No, 51% is vastly more powerful than 49%. With 51%, you essentially control the entire Blockchain because you can always create a new Blockchain that would be accepted by the network, given enough time. Always. With 49%, you can only get away with it a few times, and it's less likely you will mine the next 6 blocks.

Essentially, as time progresses, with 49% you lose out, with 51%, you keep winning.

I think I understand now. That would still be a bit of a tricky position because it isn't so much 51% when you commit the fraud that is important it is the period following the fraud.

How detectable would such an action be? Wouldn't other systems be able look at the block and say "it's verified, but it don't look right to me"

Not really. There are always lots of 'versions' of the blockchain floating around. The network only keeps track of the longest chain (broadly speaking). This means the network with 51% can determine which transactions get into the blockchain. For example, if the pool owner doesn't like you, he can essentially 'blacklist' your account, which means your Bitcoins can become unspendable, basically. The longer they have the 51% power, the more damage they can do. GHash already performed what you describe as your '49% attack' against a gambling site that accepted 0 confirmation deposits. There has been no known instance of a 51% attack yet (e.g. double spend after 6 confirmations).

You pretty. Much have to detect the fraud before/as it occurs - the dominant miner being silent for a while is a good indication they are building an alternate chain in private, for example. At least, that would be an indicator for a double-spend.

Not really. That happens regularly just because of luck.

You'd see that every single block was mined by the same pool.

That doesn't mean a 51% attack happened, it just means someone got 51% of the network hash rate.

An honest miner with 51% of the hash rate would only mine 51% of the blocks (example: https://blockchain.info/blocks ); an attacker would mine every single block.

From the bitcoin wiki:

An attacker that controls more than 50% of the network's computing power can, for the time that he is in control, exclude and modify the ordering of transactions. This allows him to:

    Reverse transactions that he sends while he's in control. This has the potential to double-spend transactions that previously had already been seen in the block chain.
    Prevent some or all transactions from gaining any confirmations
    Prevent some or all other miners from mining any valid blocks

The attacker can't:

    Reverse other people's transactions
    Prevent transactions from being sent at all (they'll show as 0/unconfirmed)
    Change the number of coins generated per block
    Create coins out of thin air
    Send coins that never belonged to him

You could theoretically mine your own chain from the genesis block right?

That won't work due to checkpoints and even if it did work it would be the equivalent of nuking the entire Bitcoin ecosystem.

It would be more profitable to do something like extending the current blockchain but charge 1% transaction fees.

Interesting. For the benefit of others unfamiliar with this.

There is a hardcoded list of checkpoint blocks in the Bitcoin client. Any new chain starting before the last checkpoint will be rejected.

https://bitcoin.stackexchange.com/questions/1061/can-a-51-at... https://bitcoin.stackexchange.com/questions/3114/which-block...

So in other words if a 51% attack were to mine starting before a checkpoint block, it would end up "just" hard-forking Bitcoin.

Wait. If the attacker did any of that while it was in control, then wouldn't it fail validation for every other bitcoin miner?

If they lose their 50% control then they generate invalid blocks. Particularly the part regarding the number of coins generated per block. That one will obviously be wrong.

What does "in control" mean in this sense? Is that an equivalent expression to "greater than 50%"?

Also the "Some or all" doesn't seem very specific. What is the factor that makes some become all?

I believe the intention in the phrasing is that they could selectively or universally exercise that action. Not that there is some spectrum of control.

The bitcoin protocol defines the longest chain as the correct/canonical chain.

At 51% you have more hashing power than the rest of the network combined, so you can start mining blocks on your own and create your own chain with the knowledge that eventually your chain will be longer than the 49% chain everyone else is working on. When that happens, the 49% will abandon their chain and start working on yours.

What kind of chain length differential is enough to cause people to switch. Is it a single block?

> Is it a single block?

No. The 'winning chain' is determined by total difficulty, not block count.

Yes.