36

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 ...


27

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....


19

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 everyone else combined can, given enough time, always build a longer chain than everyone else.


16

It's not just about the miners, it is also about people using bitcoin. If miners decide to fork the chain and keep block bounty at 50btc, they cannot use those bitcoins in any place where people have not also modified their non-mining bitcoin client. Protocol changes are not just about >50% of miners, but about >50% of user clients. In this case, since ...


15

There are too many issues rolled up into this question, I'll try to address each separately. The fact that the miner does not include transactions is not a problem. Miners have a right to exclude transactions, even all transactions. Senders can include tx fees if they want to improve the chances of being included quickly (if the miner excludes transactions ...


14

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 majority of the hashrate will cooperate and converge on a single chain, because it is most financially advantageous thing to do. When that is no longer the case, ...


11

Ripple solves the double spend problem by consensus. Everyone who wants to run a server on the Ripple network picks a set of validators and tries to reach a consensus with them on which transactions are valid. The Ripple equivalent of Bitcoin's 51% attack would be if some group obtained control over enough validators that the consensus process failed. ...


9

Yes, if you have 51% of the network you can reject all other blocks, thus making sure you receive all rewards. The difficulty will adjust to match just your own hashrate, making sure you mine the expected 144 blocks per day. In fact, with a clever strategy you can get more than your fair share of block rewards even if you're below 50%. This was discussed ...


9

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 chain of (secretly premined) child blocks? Because you can't prove that to nodes that weren't on the network at the time of the attack. Which means that either:...


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 ...


8

A double-spend would be blackletter law illegal, guys. Fraud and theft include a wide variety of intentional deception wrongfully depriving someone of property - use of a government-minted currency is not, and never was, a required element. Look at any larceny or fraud statute - it will refer to loss of or damage to person and property not "dollars." ...


8

When performing a 51% attack, you will need to find new blocks faster than the remainder of the network combined. This way, your block chain will become the longest and clients will consider it the main chain. Theoretically, this is achieved (on long term) when you have >50% of the hash power. But as we know, a lot of luck is involved in mining. It's ...


8

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 psuedocode. It's always nice answering a question written in clear code. The answer is that we want nodes to be able to agree on the best block chain based ...


8

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 the attacker must buy the hardware, then the most economical device is the Antminer S5+, which produces 7.7 TH/s (7.7e12 H/s) and costs US$2300. It draws 3.4 ...


8

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 automatically end up on the heavier chain by comparing the aggregate difficulty of the chain-tips offered by their peers due to headers-first synchronization. ...


7

There's no cooperation involved in mining, and they won't be trying the same hashes because (for example) they have different transaction pools and (possibly) different destination addresses. Mining pooling is just like non-pooled mining, except that the revenue is spread over time instead of getting blocks of 25BTC. Even when the pooled miners have a ...


7

Conceptually it isn't difficult to understand conflicts of interests (COI) principles. If a pool has over 50% of the computing power required to demonstrate adequate "proof of work" to add entries to the Blockchain Ledger, this absolute centralized power will eventually "absolutely corrupt". That centralized pool could be used to change the history of ...


7

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 is not a linear task of work that needs to be accumulated. Rather it is a random process. Instead of a pile of work you are going through that has a fixed size,...


6

EC2 would be prohibitively expensive to attack bitcoin but would be well suited against so called "GPU unfriendly" block chains. Against bitcoin EC most likely usage would be to "top off" an attackers hashing power. If a hypothetical attacker built a massive hashing farm but was slightly short of 51% hashing power the attacker could use EC2 instances to ...


6

Price comes from ratio of supply and demand. Bitcoin supply rate doesn't change, about 300 bitcoin per hour. So the price changes only when demand changes, and the difficulty to mine bitcoins adjusts to the new price, in another words price change would affect network difficulty, but not the other way around. Therefore video card price would affect ...


6

If you want to profit from Bitcoins, you would not want to work against it (as was explained by Satoshi in his original paper). A 51% attack would not bring you profit, rather it would make everyone involved in the project loose, as Bitcoin's reputation would be damaged. The other thing is getting enough hashing power. Unless you target mining-specific ...


6

DeathAndTaxes' answer covers the main point: Botnets are composed of computers with a wide range of hardware (mostly medium/low end) and you need very specific high end hardware to efficiently mine bitcoins. I just want to add a few things about botnets. The largest botnets are estimated to have more than 1 million computers. So while the math ...


6

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 with the new ones that the network is still creating. Example: Say, Eve achieved to control 51% of the hash rate and wants to unconfirm a transaction from 6 ...


6

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 together with the hash they created/solved, thus creating a valid block. See https://en.bitcoin.it/wiki/Block_hashing_algorithm for more information. The person that ...


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 ...


5

You're exactly right; there's no need to catch up if you don't start off behind. As soon as I get 51% of the network's hashing power, I start building my fork in private, then buy something on the 'real' fork and allow the purchase to get as many confirmations as it needs. It will be a lot, because I'll be buying something really expensive. I take care ...


5

If hardware cost or efficiency rises, the new mining power will enter the network, and the difficulty will rise, keeping the rate of bitcoin production stable - this is one of the key design elements of bitcoin. So, arguably nothing should happen to the price of bitcoins due to cheaper/better mining hardware.


5

Even if the dynamics are interpreted in the most basic way, it does not lead to this positive feedback, rather it maintains the status quo. If there are miners with stake of 40%, 30% and 30%, they will get 40%, 30% and 30% of the rewards respectively, so the ratios will remain at 40%, 30%, 30%. For example, if they start with 40, 30, 30 BTC, and 10 new ...


5

You could easy manipulate a pool to wreck havoc on other SHA256 coins if you owned it or managed to gain access. You don't even need to be a large pool, some altcoins are really small and vulnerable. It has affected other coins before, where a pool or a large miner created trouble solely by mining a smaller altcoin. Feathercoin hit by massive attack Only ...


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