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From: "Kenshiro []" <tensiam@hotmail.com>
To: Ethan Heilman <eth3rs@gmail.com>, Bitcoin Dev
	<bitcoin-dev@lists.linuxfoundation.org>
Thread-Topic: [bitcoin-dev] Add a moving checkpoint to the Bitcoin protocol
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Subject: Re: [bitcoin-dev] Add a moving checkpoint to the Bitcoin protocol
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Hi all,

Very good points. I did some clarifications in a private conversation, the =
new rule is making the moving checkpoint valid only if the difference in bl=
ocks between the main chain and the new fork is smaller than X blocks, like=
 for example 3 days of blocks, so after a long network split everyone can f=
inally follow the longest chain:

With the current protocol the fix to a network split is simple, the longest=
 chain win. But with the moving checkpoint I'm proposing we have a problem =
if both chains began to differ more than N blocks ago, the forks are perman=
ent. So we need an additional rule to ignore the moving checkpoint, a limit=
 of X blocks:

If a node sees a fork longer than his main chain, and the fork has at least=
 X blocks more than the main chain, then the node ignore the moving checkpo=
int rule, and it follows the fork, the longest chain.

So as an example, the moving checkpoint could be 24 hours of blocks, and th=
e limit of X blocks, the blocks of 3 days.

So we have 2 possible situations to consider:

- 51% attack:  the blocks older than 24 hours are protected against a histo=
ry rewrite during at least 3 days, in that time developers could release an=
 emergency release with another mining algorithm to stop the attack.

- Network split: if the network split is older than N blocks, we have 2 per=
manent forks (or chains), but in 3 days (or more) the blockchain heights wi=
ll differ in more than X blocks (the blocks of 3 days) because there will b=
e more miners in one chain than in the other so finally the loser chain wil=
l be abandoned and everyone will follow the longest chain.

It could be even more conservative, like 48 hours for the moving checkpoint=
 and a block limit of 7 days of blocks.

Regards,



________________________________
From: Ethan Heilman <eth3rs@gmail.com>
Sent: Friday, August 2, 2019 14:19
To: Kenshiro [] <tensiam@hotmail.com>; Bitcoin Dev <bitcoin-dev@lists.linux=
foundation.org>
Cc: Alistair Mann <al@pectw.net>
Subject: Re: [bitcoin-dev] Add a moving checkpoint to the Bitcoin protocol

Attack 1:
I partition (i.e. eclipse) a bunch of nodes from the network this partition=
 contains no mining power . I then mine 145 blocks for this partition. I do=
n't even need 51% of the mining power because I'm not competing with any ot=
her miners. Under this rule this partition will hardfork from the network p=
ermanently. Under current rules this partition will be able to rejoin the n=
etwork as the least weight chain will be orphaned.

Attack 2:
I pre-mine 145 blocks. A node goes offline for 24 hours, when it rejoins I =
feed it 145 blocks which fork off from the consensus chain. I have 24+24 ho=
urs to mine these 145 blocks so I should be able to do this with 25% of the=
 current hash rate at the time the node went offline. Under your rule each =
of these offline-->online nodes I attack this way will hardfork themselves =
from the rest of the network.

I believe a moving-checkpoint rule as describe above would make Bitcoin mor=
e vulnerable to 51% attacks.

A safer rule would be if a node detects a fork with both sides of the split=
 having  length > 144 blocks, it halts and requests user intervention to de=
termine which chain to follow.  I don't think 144 blocks is a great number =
to use here as 24 hours is very short. I suspect you could improve the secu=
rity of the rule by making the number of blocks a fork most reach to halt t=
he network proportional to the difference in time between the timestamp in =
the block prior to the fork and the current time. I am **NOT** proposing Bi=
tcoin adopt such a rule.

NXT has a fundamentally different security model as it uses Proof-of-stake =
rather than Proof-of-Work.

On Wed, Jul 31, 2019 at 2:37 PM Kenshiro [] via bitcoin-dev <bitcoin-dev@li=
sts.linuxfoundation.org<mailto:bitcoin-dev@lists.linuxfoundation.org>> wrot=
e:
P.S.: To be clearer, in this example I set an N value of 144 blocks, which =
is approximately 24 hours.

________________________________
From: Kenshiro [] <tensiam@hotmail.com<mailto:tensiam@hotmail.com>>
Sent: Wednesday, July 31, 2019 16:40
To: Alistair Mann <al@pectw.net<mailto:al@pectw.net>>; Bitcoin Protocol Dis=
cussion <bitcoin-dev@lists.linuxfoundation.org<mailto:bitcoin-dev@lists.lin=
uxfoundation.org>>
Subject: Re: [bitcoin-dev] Add a moving checkpoint to the Bitcoin protocol

>>> How would a (potentially, state-sponsored) netsplit lasting longer than=
 N be
handled?

It would be detected by the community much before reaching the reorg limit =
of N blocks (it's 24 hours) so nodes could stop until the netsplit is fixed=
.

In the extreme case no one notice the network split during more than N bloc=
ks (24 hours) and there are 2 permanent forks longer than N, nodes from one=
 branch could delete their local history so they would join the other branc=
h.

Regards,


________________________________
From: Alistair Mann <al@pectw.net<mailto:al@pectw.net>>
Sent: Wednesday, July 31, 2019 15:59
To: Kenshiro [] <tensiam@hotmail.com<mailto:tensiam@hotmail.com>>; Bitcoin =
Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org<mailto:bitcoin-d=
ev@lists.linuxfoundation.org>>
Subject: Re: [bitcoin-dev] Add a moving checkpoint to the Bitcoin protocol

On Wednesday 31 Jul 2019 12:28:58 Kenshiro [] via bitcoin-dev wrote:

> I would like to propose that a "moving checkpoint" is added to the Bitcoi=
n
> protocol. It's a very simple rule already implemented in NXT coin:
>
> - A node will ignore any new block under nodeBlockHeight - N, so the
> blockchain becomes truly immutable after N blocks, even during a 51% atta=
ck
> which thanks to the moving checkpoint can't rewrite history older than th=
e
> last N blocks.

How would a (potentially, state-sponsored) netsplit lasting longer than N b=
e
handled?
--
Alistair Mann

_______________________________________________
bitcoin-dev mailing list
bitcoin-dev@lists.linuxfoundation.org<mailto:bitcoin-dev@lists.linuxfoundat=
ion.org>
https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev

--_000_DB6PR10MB1832110D67FB7FBD1AF5E3EDA6D90DB6PR10MB1832EURP_
Content-Type: text/html; charset="iso-8859-1"
Content-Transfer-Encoding: quoted-printable

<html>
<head>
<meta http-equiv=3D"Content-Type" content=3D"text/html; charset=3Diso-8859-=
1">
<style type=3D"text/css" style=3D"display:none;"> P {margin-top:0;margin-bo=
ttom:0;} </style>
</head>
<body dir=3D"ltr">
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
Hi all,</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
Very good points. I did some clarifications in a private conversation, the =
new rule is making the moving checkpoint valid only if the difference in bl=
ocks between the main chain and the new fork is smaller than X blocks, like=
 for example 3 days of blocks, so
 after a long network split everyone can finally follow the longest chain:<=
/div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
With the current protocol the fix to a network split is simple, the longest=
 chain win. But with the moving checkpoint I'm proposing we have a problem =
if both chains began to differ more than N blocks ago, the forks are perman=
ent. So we need an additional rule
 to ignore the moving checkpoint, a limit of X blocks:<br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<div><br>
</div>
<div>If a node sees a fork longer than his main chain, and the fork has at =
least X blocks more than the main chain, then the node ignore the moving ch=
eckpoint rule, and it follows the fork, the longest chain.<br>
</div>
<div><br>
</div>
<div>So as an example, the moving checkpoint could be 24 hours of blocks, a=
nd the limit of X blocks, the blocks of 3 days.
<br>
</div>
<div><br>
</div>
<div>So we have 2 possible situations to consider:<br>
</div>
<div><br>
</div>
<div>- 51% attack: &nbsp;the blocks older than 24 hours are protected again=
st a history rewrite during at least 3 days, in that time developers could =
release an emergency release with another mining algorithm to stop the atta=
ck.<br>
</div>
<div><br>
</div>
<span>- Network split: if the network split is older than N blocks, we have=
 2 permanent forks (or chains), but in 3 days (or more) the blockchain heig=
hts will differ in more than X blocks (the blocks of 3 days) because there =
will be more miners in one chain
 than in the other so finally the loser chain will be abandoned and everyon=
e will follow the longest chain.</span><br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<span><br>
</span></div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
It could be even more conservative, like 48 hours for the moving checkpoint=
 and a block limit of 7 days of blocks.</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
Regards,</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<br>
</div>
<div style=3D"font-family: Calibri, Helvetica, sans-serif; font-size: 12pt;=
 color: rgb(0, 0, 0);">
<br>
</div>
<div>
<div id=3D"appendonsend"></div>
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
<br>
</div>
<hr tabindex=3D"-1" style=3D"display:inline-block; width:98%">
<div id=3D"divRplyFwdMsg" dir=3D"ltr"><font face=3D"Calibri, sans-serif" co=
lor=3D"#000000" style=3D"font-size:11pt"><b>From:</b> Ethan Heilman &lt;eth=
3rs@gmail.com&gt;<br>
<b>Sent:</b> Friday, August 2, 2019 14:19<br>
<b>To:</b> Kenshiro [] &lt;tensiam@hotmail.com&gt;; Bitcoin Dev &lt;bitcoin=
-dev@lists.linuxfoundation.org&gt;<br>
<b>Cc:</b> Alistair Mann &lt;al@pectw.net&gt;<br>
<b>Subject:</b> Re: [bitcoin-dev] Add a moving checkpoint to the Bitcoin pr=
otocol</font>
<div>&nbsp;</div>
</div>
<div>
<div dir=3D"auto">
<div dir=3D"ltr">Attack 1:<br>
I partition (i.e. eclipse) a bunch of nodes from the network this partition=
 contains no mining power . I then mine 145 blocks for this partition. I do=
n't even need 51% of the mining power because I'm not competing with any ot=
her miners. Under this rule this
 partition will hardfork from the network permanently. Under current rules =
this partition will be able to rejoin the network as the least weight chain=
 will be orphaned.<br>
<br>
Attack 2:<br>
I pre-mine 145 blocks. A node goes offline for 24 hours, when it rejoins I =
feed it 145 blocks which fork off from the consensus chain. I have 24&#43;2=
4 hours to mine these 145 blocks so I should be able to do this with 25% of=
 the current hash rate at the time the
 node went offline. Under your rule each of these offline--&gt;online nodes=
 I attack this way will hardfork themselves from the rest of the network.<b=
r>
<br>
I believe a moving-checkpoint rule as describe above would make Bitcoin mor=
e vulnerable to 51% attacks.<br>
<br>
A safer rule would be if a node detects a fork with both sides of the split=
 having&nbsp; length &gt; 144 blocks, it halts and requests user interventi=
on to determine which chain to follow.&nbsp; I don't think 144 blocks is a =
great number to use here as 24 hours is very
 short. I suspect you could improve the security of the rule by making the =
number of blocks a fork most reach to halt the network proportional to the =
difference in time between the timestamp in the block prior to the fork and=
 the current time. I am **NOT**
 proposing Bitcoin adopt such a rule.<br>
<br>
NXT has a fundamentally different security model as it uses Proof-of-stake =
rather than Proof-of-Work.</div>
</div>
<br>
<div class=3D"x_gmail_quote">
<div dir=3D"ltr" class=3D"x_gmail_attr">On Wed, Jul 31, 2019 at 2:37 PM Ken=
shiro [] via bitcoin-dev &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfound=
ation.org" target=3D"_blank" rel=3D"noreferrer">bitcoin-dev@lists.linuxfoun=
dation.org</a>&gt; wrote:<br>
</div>
<blockquote class=3D"x_gmail_quote" style=3D"margin:0px 0px 0px 0.8ex; bord=
er-left:1px solid rgb(204,204,204); padding-left:1ex">
<div dir=3D"ltr">
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
P.S.: To be clearer, in this example I set an N value of 144 blocks, which =
is approximately 24 hours.</div>
<div>
<div id=3D"x_m_-1010116091299868493gmail-m_6888503219858923550appendonsend"=
></div>
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
<br>
</div>
<hr style=3D"display:inline-block; width:98%">
<div id=3D"x_m_-1010116091299868493gmail-m_6888503219858923550divRplyFwdMsg=
" dir=3D"ltr">
<font face=3D"Calibri, sans-serif" color=3D"#000000" style=3D"font-size:11p=
t"><b>From:</b> Kenshiro [] &lt;<a href=3D"mailto:tensiam@hotmail.com" targ=
et=3D"_blank" rel=3D"noreferrer">tensiam@hotmail.com</a>&gt;<br>
<b>Sent:</b> Wednesday, July 31, 2019 16:40<br>
<b>To:</b> Alistair Mann &lt;<a href=3D"mailto:al@pectw.net" target=3D"_bla=
nk" rel=3D"noreferrer">al@pectw.net</a>&gt;; Bitcoin Protocol Discussion &l=
t;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" target=3D"_blank=
" rel=3D"noreferrer">bitcoin-dev@lists.linuxfoundation.org</a>&gt;<br>
<b>Subject:</b> Re: [bitcoin-dev] Add a moving checkpoint to the Bitcoin pr=
otocol</font>
<div>&nbsp;</div>
</div>
<div dir=3D"ltr">
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
&gt;&gt;&gt;&nbsp;<span>How would a (potentially, state-sponsored) netsplit=
 lasting longer than N be
<br>
</span><span>handled? &nbsp;</span></div>
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
<span><br>
</span></div>
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
<span>It would be detected by the community much before reaching the reorg =
limit of N blocks (it's 24 hours) so nodes could stop until the netsplit is=
 fixed.&nbsp;</span></div>
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
<span><br>
</span></div>
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
<span>In the extreme case no one notice the network split during more than =
N blocks (24 hours) and there are 2 permanent forks longer than N, n</span>=
odes from one branch could delete their local history so they would join th=
e other branch.</div>
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
<br>
</div>
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
Regards,</div>
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
<br>
</div>
<div>
<div id=3D"x_m_-1010116091299868493gmail-m_6888503219858923550x_appendonsen=
d"></div>
<div style=3D"font-family:Calibri,Helvetica,sans-serif; font-size:12pt; col=
or:rgb(0,0,0)">
<br>
</div>
<hr style=3D"display:inline-block; width:98%">
<div id=3D"x_m_-1010116091299868493gmail-m_6888503219858923550x_divRplyFwdM=
sg" dir=3D"ltr">
<font face=3D"Calibri, sans-serif" color=3D"#000000" style=3D"font-size:11p=
t"><b>From:</b> Alistair Mann &lt;<a href=3D"mailto:al@pectw.net" target=3D=
"_blank" rel=3D"noreferrer">al@pectw.net</a>&gt;<br>
<b>Sent:</b> Wednesday, July 31, 2019 15:59<br>
<b>To:</b> Kenshiro [] &lt;<a href=3D"mailto:tensiam@hotmail.com" target=3D=
"_blank" rel=3D"noreferrer">tensiam@hotmail.com</a>&gt;; Bitcoin Protocol D=
iscussion &lt;<a href=3D"mailto:bitcoin-dev@lists.linuxfoundation.org" targ=
et=3D"_blank" rel=3D"noreferrer">bitcoin-dev@lists.linuxfoundation.org</a>&=
gt;<br>
<b>Subject:</b> Re: [bitcoin-dev] Add a moving checkpoint to the Bitcoin pr=
otocol</font>
<div>&nbsp;</div>
</div>
<div class=3D"x_m_-1010116091299868493gmail-m_6888503219858923550x_BodyFrag=
ment"><font size=3D"2"><span style=3D"font-size:11pt">
<div class=3D"x_m_-1010116091299868493gmail-m_6888503219858923550x_PlainTex=
t">On Wednesday 31 Jul 2019 12:28:58 Kenshiro [] via bitcoin-dev wrote:<br>
<br>
&gt; I would like to propose that a &quot;moving checkpoint&quot; is added =
to the Bitcoin<br>
&gt; protocol. It's a very simple rule already implemented in NXT coin:<br>
&gt; <br>
&gt; - A node will ignore any new block under nodeBlockHeight - N, so the<b=
r>
&gt; blockchain becomes truly immutable after N blocks, even during a 51% a=
ttack<br>
&gt; which thanks to the moving checkpoint can't rewrite history older than=
 the<br>
&gt; last N blocks.<br>
<br>
How would a (potentially, state-sponsored) netsplit lasting longer than N b=
e <br>
handled?&nbsp; <br>
-- <br>
Alistair Mann<br>
<br>
</div>
</span></font></div>
</div>
</div>
</div>
</div>
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rel=3D"noreferrer">bitcoin-dev@lists.linuxfoundation.org</a><br>
<a href=3D"https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev" =
rel=3D"noreferrer noreferrer" target=3D"_blank">https://lists.linuxfoundati=
on.org/mailman/listinfo/bitcoin-dev</a><br>
</blockquote>
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