Date: Thu, 1 Sep 2016 07:29:29 -0400 (EDT)
From: Johnson Lau <jl2012@xbt.hk>
Reply-To: Johnson Lau <jl2012@xbt.hk>
To: Sergio Demian Lerner <sergio.d.lerner@gmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Attack by modifying non-segwit transactions after
 segwit is accepted ?
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Thank you so much for taking time to actually review the codes. I hope you will keep raising questions when you feel something might be wrong. This is how things supposed to work and we should not be affected by some forum discussions.

> On August 26, 2016 at 9:16 AM Sergio Demian Lerner <sergio.d.lerner@gmail.com> wrote:
> 
>     Because there was a discussion on reddit about this topic, I want to clarify that Johnson Lau explained how a check in the code prevents this attack.
>     So there is no real attack.
> 
>     Also note that the subject of this thread has a question mark, which means that I'm asking the community for clarification, not asserting the existence of a vulnerability.
> 
>     The segwit code is complex, and some key parts of the consensus code are spread over the source files (such as state.CorruptionPossible() relation to DoS banning, IsNull() check in witness program serialization, etc.).
> 
>     Thanks again Johnson for your clarifications.
> 
> 
>     On Wed, Aug 24, 2016 at 10:49 PM, Johnson Lau <jl2012@xbt.hk mailto:jl2012@xbt.hk > wrote:
> 
>         > > 
> >         Adding witness data to a non-segwit script is invalid by consensus:
> > 
> >         https://github.com/bitcoin/bitcoin/blob/d612837814020ae832499d18e6ee5eb919a87907/src/script/interpreter.cpp#L1467 https://github.com/bitcoin/bitcoin/blob/d612837814020ae832499d18e6ee5eb919a87907/src/script/interpreter.cpp#L1467
> > 
> > 
> >         This PR will detect such violation early and ban the peer:
> > 
> >         https://github.com/bitcoin/bitcoin/pull/8499 https://github.com/bitcoin/bitcoin/pull/8499
> > 
> >          
> > 
> > 
> >         Another approach is to run the scripts of all incoming transactions. That's not too bad as you have already fetched the utxos which is a major part of validation.
> > 
> >     > 
> 

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<!DOCTYPE html>
<html><head>
    <meta charset=3D"UTF-8">
</head><body><p>Thank you so much for taking time to actually review the co=
des. I hope you will&#160;keep raising questions when you feel something mi=
ght be wrong. This is how things supposed to work and we should&#160;not be=
 affected by some forum discussions.</p><blockquote type=3D"cite">On August=
 26, 2016 at 9:16 AM Sergio Demian Lerner &#60;sergio.d.lerner@gmail.com&#6=
2; wrote:<br><br><div dir=3D"ltr"><div>Because there was a discussion on re=
ddit about this topic, I want to clarify that Johnson Lau explained how a c=
heck in the code prevents this attack.<br></div><div>So there is no real at=
tack.<br></div><div><br></div><div>Also note that the subject of this threa=
d has a question mark, which means that I&#39;m asking the community for cl=
arification, not asserting the existence of a vulnerability.<br><br></div><=
div>The segwit code is complex, and some key parts of the consensus code ar=
e spread over the source files (such as state.CorruptionPossible() relation=
 to DoS banning, IsNull() check in witness program serialization, etc.). <b=
r><br></div><div>Thanks again Johnson for your clarifications.<br></div><di=
v><br></div></div><div class=3D"ox-455267db53-gmail_extra"><br><div class=
=3D"ox-455267db53-gmail_quote">On Wed, Aug 24, 2016 at 10:49 PM, Johnson La=
u &#60;<a href=3D"mailto:jl2012@xbt.hk" target=3D"_blank">jl2012@xbt.hk</a>=
&#62; wrote:<br><blockquote><u></u><div><p>Adding witness data to a non-seg=
wit script is invalid by consensus:</p><p><a href=3D"https://github.com/bit=
coin/bitcoin/blob/d612837814020ae832499d18e6ee5eb919a87907/src/script/inter=
preter.cpp#L1467" target=3D"_blank">https://github.com/bitcoin/<wbr/>bitcoi=
n/blob/<wbr/>d612837814020ae832499d18e6ee5e<wbr/>b919a87907/src/script/<wbr=
/>interpreter.cpp#L1467</a></p><p><br></p><p>This PR will detect such viola=
tion early and ban the peer:</p><p><a href=3D"https://github.com/bitcoin/bi=
tcoin/pull/8499" target=3D"_blank">https://github.com/bitcoin/<wbr/>bitcoin=
/pull/8499</a></p><p>&#160;<br></p><p><br></p><p>Another approach is to run=
 the scripts of all incoming transactions. That&#39;s not too bad as you ha=
ve already&#160;fetched the utxos which is a major part of validation.</p><=
/div></blockquote></div><br></div></blockquote></body></html>
=20
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