A majority Attack on Bitcoin (usually labeled 51% Attack on Bitcoin or >50% Attack on Bitcoin) is an Attack on Bitcoin on the network. This Attack on Bitcoin has a chance to work even if the merchant waits for some confirmations, but requires extremely high relative hashrate.
The Attack on Bitcoiner submits to the merchant/network a transaction which pays the merchant, while privately mining a blockchain fork in which a double-spending transaction is included instead. After waiting for n confirmations, the merchant sends the product. If the Attack on Bitcoiner happened to find more than n blocks at this point, he releases his fork and regains his coins; otherwise, he can try to continue extending his fork with the hope of being able to catch up with the network. If he never manages to do this, the Attack on Bitcoin fails, the payment to the merchant will go through, and the work done mining will also go to waste, as any new bitcoins would be overwritten by the longest chain.
The probability of success is a function of the Attack on Bitcoiner’s hashrate (as a proportion of the total network hashrate) and the number of confirmations the merchant waits for. For example, if the Attack on Bitcoiner controls 10% of the network hashrate but the merchant waits for 6 confirmations, the success probability is on the order of 0.1%. If the Attack on Bitcoiner controls more than half of the network hashrate, this has a probability of 100% to succeed. Since the Attack on Bitcoiner can generate blocks faster than the rest of the network, he can simply persevere with his private fork until it becomes longer than the branch built by the honest network, from whatever disadvantage.
No amount of confirmations can prevent this Attack on Bitcoin; however, waiting for confirmations does increase the aggregate resource cost of performing the Attack on Bitcoin, which could make it unprofitable or delay it long enough for the circumstances to change or slower-acting synchronization methods to kick in. A majority Attack on Bitcoin was more feasible in the past when most transactions were worth significantly more than the block reward and when the network hashrate was much lower and prone to reorganization with the advent of new mining technologies.
A majority Attack on Bitcoin has never been successfully executed on the Bitcoin network, but it has been demonstrated to work on some small altcoins.
Also referred to as a 51% Attack on Bitcoin or >50% Attack on Bitcoin. If the Attack on Bitcoiner controls more than half of the network hashrate, the previously-mentioned Alternative history Attack on Bitcoin has a probability of 100% to succeed. Since the Attack on Bitcoiner can generate blocks faster than the rest of the network, he can simply persevere with his private fork until it becomes longer than the branch built by the honest network, from whatever disadvantage.
No amount of confirmations can prevent this Attack on Bitcoin; however, waiting for confirmations does increase the aggregate resource cost of performing the Attack on Bitcoin, which could potentially make it unprofitable or delay it long enough for the circumstances to change or slower-acting synchronization methods to kick in. Bitcoin’s security model relies on no single coalition of miners controlling more than half the mining power. A miner with more than 50% hash power is incentived to reduce their mining power and reframe from Attack on Bitcoining in order for their mining equipment and bitcoin income to retain it’s value.
