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From: Tier Nolan <tier.nolan@gmail.com>
Date: Thu, 18 May 2017 15:59:50 +0100
Message-ID: <CAE-z3OX2b4V+ERAYszokAUrSRPqpOCd2TovxBiqfeRTj4yuVpw@mail.gmail.com>
Cc: Bitcoin Dev <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev]
	=?utf-8?b?VHJlYXRpbmcg4oCYQVNJQ0JPT1NU4oCZIGFzIGEg?=
	=?utf-8?q?Security_Vulnerability?=
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On Thu, May 18, 2017 at 2:44 PM, Cameron Garnham via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> 1.     Significant deviations from the Bitcoin Security Model have been
> acknowledged as security vulnerabilities.
>
> The Bitcoin Security Model assumes that every input into the Proof-of-Work
> function should have the same difficulty of producing a desired output.
>

This isn't really that clear.

Arguably as long as the effort to find a block is proportional to the block
difficulty parameter, then it isn't an exploit.  It is just an optimisation.

A quantum computer, for example, could find a block with effort
proportional to the square root of the difficulty parameter, so that would
count as an attack.  Though in that case, the fix would likely be to tweak
the difficulty parameter update calculation.

A better definition would be something like "when performing work, each
hash should be independent".

ASICBOOST does multiple checks in parallel, so would violate that.

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<div dir=3D"ltr"><div class=3D"gmail_extra"><div class=3D"gmail_quote">On T=
hu, May 18, 2017 at 2:44 PM, Cameron Garnham via bitcoin-dev <span dir=3D"l=
tr">&lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"=
_blank">bitcoin-dev@lists.linuxfoundation.org</a>&gt;</span> wrote:<br><blo=
ckquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #c=
cc solid;padding-left:1ex">
1.=C2=A0 =C2=A0 =C2=A0Significant deviations from the Bitcoin Security Mode=
l have been acknowledged as security vulnerabilities.<br>
<br>
The Bitcoin Security Model assumes that every input into the Proof-of-Work =
function should have the same difficulty of producing a desired output.<br>=
</blockquote><div><br></div>This isn&#39;t really that clear.<br><br></div>=
<div class=3D"gmail_quote">Arguably as long as the effort to find a block i=
s proportional to the block difficulty parameter, then it isn&#39;t an expl=
oit.=C2=A0 It is just an optimisation.<br><br></div><div class=3D"gmail_quo=
te">A quantum computer, for example, could find a block with effort proport=
ional to the square root of the difficulty parameter, so that would count a=
s an attack.=C2=A0 Though in that case, the fix would likely be to tweak th=
e difficulty parameter update calculation.<br><br></div><div class=3D"gmail=
_quote">A better definition would be something like &quot;when performing w=
ork, each hash should be independent&quot;.=C2=A0 <br><br></div><div class=
=3D"gmail_quote">ASICBOOST does multiple checks in parallel, so would viola=
te that.<br></div></div></div>

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