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Date: Mon, 28 Dec 2015 11:35:34 -0800
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Subject: Re: [bitcoin-dev] We need to fix the block withholding attack
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On Mon, Dec 28, 2015 at 11:30 AM, Emin G=C3=BCn Sirer <
bitcoin-dev@lists.linuxfoundation.org> wrote:

>
>
> On Mon, Dec 28, 2015 at 2:12 PM, Peter Todd via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> Do you specifically mean selfish mining as defined in Emin G=C3=BCn
>> Sirer/Ittay Eyal's paper? Keep in mind that attack is only a significant
>> issue in a scenario - one malicious miner with >30% hashing power -
>> where you're already very close to the margins anyway; the difference
>> between a 50% attack threshold and a 30% attack threshold isn't very
>> significant.
>>
>
> This is not quite right: we know that selfish mining is a guaranteed win
> at 34%. We do not know when exactly it begins to pay off. The more
> consolidated and centralized the other mining pools, the less of a threat
> it is below 34%; the more decentralized, the more likely it is to pay off
> at lower thresholds.
>

Exactly.


> Far more concerning is network propagation effects between large and
>> small miners.
>
>
> On a related note, the Bitcoin-NG paper took a big step towards moving
> these kinds of concerns out of the realm of gut-feelings and wavy hands
> into science. In particular, it introduced metrics for fairness (i.e.
> differential
> rate in orphans experienced by small and large miners), hash power
> efficiency, as well as consensus delay.
>
>
>> For that class of issues, if you are in an environemnt
>> where selfish mining is possible - a fairly flat, easily DoS/sybil
>> attacked network topology - the profitability difference between small
>> and large miners even *without* attacks going on is a hugely worrying
>> problem.
>
>
> Indeed, there is a slight, quantifiable benefit to larger pools. Which is
> why
> we need to be diligent about not letting pools get too big.
>
>
>> Note though that Eligius is *not* the only pool to have had problems
>>
> with block withholding, though AFAIK Eligius is the only one who has
>> gone on record so far. (as I said in my original post, I'm relaying
>> information given to me under condition of confidentiality)
>>
>
> I can see why they don't want to go public with this: it means that they
> are less profitable than other pools.
>

This I disagree with -- if they know that they have been attacked, then
there is every reason to come forward with this information.

First of all, it offers an explanation for poor profits (this is better
than unexplained poor profits).

Second of all, if one pool can be attacked then any pool can be attacked --
this is not a reason not to mine on a particular pool.  If anything, it's a
reason to diversify hashrate among many pools.

--Adam

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<div dir=3D"ltr"><div class=3D"gmail_extra"><br><div class=3D"gmail_quote">=
On Mon, Dec 28, 2015 at 11:30 AM, Emin G=C3=BCn Sirer <span dir=3D"ltr">&lt=
;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_blank"=
>bitcoin-dev@lists.linuxfoundation.org</a>&gt;</span> wrote:<br><blockquote=
 class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc soli=
d;padding-left:1ex"><div dir=3D"ltr"><br><div class=3D"gmail_extra"><br><di=
v class=3D"gmail_quote"><span class=3D"">On Mon, Dec 28, 2015 at 2:12 PM, P=
eter Todd via bitcoin-dev <span dir=3D"ltr">&lt;<a href=3D"mailto:bitcoin-d=
ev@lists.linuxfoundation.org" target=3D"_blank">bitcoin-dev@lists.linuxfoun=
dation.org</a>&gt;</span> wrote:<br><blockquote class=3D"gmail_quote" style=
=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Do you s=
pecifically mean selfish mining as defined in Emin G=C3=BCn<br>
Sirer/Ittay Eyal&#39;s paper? Keep in mind that attack is only a significan=
t<br>
issue in a scenario - one malicious miner with &gt;30% hashing power -<br>
where you&#39;re already very close to the margins anyway; the difference<b=
r>
between a 50% attack threshold and a 30% attack threshold isn&#39;t very<br=
>
significant.<br></blockquote><div><br></div></span><div>This is not quite r=
ight: we know that selfish mining is a guaranteed win</div><div>at 34%. We =
do not know when exactly it begins to pay off. The more=C2=A0</div><div>con=
solidated and centralized the other mining pools, the less of a threat</div=
><div>it is below 34%; the more decentralized, the more likely it is to pay=
 off=C2=A0</div><div>at lower thresholds.</div></div></div></div></blockquo=
te><div><br></div><div>Exactly.</div><div>=C2=A0</div><blockquote class=3D"=
gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-=
left:1ex"><div dir=3D"ltr"><div class=3D"gmail_extra"><div class=3D"gmail_q=
uote"><span class=3D""><blockquote class=3D"gmail_quote" style=3D"margin:0 =
0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">Far more concerning i=
s network propagation effects between large and<br>
small miners. </blockquote><div><br></div></span><div>On a related note, th=
e Bitcoin-NG paper took a big step towards moving</div><div>these kinds of =
concerns out of the realm of gut-feelings and wavy hands=C2=A0</div><div>in=
to science. In particular, it introduced metrics for fairness (i.e. differe=
ntial</div><div>rate in orphans experienced by small and large miners), has=
h power=C2=A0</div><div>efficiency, as well as consensus delay.=C2=A0</div>=
<span class=3D""><div>=C2=A0</div><blockquote class=3D"gmail_quote" style=
=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">For that=
 class of issues, if you are in an environemnt<br>
where selfish mining is possible - a fairly flat, easily DoS/sybil<br>
attacked network topology - the profitability difference between small<br>
and large miners even *without* attacks going on is a hugely worrying<br>
problem. </blockquote><div><br></div></span><div>Indeed, there is a slight,=
 quantifiable benefit to larger pools. Which is why</div><div>we need to be=
 diligent about not letting pools get too big.</div><span class=3D""><div>=
=C2=A0<br></div><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8e=
x;border-left:1px #ccc solid;padding-left:1ex">Note though that Eligius is =
*not* the only pool to have had problems<br></blockquote><blockquote class=
=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padd=
ing-left:1ex">
with block withholding, though AFAIK Eligius is the only one who has<br>
gone on record so far. (as I said in my original post, I&#39;m relaying<br>
information given to me under condition of confidentiality)<br></blockquote=
><div><br></div></span><div>I can see why they don&#39;t want to go public =
with this: it means that they</div><div>are less profitable than other pool=
s.=C2=A0</div></div></div></div></blockquote><div><br></div><div>This I dis=
agree with -- if they know that they have been attacked, then there is ever=
y reason to come forward with this information.</div><div><br></div><div>Fi=
rst of all, it offers an explanation for poor profits (this is better than =
unexplained poor profits).</div><div><br></div><div>Second of all, if one p=
ool can be attacked then any pool can be attacked -- this is not a reason n=
ot to mine on a particular pool.=C2=A0 If anything, it&#39;s a reason to di=
versify hashrate among many pools.</div><div><br></div><div>--Adam</div></d=
iv></div></div>

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