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In-Reply-To: <20180609124516.6ms6t7r5t7ikved6@petertodd.org>
From: Sergio Demian Lerner <sergio.d.lerner@gmail.com>
Date: Sat, 9 Jun 2018 14:51:55 +0200
Message-ID: <CAKzdR-omk7wCk5c_9T28O2Hc-Uzui5S4B_BuDLFt27RWSFzdJw@mail.gmail.com>
To: Peter Todd <pete@petertodd.org>
Content-Type: multipart/alternative; boundary="00000000000060c77a056e34fdd1"
Cc: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Trusted merkle tree depth for safe tx inclusion
 proofs without a soft fork
Precedence: list

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Yo can fool a SPV wallet even if it requires a thousands confirmations
using this attack, and you don't need a Sybil attack, so yes, it impacts
SPV wallets also. The protections a SPV node should have to prevent this
attack are  different, so it must be considered separately.

It should be said that a SPV node can avoid accepting payments if any
Merkle node is at the same time a valid transaction, and that basically
almost eliminates the problem.

SPV Wallet would reject valid payments with a astonishingly low probability.


On Sat, Jun 9, 2018 at 2:45 PM Peter Todd <pete@petertodd.org> wrote:

> On Sat, Jun 09, 2018 at 02:21:17PM +0200, Sergio Demian Lerner wrote:
> > Also it must be noted that an attacker having only 1.3M USD that can
> > brute-force 72 bits (4 days of hashing on capable ASICs) can perform the
> > same attack, so the attack is entirely feasible and no person should
> accept
> > more than 1M USD using a SPV wallet.
>
> That doesn't make any sense. Against a SPV wallet you don't need that
> attack;
> with that kind of budget you can fool it by just creating a fake block at
> far
> less cost, along with a sybil attack. Sybils aren't difficult to pull off
> when
> you have the budget to be greating fake blocks.
>
> > Also the attack can be repeated: once you create the "extension point"
> > block, you can attack more and more parties without any additional
> > computation.
>
> That's technically incorrect: txouts can only be spent once, so you'll
> need to
> do 2^40 work each time you want to repeat the attack to grind the matching
> part
> of the prevout again.
>
> --
> https://petertodd.org 'peter'[:-1]@petertodd.org
>

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<div dir=3D"ltr">Yo can fool a SPV wallet even if it requires a thousands c=
onfirmations using this attack, and you don&#39;t need a Sybil attack, so y=
es, it impacts SPV wallets also. The protections a SPV node should have to =
prevent this attack are=C2=A0 different, so it must be considered separatel=
y.<div><br></div><div>It should be said that a SPV node can avoid accepting=
 payments if any Merkle node is at the same time a valid transaction, and t=
hat basically almost eliminates the problem.=C2=A0</div><div><br></div><div=
>SPV Wallet would reject valid payments with a astonishingly low probabilit=
y.</div><div><br></div><div><div><div><br></div></div></div></div><br><div =
class=3D"gmail_quote"><div dir=3D"ltr">On Sat, Jun 9, 2018 at 2:45 PM Peter=
 Todd &lt;<a href=3D"mailto:pete@petertodd.org">pete@petertodd.org</a>&gt; =
wrote:<br></div><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8e=
x;border-left:1px #ccc solid;padding-left:1ex">On Sat, Jun 09, 2018 at 02:2=
1:17PM +0200, Sergio Demian Lerner wrote:<br>
&gt; Also it must be noted that an attacker having only 1.3M USD that can<b=
r>
&gt; brute-force 72 bits (4 days of hashing on capable ASICs) can perform t=
he<br>
&gt; same attack, so the attack is entirely feasible and no person should a=
ccept<br>
&gt; more than 1M USD using a SPV wallet.<br>
<br>
That doesn&#39;t make any sense. Against a SPV wallet you don&#39;t need th=
at attack;<br>
with that kind of budget you can fool it by just creating a fake block at f=
ar<br>
less cost, along with a sybil attack. Sybils aren&#39;t difficult to pull o=
ff when<br>
you have the budget to be greating fake blocks.<br>
<br>
&gt; Also the attack can be repeated: once you create the &quot;extension p=
oint&quot;<br>
&gt; block, you can attack more and more parties without any additional<br>
&gt; computation.<br>
<br>
That&#39;s technically incorrect: txouts can only be spent once, so you&#39=
;ll need to<br>
do 2^40 work each time you want to repeat the attack to grind the matching =
part<br>
of the prevout again.<br>
<br>
-- <br>
<a href=3D"https://petertodd.org" rel=3D"noreferrer" target=3D"_blank">http=
s://petertodd.org</a> &#39;peter&#39;[:-1]@<a href=3D"http://petertodd.org"=
 rel=3D"noreferrer" target=3D"_blank">petertodd.org</a><br>
</blockquote></div>

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