What happens when all nodes on the blockchain go offline?

[yo_mama]

So, for the decentralized network to run, there must be some nodes online.
What happenes, when at some time, all nodes on the blockchain go offline? Like for some unpopular coins, all node operators lose interest and stop running the node one day?
Will the blockchain be dead forever?
Or is there a way to restore the blockchain network and make it "alive" again, if at a later time some nodes come back to run the blockchain again?

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[shunsaitakahashi]

What happenes, when at some time, all nodes on the blockchain go offline?

While the answer depends on the exact consensus protocol used by the blockchain, one can expect the following:
1. No transaction will be recorded while all nodes are offline.
2. Previous transactions (in the last few blocks) will not be finalized for longer period if ever.
3. An adversary may be able to gain control of the blockchain with less effort/resources than usual.

Will the blockchain be dead forever?

The answer depends upon
1. If the offline event is caused by network outage, then the blockchain will likely resume operation when the connectivity is restored.
2. If the nodes have lost interest in the blockchain, it may never come back to life.

[yg10]

If at least one node still has blockchain data on disk, network can be restarted if such node (or multiple nodes) comes back online.
Otherwise this blockchain is dead.

[yg10]

What happenes, when at some time, all nodes on the blockchain go offline?

While the answer depends on the exact consensus protocol used by the blockchain, one can expect the following:
1. No transaction will be recorded while all nodes are offline.
2. Previous transactions (in the last few blocks) will not be finalized for longer period if ever.
3. An adversary may be able to gain control of the blockchain with less effort/resources than usual.

Will the blockchain be dead forever?

The answer depends upon
1. If the offline event is caused by network outage, then the blockchain will likely resume operation when the connectivity is restored.
2. If the nodes have lost interest in the blockchain, it may never come back to life.

How this adversary can get into the network if there are no running nodes?

[shunsaitakahashi]

How this adversary can get into the network if there are no running nodes?

Let's take example of Bitcoin. Assume all nodes of Bitcoin network go offline for some reason. Now assume an adversary (a party who wants to do harm to Bitcoin) detects the situation and starts mining blocks. Since no one else is mining at that time, the adversary can create selfish (illegal) transactions and include them in his/her blocks. If he is able to build many such blocks on top of each other, when the parties start getting back online, they may have to accept those blocks with selfish transactions (since the longest chain wins). If those blocks are accepted, the adversary has been able to successfully do illegal transactions.

[yg10]

How this adversary can get into the network if there are no running nodes?

Let's take example of Bitcoin. Assume all nodes of Bitcoin network go offline for some reason. Now assume an adversary (a party who wants to do harm to Bitcoin) detects the situation and starts mining blocks. Since no one else is mining at that time, the adversary can create selfish (illegal) transactions and include them in his/her blocks. If he is able to build many such blocks on top of each other, when the parties start getting back online, they may have to accept those blocks with selfish transactions (since the longest chain wins). If those blocks are accepted, the adversary has been able to successfully do illegal transactions.

How he can to start mining if he does not have previous blocks?
This will be just a new blockchain.

[Kogs]

Let's take example of Bitcoin. Assume all nodes of Bitcoin network go offline for some reason. Now assume an adversary (a party who wants to do harm to Bitcoin) detects the situation and starts mining blocks. Since no one else is mining at that time, the adversary can create selfish (illegal) transactions and include them in his/her blocks. If he is able to build many such blocks on top of each other, when the parties start getting back online, they may have to accept those blocks with selfish transactions (since the longest chain wins). If those blocks are accepted, the adversary has been able to successfully do illegal transactions.

Your explained way to cheat the system is not possible.
There is more than that the longest chain wins, it's actually the longest valid chain wins. If only the longest chain would win, I could start mining now with a very low difficulty and broadcast new blocks every 5 minutes. I would then have the longest chain and would, in your logic, force all other participants to change to my side of the blockchain, because it's the longest. That's not how it works.
If someone creates transactions or blocks which other nodes don't accept, because they go against the consensus rules, they just ignore those transactions and blocks and also ignore the node which is broadcasting those.
This is how it works at the moment with 100000 of participants, this is how it will work with only 2 participants in the bitcoin network.
The only way to be successful with such an attack is when the attacker change the bitcoin client and change the consensus rules so that his illegal transactions looks like valid transactions and all other participants of the network download this bitcoin client.
If other participants work with the original client and original consensus rules the only thing what happens is, that the attacker creates a fork of the blockchain with illegal transactions and blocks. All other will just ignore this side of the fork and continue on the other valid side of the fork.

So it's actually a dump way to cheat in this way.
If you are the only one left in the network you could just start mining normal and valid blocks and be happy with the block reward and hope that the coin is coming back alive after some time to be able to sell those coins. But it depends with which difficulty the last block was mined. If it was too high, you might not be able to mine any block within a reasonable time frame. Difficulty adjustments in Bitcoin only happens after 2016 mined blocks. So it's possible you are not able to mine those much blocks to reduce the difficulty.
Then the only way to bring the blockchain back to life would be a hardfork to change the difficulty adjustment. But then again, if other nodes start to join the network again, they will see your blocks as invalid unless they also update to your new client with new difficulty adjustment.

How he can to start mining if he does not have previous blocks?
This will be just a new blockchain.

In the example of Bitcoin there are currently a lot of full nodes which have the full blockchain saved. So even if all nodes go offline that does not mean that everyone is deleting the blockchain.
In the best case someone who want to continue mining already have the previous blocks. If not he might find someone who did run a full node in the past and get the blocks from this person.

[Kakmakr]

Why would people distributed all over the world, suddenly decide not to run their nodes? One reason might be that they found out that the government is behind this experiment or that some major flaw is going to threaten their coins. So this will require a immediate global shutdown of all nodes.

What will stop the enemy from running their node? Nothing at all. As long as there are people running a node, somewhere on this planet, Bitcoin would survive and this gives it the power that it has over other centralized payment networks.

[shunsaitakahashi]

There is more than that the longest chain wins, it's actually the longest valid chain wins.

Completely agree that it's the longest valid chain that wins. But it is not always possible to know if a selfish transaction is invalid. The validity rules embedded in the software can check for some invalid situations but not for every possible invalid situation. If validity rules were able to check for every invalid situation there would be no 51% attack (https://www.investopedia.com/terms/1/51-attack.asp).

... they may have to accept those blocks with selfish transactions ...

Note the use of may purely for that reason 

[ranochigo]

Completely agree that it's the longest valid chain that wins. But it is not always possible to know if a selfish transaction is invalid. The validity rules embedded in the software can check for some invalid situations but not for every possible invalid situation. If validity rules were able to check for every invalid situation there would be no 51% attack (https://www.investopedia.com/terms/1/51-attack.asp).

Kog is correct, you're not.

The validation in Bitcoin Core and similar other wallets will check each and every transactions for their validity. It is simply impossible for someone to broadcast a transaction that does not follow the network rules to the network and expect anyone to accept it. It won't be relayed and it won't be accepted, its that simple.

51% attack doesn't involve anything invalid or the breaking of rules. 51% attack is just the downside with the way that the system is designed and it is not possible to be avoided. Bitcoin hinges on the fact that whoever has the largest POW is the correct person. 51% attack is when a miner attempts to spend the inputs of a previous transaction and use their block to essentially "replace" it. You cannot break any of the network's rule (ie. create coins out of thin air), or else you're on your own fork. Please read the post that has been written thoroughly.

[shunsaitakahashi]

It is simply impossible for someone to broadcast a transaction that does not follow the network rules to the network and expect anyone to accept it...

... 51% attack doesn't involve anything invalid or the breaking of rules.

I am looking at https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power. Are you saying that

Reverse transactions that he sends while he's in control. This has the potential to double-spend transactions that previously had already been seen in the block chain.

is not possible?

[bob123]

It is simply impossible for someone to broadcast a transaction that does not follow the network rules to the network and expect anyone to accept it...

... 51% attack doesn't involve anything invalid or the breaking of rules.

I am looking at https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power. Are you saying that

Reverse transactions that he sends while he's in control. This has the potential to double-spend transactions that previously had already been seen in the block chain.

is not possible?

Reversing transactions (with 51% hash rate) made from an attack has nothing to do with breaking consensus rules.
The attacker has to have control over those 51% hashing power while sending an transaction.
For every confirmation of a TX (he is trying to reverse) he has to mine one block additionally more than the honest miner (in a shorter amount of time).

An attacker still has to broadcast blocks which meet the consensus rules. He can't just break them. And thats what ranochigo meaned.
If blocks will be mined 'on top' of invalid blocks (those who don't meed these rules) this will create a fork with only one person participating, the attacker.

[ranochigo]

I am looking at https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power. Are you saying that

Reverse transactions that he sends while he's in control. This has the potential to double-spend transactions that previously had already been seen in the block chain.

is not possible?

I didn't say that. It is not possible to prevent against double spending since it doesn't violate any of the protocol rules. You have to quote my entire post to understand what I'm referring to.

Anyhow, I've re-read your posts again and I've understood that your point is that someone can effectively execute a 51% attack if no one else is mining. The probability of anyone executing that is close to zero. One would have to have a significant hashpower before they can even mine a block. Depending on how many blocks ago that is, the cost of such an attack would outweigh the benefits. The effort/resources needed to execute said attack is the same.

[Cryptagio]

What happenes, when at some time, all nodes on the blockchain go offline?

While the answer depends on the exact consensus protocol used by the blockchain, one can expect the following:
1. No transaction will be recorded while all nodes are offline.
2. Previous transactions (in the last few blocks) will not be finalized for longer period if ever.
3. An adversary may be able to gain control of the blockchain with less effort/resources than usual.

Will the blockchain be dead forever?

The answer depends upon
1. If the offline event is caused by network outage, then the blockchain will likely resume operation when the connectivity is restored.
2. If the nodes have lost interest in the blockchain, it may never come back to life.

It would still be very hard for an adversary to gain control over the blockchain since he/she would need enormous amount of hash rate to have significant chances to mine a block alone. And he/she would have to do it until the network reajusts difficulty (which is 2 weeks for BTC)

[Kogs]

There is more than that the longest chain wins, it's actually the longest valid chain wins.

Completely agree that it's the longest valid chain that wins. But it is not always possible to know if a selfish transaction is invalid. The validity rules embedded in the software can check for some invalid situations but not for every possible invalid situation. If validity rules were able to check for every invalid situation there would be no 51% attack (https://www.investopedia.com/terms/1/51-attack.asp).

... they may have to accept those blocks with selfish transactions ...

Note the use of may purely for that reason 

As ranochigo and bob123 already mentioned, the 51% attack has nothing todo with consensus rules.
Even with a 51% attack you would not be able to create invalid transactions as they would be rejected by any other participants.

But you could do the following, even if it is very hard to do because you really need massive amount of hash power even to lower the difficulty. If you don't manage it to mine 2016 blocks the difficulty will not decrease. And with a high enough difficulty to begin with, you might need 1 week or longer to just mine one single block. To mine 2016 blocks would then take years.

But lets say just for fun the attacker does have the necessary hash power.
What could he do with a 51% attack?

1) He could prevent transactions which are broadcasted and are inside the mempool to get into a block so that the transaction keeps unconfirmed as long as he keeps the 51% advantage.
2) If he really is the only one who mines this coin he could rollback transactions. For this he defines a specific blockhight from the past, lets say he start 2 weeks in the past (about 2000 blocks before the current block hight) and continue mining from this block and create a fork. At some point his fork might be longer than the original one and then all other participants would need to reorg their chain and your chain become the longest valid one.
The only reason why someone would do this, is when he plans to replay one of his own transactions.
Lets say he made a transactions 2 weeks ago where he transmitted 10 coins. The transaction was confirmed by the network and got into one block. If he starts mining one block before the block of his transaction he could create a replay transaction which use one of the same UTXOs from the original transaction and send the coins to one of his address instead of the original target. In this case the orignal transaction would become invalid and the original receiver of the coins would not get it.

But you can do this only with transactions where you control the private key. All other transactions which were inside the blocks you rollbacked would just go back to the mempool and wait for anybody to mine them again. They are not lost. Either the attacker himself mines those reorganized transactions on his side of the fork or like in point 1 described, he just ignores them and they stay in the mempool forever or until any other miner starts mining and he looses the 51% advantage.

Actually there is a chance that those rollbacked transaction are lost forever if the other participants of the network did not had the original rollbacked blocks in their local blockchain. For example they start syncing the blockchain from the beginning or from an older point than the attacker started the reorg. But if anybody did have this reorg blocks in their local blockchain, those transaction would be broadcasted again (through the reorg process of the bitcoin client) and later mined again by any other miner.

That's actually the only 2 attack points you have with a 51% attack.
Point 2 above (to get already spent coins back) makes only sense if the coins do have any value (but if no one is interested in those coins - no node is online, no one is mining then the value of this coin will be 0). And then it's maybe still more profitable to just mine as much blocks as possible on top of the last block to get the block reward.

In all other situations where you start creating invalid blocks/transactions you would just create an invalid fork of the blockchain which does not give you any advantage as all other participants would just reject your blocks.

[Tony Long]

So, for the decentralized network to run, there must be some nodes online.
What happenes, when at some time, all nodes on the blockchain go offline? Like for some unpopular coins, all node operators lose interest and stop running the node one day?
Will the blockchain be dead forever?
Or is there a way to restore the blockchain network and make it "alive" again, if at a later time some nodes come back to run the blockchain again?

Now, generally, it relies on the internet.

[mike_spencer]

So, for the decentralized network to run, there must be some nodes online.
What happenes, when at some time, all nodes on the blockchain go offline? Like for some unpopular coins, all node operators lose interest and stop running the node one day?
Will the blockchain be dead forever?
Or is there a way to restore the blockchain network and make it "alive" again, if at a later time some nodes come back to run the blockchain again?

Yes it can happen with multiple reasons,

1. People losses interest in crypto.
2. or Some regulatory policy imposed which enables this ti do.

if all nodes looses interest impossible to back to life again, but if internet fluctuation it can be alive once online again.

[Ucy]

I guess Nodes monetization would make it almost impossible for people to lose interest. I still don't understand why people aren't paid for Decentralizing Cryptocurrencies.The lack of monetary rewards for Nodes just doesn't make sense to me.

Maybe he was worried that one person could host as many Nodes as possible.

[V-t.Ester]

Maybe only global space cataclysm or aliens that will destroy all Earth's orbit satellites can force all nodes to stop working. That will mean the total end of blockchain, Bitcoin, Internet etc. But this is an incredible stuff.

[bob123]

1. People losses interest in crypto.

This still would not mean the death of bitcoin. If there are 2+ nodes online, it can be considered as a network (a pretty small one obviously).

2. or Some regulatory policy imposed which enables this ti do.

Regulations can't shut down bitcoin. Thats not how it works.
Regulators can force provider to discard packets which have been flagged as BTC packets. But this can't be forced on 'all' routers.
So thats not a way of shutting the BTC network down.

if all nodes looses interest impossible to back to life again

It is still not impossible. If a miner/node goes back online with the latest copy of the blockchain, it would be 'back to life again'.
Like resuming a paused movie.