What would be the result of a 51% attack?

[BitNerd]

Would it destroy bitcoin for good? Cause inflation beyond 21M with double-spending? Or something else? Would the bitcoin survive?

[dothebeats]

Bitcoin would succumb.

If a person possess more than 50% of the entire network's hashrate, he could reverse transactions that he made, prevent transactions from gaining confirmations if he likes and prevent generators from gaining generations if he likes. Though scary and seems unstoppable, seeing the things an attacker could do, he can't reverse transactions made by other people, create coins out of nowhere, send coins that never belonged to him in the first place and prevent or fully stop a transaction (the network would display it as unconfirmed).

So long as you are in control, you can modify the recent chain any way you like.

[cjmoles]

Bitcoin would succumb.

If a person possess more than 50% of the entire network's hashrate, he could reverse transactions that he made, prevent transactions from gaining confirmations if he likes and prevent generators from gaining generations if he likes. Though scary and seems unstoppable, seeing the things an attacker could do, he can't reverse transactions made by other people, create coins out of nowhere, send coins that never belonged to him in the first place and prevent or fully stop a transaction (the network would display it as unconfirmed).

So long as you are in control, you can modify the recent chain any way you like.

I believe that that the worse consequence of a 51% attack would be the confidence lost in the security of the network.  It would very quickly be uncovered and the attacker would lose money because the prices of bitcoin would crash leaving the miner with a huge electricity bill and a bag of valueless bitcoin.  The profit incentive would be low.  However, if another technology with deep pockets wanted to ruin bitcoin by attacking the network with a 51% attack to ruin the bitcoin economy, then there might be incentive to spend the extra money to cause such an event if that would mean a transfer of wealth into their platform.  In fact, I wouldn't put it past an organization such as R3CEV to try such a tactic someday!

[Holliday]

Temporary disruption. We can argue how long "temporary" is though.

As long as you don't make any transactions during the attack, your coins would be fine. Bitcoin would survive.

[Superbitzz]

if the panic is not created and the people remain calm. they do not sell their bitcoin and hold the bitcoin i am sure that the bitcion will survival.

[serjent05]

I think it will  build a new chain faster than the network, and broadcast it whenever the attacker like.  Can cause panick to the people and may cause hundreds of million dollar damage to the Bitcoin economy.  The long years of promotion and advertisement campaign will be in vain since trust will be lost if this thing happen.

[a7mos]

If someone could have control on 51% of bitcoin network, why would he think about harming bitcoin? he must be believing in bitcoin as he invests all these money and effort to be able to do so

[DarkStar_]

Nothing. Nothing will happen to bitcoin, except maybe a few transactions getting reversed. The miners could not allow any transactions to go through with 51% of the hashing power, but they wouldn't do that. It would cause bitcoin's price to go down a lot, because of the panic and problems. Once that happens, the millions spent on hashing power to get to 51% of the network would be wasted, since mining bitcoin will be very unprofitable.

if the panic is not created and the people remain calm. they do not sell their bitcoin and hold the bitcoin i am sure that the bitcion will survival.

Selling/holding bitcoin won't affect what happens in the 51% attack. Even if people hold, and don't sell, the person could not allow any transactions to be confirmed, which would cause panic.

[odolvlobo]

There are two possible effects that I can think of:

• Control of mining: Someone with 51% would be able to mine all blocks and therefore be able to control what transactions are confirmed and what transactions are not. Also, note that all other miners would probably drop out because they would get no blocks.
• Ability to revise history: Someone with 51% would be able to roll-back the block chain and remove transactions that were previously confirmed, allowing them to double spend. The number of blocks they can roll back potentially depends on how much more hash power they have than everyone else.

Bitcoin could otherwise work normally (except for any ramifications I may have missed).

Note also that if the state of Bitcoin got really bad, a successful 51% attack might be seen as a positive outcome.

[lottery248]

There are two possible effects that I can think of:

• Control of mining: Someone with 51% would be able to mine all blocks and therefore be able to control what transactions are confirmed and what transactions are not. Also, note that all other miners would probably drop out because they would get no blocks.
• Ability to revise history: Someone with 51% would be able to roll-back the block chain and remove transactions that were previously confirmed, allowing them to double spend. The number of blocks they can roll back potentially depends on how much more hash power they have than everyone else.

Bitcoin would otherwise work normally (Except for any ramifications I may have missed).

Note also that if the state of Bitcoin got really bad, a successful 51% attack might be seen as a positive outcome.

someone written the book about bitcoin, claiming that the 51% attacks cannot block the transactions, and cannot easily remove blocks.
double spend to be success requires a high ability of decoding AFAIK.

[Herbert2020]

Temporary disruption. We can argue how long "temporary" is though.

As long as you don't make any transactions during the attack, your coins would be fine. Bitcoin would survive.

correct me if i am wrong but since 51% attack is only letting double spend happen, you only have to wait for more confirmation on your transactions, you don't need to stay away.
i don't know what that number should be but i think something around 30 according to bitcoin.org would suffice

[beastmodeBiscuitGravy]

We'd quickly download The Realest Bitcoin Client 0.9.99.99 from bitcoin.org, look for a statement here or on r/bitcoin, and the crisis of centralisation would be averted.

[nanonymousx]

He can create a special address, then spend money from that address and never confirm it, double, triple quadruple spend the money theoretically.