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40 votes

51% attack - apparently very easy? refering to CZ's "rollback btc chain" - How to make sure such corruptible scenario can never happen so easily?

Disclaimer: I believe this question may be primarily opinion-based and not very appropriate for this site, but there are a number of technical misunderstandings that can be clarified along with it, so ...
Pieter Wuille's user avatar
27 votes
Accepted

Why do we call it a 51% attack instead of a 50% attack?

The distinction is of theoretical importance only. But if the attacker controls exactly 50%, then it's true that the attacker will eventually catch up, but he won't stay caught up: the honest ...
Nate Eldredge's user avatar
25 votes

Why does Bitcoin no longer have checkpoints?

The issue is that you assume a majority attack is an attack that can be prevented. It is not. It is a fundamental breakdown of the security assumptions. Proof of work (PoW)'s assumption is that the ...
Pieter Wuille's user avatar
10 votes

51% attack and rewriting to the latest checkpoint

As long as both competing chain-tips are adhering to the same rules, the chain with the most aggregate difficulty ("heavier") will win, regardless of height. Nodes performing the initial sync would ...
Murch's user avatar
  • 76k
10 votes

51% attack - apparently very easy? refering to CZ's "rollback btc chain" - How to make sure such corruptible scenario can never happen so easily?

(adding some color) Some discussion I saw suggested that people promoting this believed they only needed to achieve >50% hashpower, which caused them to overestimate the feasibility. Reorging with ...
G. Maxwell's user avatar
  • 7,727
9 votes
Accepted

How does the network protect itself against attacks where one miner with a lot of hashing power lies that it takes 10 minutes to create a block?

What you describe is a (variant of a) majority attack: if an attacker can construct a block every 3 minutes while the rest of the network can only construct a block every 10 minutes, that means the ...
Pieter Wuille's user avatar
7 votes

Why don't the miners just take profit from our coins and make the coin worthless?

Because miners do not actually own/ or have access to the bitcoins which are being spent in a transaction. They simply choose a number of transactions which have valid signatures, and put them ...
Rutger Versteegden's user avatar
6 votes

Incentive and 51% attack

Bitcoin has a huge incentive system to discourage a 51% attack. Bitcoin mining with CPUs is impractical. You have to use dedicated hardware that can only be used to mine Bitcoin and other coins that ...
David Schwartz's user avatar
6 votes

Why do we call it a 51% attack instead of a 50% attack?

This is to impart the need for a higher hash rate than the rest of the (honest) miners. Mining success is probabilistic, however, so this 50% or 51% is an indication of the expected behaviour given an ...
user36303's user avatar
  • 786
6 votes
Accepted

Couldn't Microsoft execute 51% attack?

Microsoft's software runs on a very large number of PCs. However the computers in question generally don't have specialised hashing hardware - they only have typical central processing units (CPUs) ...
RedGrittyBrick's user avatar
5 votes
Accepted

51% attack clarification

Yes, the attacker needs to be faster than the rest of the network combined. Being faster than the second-fastest miner is not enough. I think this is where you are mistaken: Blocks in the ...
Jestin's user avatar
  • 8,812
5 votes

Is "fee higher than block reward" attack possible?

An attack like this is possible, it is just a way to profit off a 51% attack. However you must remember several things: if a miner controls less than 51% of the hashpower, then chances are they will ...
chytrik's user avatar
  • 18.3k
5 votes

Why has BCH not received a 51% attack yet?

Because of the cost, specifically the opportunity cost. If a miner has 10% of the BTC hashrate, then if they pointed all of that mining power to BCH, they would still be losing money. With 50% of the ...
Ava Chow's user avatar
  • 71k
5 votes
Accepted

Can a bitcoin transaction from a specific bitcoin address be banned? (via mining pools)

Yes, miners can censor transactions. A mining pool (or solo miner) can choose to not add a transaction to any blocks that they create. And, of course, this can be done dynamically so that transactions ...
Ava Chow's user avatar
  • 71k
4 votes

Why wouldn't miners change the protocol? (e.g. change difficulty)

The rest of the network, which uses the existing difficulty parameters, will reject the new blocks generated by your modified miners. There will be a longer blockchain, but there will not be a longer ...
Greg Hewgill's user avatar
  • 3,421
4 votes
Accepted

How exactly are Bitcoin's consensus rules enforced?

But how exactly the consensus rules are forced? Every single full node enforces them. Internet says that the consensus rules are the rules thet every full node follows. And rejects blocks that do ...
David Schwartz's user avatar
4 votes

51% attack - apparently very easy? refering to CZ's "rollback btc chain" - How to make sure such corruptible scenario can never happen so easily?

I want to add a little bit to the coordination problem that @PiererWuille mentioned. Not only does the probabilistic success of a rollback depend on the ability of miners and affected parties to ...
chytrik's user avatar
  • 18.3k
4 votes

Don't miners control who is allowed to make transactions?

The stakeholders of bitcoin don't want miners to do that. If miners do that, the stakeholders (the people who are paying for the miners to mine) would change the rules. For example, they'd change the ...
David Schwartz's user avatar
4 votes
Accepted

Can a 51% attacker earn mining rewards on the blocks they tampered with?

When the attacker broadcasts the chain they had been working on in private, will they get mining rewards for all those new blocks after the reorg? Yes, the majority attacker can create the longest ...
chytrik's user avatar
  • 18.3k
4 votes

Can someone with majority hashing power decide what transactions are included in my block?

If I somehow was able to mine a block, How can someone with majority hashing power can decide transactions to include in my block Nobody else can influence the contents of the block you are working ...
chytrik's user avatar
  • 18.3k
4 votes

Attack the network by having 51% of full nodes?

The question has the appearance of mixing up two different types of attack The so-called "51% attack" The "eclipse attack" The first is where an attacker controls >50% of the ...
RedGrittyBrick's user avatar
4 votes
Accepted

Can the two biggest bitcoin mining pools lock out all others?

It's certainly not an ideal situation. However them cooperating to censor outsiders' blocks doesn't make it impossible for anyone else to "squeeze in a block" ever again. In order to prevent ...
Antoine Poinsot's user avatar
4 votes
Accepted

How can an external party be sure that node's response is valid?

A Merkle inclusion proof is a proof that a transaction is included in a block with a particular hash, nothing more. Whoever gave you that proof can lie by omission (claiming no transactions you asked ...
Pieter Wuille's user avatar
3 votes

What is "If ghash.io hits 51% people would just leave them?" referring to?

GHash.io was a big mining pool back when mining pools were mostly just a way to organize for large numbers of small private mining operations. (Whereas today, the biggest mining pools own huge mining ...
Murch's user avatar
  • 76k
3 votes

What is "If ghash.io hits 51% people would just leave them?" referring to?

No, the network wouldn't disconnect them. There's no way for that to happen. Recall that a pool is an association of miners who agree to share the proceeds of their mining; whenever anyone mines a ...
Nate Eldredge's user avatar
3 votes
Accepted

How to extract maximum value from a 51% attack

If an attacker were to gain 51% of the network, they can essentially forge arbitrary transactions. With control over a majority of hash rate (either directly, or by using something like selfish ...
David A. Harding's user avatar
3 votes
Accepted

Is possible to destroy the Bitcoin for just $500K? (51% attack)

TL/DR: No. The long form of the answer: Online mining is that grey area where you don't invest until you're 100% sure. The aforementioned website is likely to scam you if they are providing such a ...
Junaid Shaikh's user avatar
3 votes

Why can a 51% attacker not change the block reward?

Even you have 51% of hashpower you can not change the software on other computers to accept your blocks.
amaclin's user avatar
  • 6,780
3 votes

Why would more confirmations help in a 51% attack scenario?

If the exchange waits for 6 confirmations (1 hour) and sends the dollars then, it could be possible that no one even noticed that there was such an attack. Then a scammer could pay out twice. But if ...
hardfork's user avatar
  • 2,117
3 votes

Do you really need more than 50% mining power in order to attack Bitcoin blockchain?

A majority attack (51% attack) was given this name for a reason: in order for the attack to succeed on average, the attacker needs 51% of the hashpower. Reading your question, I can see a ...
chytrik's user avatar
  • 18.3k

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