Can Bitcoin blockchain be altered with a 51% attack?

[troiste]

Hi everyone,

Can Bitcoin blockchain be altered with a 51% attack? (old transactions / new ones) By this I'm asking if a 51% attacker would be able to move balances or coins from one address to another or something similar...

...Or a 51% attacker would just be able to double spend his coins (and that's it)?

Thanks in advance for clarification!

[remotemass]

Hope this article answers your question:
http://learncryptography.com/51-attack/

[Holliday]

https://en.bitcoin.it/wiki/Weaknesses#Attacker_has_a_lot_of_computing_power

[Mrrr]

http://lmgtfy.com/?q=51%25

[porcupine87]

Imagine a miner with 2/3 of the network power and he want's to change a transaction 100 blocks ago. So he has to make a fork beginning with this block. While the other miners make 1 block, our evil miner makes 2 blocks. So 100 blocks = 16.6 hours later the manipulated chain would be taller and all miner build up the new one.

But there are limitations. The evil miner could no move coin from one address to another without have the private key to the address. But he could recall his own transaction, or make confirmed transaction unconfirmed again and so on.

[troiste]

I see, means that for the others what an attacker can do is:

- Prevent some or all transactions from gaining any confirmations
- Prevent some or all other miners from mining any valid blocks

If the attacker prevents transactions from gaining confirmations, what does this mean exactly? Lets say if I send 25 BTC and an attack comes up at that moment, means that neither me or the receiver will effectivly hold the coins? (while the attack takes places...?) and when the attack is over all would be back to normal or is there possibility of irreversible damage?

[DeathAndTaxes]

I see, means that for the others what an attacker can do is:

- Prevent some or all transactions from gaining any confirmations
- Prevent some or all other miners from mining any valid blocks

If the attacker prevents transactions from gaining confirmations, what does this mean exactly? Lets say if I send 25 BTC and an attack comes up at that moment, means that neither me or the receiver will effectivly hold the coins? (while the attack takes places...?) and when the attack is over all would be back to normal are is there possibility of irreversible damage?

Your wallet reduces your balance for convenience but although we say "sending" transactions are never in transit.  A transaction signs over "ownership" to the receiver's key.  Either the tx confirms at which point the receiver has the "coins" or it doesn't and you still do.  There is never a single millisecond where no party controls the coins and a denial attack doesn't change that.

That being said a denial attack would damage Bitcoin.  It may not do any damage at the protocol level, coins couldn't be erased.  It would however undermine confidence in the system, potentially fatally.  Imagine tomorrow no Bitcoin transactions could be confirmed and it was 60 days before the attack was defeated and tx could start being confirmed again.  What do you think would happen to user base, confidence, merchant adoption, exchange rate, etc?

[Agent Provocateur]

Somewhere I've read that one just needs 1/3 of total hashing power to gain control over the network. He withholds a selfmined valid block until he recognize another one publishing "his" block, then releasing "his" one gaining a time benefit. He lets all other participants mine "his" block while he's mining a new valid one, therefore gaining control over the blockchain. Dunno the exact details and it's been a while since I've read about that issue but I find it quite scary. The part I don't get is how he should be able to publish "his" block after another one does and still "wins". Maybe this is old news and has already been fixed, would be nice if some tek could drop a line on this. Thanks.

[troiste]

I see, means that for the others what an attacker can do is:

- Prevent some or all transactions from gaining any confirmations
- Prevent some or all other miners from mining any valid blocks

If the attacker prevents transactions from gaining confirmations, what does this mean exactly? Lets say if I send 25 BTC and an attack comes up at that moment, means that neither me or the receiver will effectivly hold the coins? (while the attack takes places...?) and when the attack is over all would be back to normal are is there possibility of irreversible damage?

Your wallet reduces your balance for convenience but although we say "sending" transactions are never in transit.  A transaction signs over "ownership" to the receiver's key.  Either the tx confirms at which point the receiver has the "coins" or it doesn't and you still do.  There is never a single millisecond where no party controls the coins and a denial attack doesn't change that.

That being said a denial attack would damage Bitcoin.  It may not do any damage at the protocol level, coins couldn't be erased.  It would however undermine confidence in the system, potentially fatally.  Imagine tomorrow no Bitcoin transactions could be confirmed and it was 60 days before the attack was defeated and tx could start being confirmed again.  What do you think would happen to user base, confidence, merchant adoption, exchange rate, etc?

Is there any way to prevent this from happening?

[DeathAndTaxes]

I see, means that for the others what an attacker can do is:

- Prevent some or all transactions from gaining any confirmations
- Prevent some or all other miners from mining any valid blocks

If the attacker prevents transactions from gaining confirmations, what does this mean exactly? Lets say if I send 25 BTC and an attack comes up at that moment, means that neither me or the receiver will effectivly hold the coins? (while the attack takes places...?) and when the attack is over all would be back to normal are is there possibility of irreversible damage?

Your wallet reduces your balance for convenience but although we say "sending" transactions are never in transit.  A transaction signs over "ownership" to the receiver's key.  Either the tx confirms at which point the receiver has the "coins" or it doesn't and you still do.  There is never a single millisecond where no party controls the coins and a denial attack doesn't change that.

That being said a denial attack would damage Bitcoin.  It may not do any damage at the protocol level, coins couldn't be erased.  It would however undermine confidence in the system, potentially fatally.  Imagine tomorrow no Bitcoin transactions could be confirmed and it was 60 days before the attack was defeated and tx could start being confirmed again.  What do you think would happen to user base, confidence, merchant adoption, exchange rate, etc?

Is there any way to prevent this from happening?

Yes and no.
What reduces the likelihood of this type of attack is the cost to perform the attack.  In theory as Bitcoin becomes more widely adopted, miners are compensated more, the size of the network grows and the cost to attack it grows as well.  If it costs $10B to pull of this type of attack it is less likely to happen then if a couple griefers with a botnet can do it for free.

On the other hand there is no absolute way to prevent it.  Satoshi's solution to the Generals Problem is the proof of work however it is a limited solution.  The limitation is that a decentralized trustless network of nodes can secure the network as long as the attacker has 50% or less of the network computing power.

[troiste]

I see, means that for the others what an attacker can do is:

- Prevent some or all transactions from gaining any confirmations
- Prevent some or all other miners from mining any valid blocks

If the attacker prevents transactions from gaining confirmations, what does this mean exactly? Lets say if I send 25 BTC and an attack comes up at that moment, means that neither me or the receiver will effectivly hold the coins? (while the attack takes places...?) and when the attack is over all would be back to normal are is there possibility of irreversible damage?

Your wallet reduces your balance for convenience but although we say "sending" transactions are never in transit.  A transaction signs over "ownership" to the receiver's key.  Either the tx confirms at which point the receiver has the "coins" or it doesn't and you still do.  There is never a single millisecond where no party controls the coins and a denial attack doesn't change that.

That being said a denial attack would damage Bitcoin.  It may not do any damage at the protocol level, coins couldn't be erased.  It would however undermine confidence in the system, potentially fatally.  Imagine tomorrow no Bitcoin transactions could be confirmed and it was 60 days before the attack was defeated and tx could start being confirmed again.  What do you think would happen to user base, confidence, merchant adoption, exchange rate, etc?

Is there any way to prevent this from happening?

Yes and no.
What reduces the chance is the cost to perform that attack.  In theory as Bitcoin becomes more widely adopted, miners are compensated more, the size of the network grows and the cost to attack it grows as well.

As for in is there an absolute way to prevent it.  No.   Satoshi solution to the Generals Problem is the proof of work however it is a limited solution in that it only works as long as the attacker has 50% or less of the network computing power.

Does PPC (PeerCoin) solve the problem? I heard they have Proof of Stake and checkpoints (I don't really know what this means).

[DeathAndTaxes]

It is more like PeerCoin attempts to solve it in a different way and with different limitations.  Personally I do not have as much confidence in PoS systems.

[HorseCoin]

are u sure someone with 51% power can't take the blockchain and reroute it so they can buy the Delorean from Bitcoinmotors for free?

[Sheldor333]

In essence yes. Will that problem be solved, I hope so. Like with everything you can make changes when you have majority.