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charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable 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 Sent: Friday, August 2, 2019 14:19 To: Kenshiro [] ; Bitcoin Dev Cc: Alistair Mann 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 > 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 [] > Sent: Wednesday, July 31, 2019 16:40 To: Alistair Mann >; Bitcoin Protocol Dis= cussion > 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 > Sent: Wednesday, July 31, 2019 15:59 To: Kenshiro [] >; Bitcoin = Protocol Discussion > 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 https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev --_000_DB6PR10MB1832110D67FB7FBD1AF5E3EDA6D90DB6PR10MB1832EURP_ Content-Type: text/html; charset="iso-8859-1" Content-Transfer-Encoding: quoted-printable
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 finally follow the longest chain:<= /div>

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 ch= eckpoint rule, and it follows the fork, the longest chain.

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.

So we have 2 possible situations to consider:

- 51% attack:  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.

- 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.

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 <eth= 3rs@gmail.com>
Sent: Friday, August 2, 2019 14:19
To: Kenshiro [] <tensiam@hotmail.com>; Bitcoin Dev <bitcoin= -dev@lists.linuxfoundation.org>
Cc: Alistair Mann <al@pectw.net>
Subject: Re: [bitcoin-dev] Add a moving checkpoint to the Bitcoin pr= otocol
 
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 permanently. Under current rules = this partition will be able to rejoin the network 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+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-->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 interventi= on to determine 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 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.

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 Ken= shiro [] via bitcoin-dev <bitcoin-dev@lists.linuxfoun= dation.org> wrote:
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>
Sent: Wednesday, July 31, 2019 16:40
To: Alistair Mann <al@pectw.net>; Bitcoin Protocol Discussion &l= t;bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Add a moving checkpoint to the Bitcoin pr= otocol
 
>>> 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 blocks (24 hours) and there are 2 permanent forks longer than N, n= odes from one branch could delete their local history so they would join th= e other branch.

Regards,


From: Alistair Mann <al@pectw.net>
Sent: Wednesday, July 31, 2019 15:59
To: Kenshiro [] <tensiam@hotmail.com>; Bitcoin Protocol D= iscussion <bitcoin-dev@lists.linuxfoundation.org&= gt;
Subject: Re: [bitcoin-dev] Add a moving checkpoint to the Bitcoin pr= otocol
 
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 Bitcoin
> 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% a= ttack
> which thanks to the moving checkpoint can't rewrite history older than= the
> 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
https://lists.linuxfoundati= on.org/mailman/listinfo/bitcoin-dev
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