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 ...
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27 votes
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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 ...
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21 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 ...
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19 votes
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Supermajority to prevent 51% attack?

The 51% attack is emergent behavior of the system. It's not because there's a "50%" buried somewhere in the protocol that can just be changed to 60% or 75%. Someone with more hashing power than ...
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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 ...
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  • 7,439
9 votes
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Why doesn't bitcoin place additional constraints on competing block chain forks, other than length of chain? (e.g., time and confirmation count)

My question then is: why doesn't bitcoin specify a maximum duration of time and/or a maximum number of confirmations, after which a competing/forking block is rejected even if it's backed by a longer ...
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  • 28.6k
8 votes

Why doesn't bitcoin place additional constraints on competing block chain forks, other than length of chain? (e.g., time and confirmation count)

First, a quick clarification: assuming two chains both have valid blocks, it's the chain with the most proof of work that wins, not necessarily the chain with the most blocks. Second, thanks for the ...
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8 votes
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How much would it cost to do a 51% attack

There is a handy chart at https://en.bitcoin.it/wiki/Mining_hardware_comparison. If we assume that it is complete, and that the figures listed there are accurate (I have not verified them), and that ...
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8 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 ...
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  • 62.4k
7 votes
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Why does an adversary have to control 50% of the computing power to double spend?

There are two assumptions in your question that aren't completely correct. 1) Each node would then require 68 minutes to find a proof of work (trying 2^52 hashes). The process of finding a new block ...
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  • 62.4k
7 votes
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Can an attacker with 51% of hash power change old blocks?

An attacker has a hard time changing the past An attacker has very limited influence to change old blocks, because he has to replace all blocks that confirm the event he wants to change and keep up ...
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  • 62.4k
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 ...
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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 ...
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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 ...
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  • 786
5 votes

Supermajority to prevent 51% attack?

David Schwartz has a good answer. But to explore a little further, you can roughly think of Bitcoin's proof-of-work system, in the long run, as an election to decide which is the "true" block chain, ...
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5 votes

Can I detect a 51% attack as a developer

Generally, no, not until it's too late. A typical 51% attack would look like this: Attacker privately starts mining their own chain, which diverges from the main chain at some block N. Attacker ...
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5 votes

Does Bitcoin's SHA256 proof make a 51% attack more likely?

I think what most people making the 51% attack argument for memory hardness miss is that the base unit is meaningless when you're talking about percentages. Whether your mining algorithm is ...
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  • 14.3k
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 ...
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  • 60.5k
5 votes
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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 ...
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  • 60.5k
4 votes

What can an attacker with 51% of hash power do?

The answers so far focus on the algorithm itself, I have a few social economic thoughts to add. Let's assume Bitcoin is massively popular and indeed becomes THE global go-to currency, at this point ...
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  • 141
4 votes

Is it possible for more than 21 million bitcoins to exist if 51% agree?

Maybe. It would require a split in bitcoin users. Those who wanted the change could switch to a new version of the bitcoin software with the code changed to allow it. The problem comes from the fact ...
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  • 591
4 votes

Supermajority to prevent 51% attack?

I think you should state exactly what you mean by "51% attack". If by attack you mean double-spending, the original Bitcoin paper by Satoshi outlines that it brings more reward to mine all the blocks ...
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  • 5,114
4 votes
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Would it be possible to overtake the network by running a secluded mining operation for many years?

'Longer' means 'more total work spent'. You're not going to be working harder than all miners put together are you? Then the only way to get some money is to join the existing miners and work with ...
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  • 6,169
4 votes
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When an attacker publishes a longer chain, does Bitcoin Core emit a warning?

When a previously unknown chain fork of at least 7 blocks length is published, how does Bitcoin Core react? It reorganizes to that chain, any transactions that were lost are returned to the nodes ...
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  • 15k
4 votes
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Can a malicious majority miner rewrite history?

Let's assume for a minute that your theory is true, and that actual nodes on the network would not accept this new, valid, and longer block chain. That on itself would be a bug. Maybe it isn't ...
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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 ...
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  • 3,401
4 votes
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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 ...
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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 ...
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  • 17.2k
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 ...
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4 votes
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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 ...
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  • 17.2k

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