to grind out.<br>
&gt;<br>
<br>
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------------------------------<br>
<br>
Message: 4<br>
Date: Thu, 7 Sep 2017 22:39:17 +0200<br>
From: Thomas Voegtlin &lt;<a href=3D"mailto:thomasv@electrum.org">thomasv@e=
lectrum.org</a>&gt;<br>
To: &quot;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org">bitcoin-=
dev@lists.<wbr>linuxfoundation.org</a>&quot;<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfo=
undation.org">bitcoin-dev@lists.<wbr>linuxfoundation.org</a>&gt;<br>
Subject: Re: [bitcoin-dev] Proposal: Extended serialization format for<br>
=C2=A0 =C2=A0 =C2=A0 =C2=A0 BIP-32 wallets<br>
Message-ID: &lt;<a href=3D"mailto:9e74dc17-105c-b43c-7780-4fa690043fe2@elec=
trum.org">9e74dc17-105c-b43c-7780-<wbr>4fa690043fe2@electrum.org</a>&gt;<br=
>
Content-Type: text/plain; charset=3Dwindows-1252<br>
<br>
<br>
<br>
On 07.09.2017 18:23, Pavol Rusnak wrote:<br>
&gt; On 07/09/17 06:29, Thomas Voegtlin via bitcoin-dev wrote:<br>
&gt;&gt; A solution is still needed to wallets who do not wish to use BIP43=
<br>
&gt;<br>
&gt; What if we added another byte field OutputType for wallets that do not=
<br>
&gt; follow BIP43?<br>
&gt;<br>
&gt; 0x00 - P2PKH output type<br>
&gt; 0x01 - P2WPKH-in-P2SH output type<br>
&gt; 0x02 - native Segwit output type<br>
&gt;<br>
&gt; Would that work for you?<br>
&gt;<br>
&gt; The question is whether this field should be present only if depth=3D=
=3D0x00<br>
&gt; or at all times. What is your suggestion, Thomas?<br>
&gt;<br>
<br>
<br>
well, in my initial proposal, I wrote that this value should be user<br>
visible. That is why I used version bytes. If you create an extra byte<br>
field, and then use base58 or bech32 encoding, the value will not be<br>
user visible anymore.<br>
<br>
The initial implementation of segwit xpub/xprv in Electrum used a flag<br>
that was not user visible (I added 1 to the bip32 version bytes, which<br>
leaves the xpub/xprv prefix unchanged). I have experimented with that<br>
invisible flag for more than 6 months now, and I am now convinced that<br>
it is better to make that flag user visible.<br>
<br>
The reason is that when users create wallets with multisig scripts, they<br=
>
need to combine several master public keys. However, these master public<br=
>
keys should all be of the same type: it would not make sense to create a<br=
>
2 of 3 multisig wallet with a one xpub, one ypub and one zpub. By<br>
imposing that all master keys are of the same type, we ensure that all<br>
cosigners agree on the script type that will be used to derive addresses.<b=
r>
<br>
In other words, if users are exposed to master keys and need to<br>
manipulate them, it is better to let them see what they are doing.<br>
<br>
OTOH if you do not plan to expose your users to these keys, you probably<br=
>
do not need a serialization format.<br>
<br>
<br>
------------------------------<br>
<br>
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<br>
<br>
End of bitcoin-dev Digest, Vol 28, Issue 17<br>
******************************<wbr>*************<br>
</blockquote></div><br></div></div></div></div></div></div></div></div></di=
v></div>

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