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35

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 I'll give it a shot. There are many nuances involved here, and I fear that a large part of them didn't reach as much of an audience as the exchange announcing ...


28

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 population will eventually overtake his chain, and we'll be in an unstable situation where control of the "best" chain will bounce back and forth between them forever....


9

(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 only slightly over 50% would take weeks-- even months, creating massive disruption if successful, and virtually guaranteeing an effective public initiative to ...


6

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 infinite number of attempts. Using the scenario where an attacker tries to build their own chain to eventually replace the original one: If the attacker has ...


4

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 coordinate, it also requires that individual miners must trust one another to participate in the rollback. Consider this: If I had a significant portion of the ...


3

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 misunderstanding of how mining works, in that you postulate two miners working together with 40% of the hashpower will outcompete 3 miners with 20% of the hashpower each (...


2

There a are a few things to take into account when we consider the probabilities of a reorg attack (destroying net value) to be successful: If the reorg is shallower than 100 blocks (COINBASE_MATURITY), the attacker can minimize the amount of direct victims. But if we are talking about a deeper one, not only the payments he wants to revert may have been ...


2

For example if I owned 51%+ of the network, mined 1000 blocks privately and the rest of the network was at block 990 and then I broadcast the data would everyone switch to my chain and would the last 990 blocks be orphaned? This can certainly happen. There is no limit on how many blocks can be reorganized. However, while nodes will still perform the action ...


2

I believe you are asking if the inter-block interval matters for the success rate of a high hashpower reorg attack. It does, because if it is too fast the honest participants will spend a substantial fraction of their time not working on the same (honest) chain, because they haven't heard of each others blocks or have created their own blocks before other ...


2

An honest node is a node that does not try to modify history. This is opposed to an attacker node which tries to modify a past block in the blockchain presumably to benefit them financially. Note that because transactions are cryptographically signed, "An attacker can only try to change one of his own transactions to take back money he recently spent," ...


1

No, each node (assuming that it is a miner) can pick a different set of transactions. Eventually one block will get more work built on top of it than all the others and the other competing blocks at the same height in the chain will become stale and be discontinued. Effectively the blockchain forks but eventually one branch survives and the others die. ...


1

Yes. The owner of 51% can get 100%. This is attack-51 itself. Is there some measure that would stop this? no


1

This question is somewhat like asking, "how can I steal money from myself?" Since you are the only person participating in the consensus of your altcoin, you control 100% of the hashpower. A 51% attack creates an alternative ledger, breaking consensus on which coins have been transferred, allowing the attacker to create a double spend. First mine two ...


1

Your answer is Incentives. Let's imagine a bad guy, called Mallory, has an interest in attacking Bitcoin and has either created for himself, or desires to, 51% of the hashing power required to start shenanigans. (Start, because he has to maintain and grow that hashing power for sufficiently long to outpace the 'good guys'.) To start with, there's no ...


1

One may speculate on this, but this was already taken into account in the design of Bitcoin. Essentially an attacker would not choose to attack the network because it would hurt their own investment. In the whitepaper, Satoshi states: if a greedy attacker is able to assemble more CPU power than all the honest nodes, he would have to choose between using ...


1

An honest node is one which behaves in the way that we expect nodes to behave. This means that the node does not try to modify history, serves blocks and transactions correctly, transmits messages correctly, transmits correctly formatted messages and data, etc. These are all things that have a correct way of doing and honest nodes are ones which do all of ...


1

Yes, many small coins are susceptible to such attacks. However, the value of the coins is so small that performing a 51% attack on the coin has almost no real use whatsoever. The coins that do actually have value where performing a 51% attack would actually result in non-negligible gains for the attacker are more difficult to attack because they are actually ...


1

An attacker with 51% hash power could perform a double-spend (or several). He could send a large payment to someone and then use his hash power to create a longer chain that spends the same coins but sends them to himself instead, making the first transaction invalid. It would be against his interest because playing by the rules would net him roughly 1 ...


1

The attack you are describing is correct. This is an attack against the "Common Prefix" property of the blockchain, which roughly states that it should be hard for an adversary to cause two honest parties to adopt two different blockchains at the same time when they have diverged for more than k blocks, where k is a parameter. In your case, you have set k = ...


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