Bitcoin Forum

if anyone can control 51% he will control the bitcoin network
no need to be stealthy about it, they can just not include any transactions in their blocks and reject blocks mined by others
but this wont be profitable
what they can do is set the transaction fee to whatever they want , since only they can add new transactions to the chain


Edit: what lock in points? the longest chain wins.

I think certain block hashes get hardcoded into the client with every new release.
This way if the longer blockchain doesn't satisfy these conditions it will get rejected.

is till means an attacker can do watever he wants between releases
and it would be kinda strange if this was true, any sources for this?

to clifford: if somebody gets 51% he can do (almost) whatever he wants from that point and on.

edit:
but it would be quite hard to get to 51% without being noticed - he will have to take over deepbit and another pool, or create a larger one himself - which will either take a lot of time or a lot of money (probably more than the net worth of bitcoin)

but old enough coins are safe anyway - you can replace a block from the middle or something
the only non safe ones are from the time somebody got 51% and later

If someone had a 51% hashrate, what would stop them from solving blocks but not actually publishing the answers?

In this way, they could create a fork of the blockchain which would be longer than the real published one,  and obviously they could
publish this fork whenever -- thereby re-writing the existing blockchain, which could be many blocks long.

You wouldn't know about this until it happened, and it could be very damaging -- Imagine if the last 200 blocks were suddenly re-written...

Is there a way of mitigating against a stealth attack like this ?

A withholding attack may only be marginally successful. My understanding is that the client will refuse to parse so many blocks in such a short space of time. If you try to dump a large pile of blocks on the client, it will probably refuse them.

I wished people knew statistics and realized that you need far more than 51% in order to pull of what you're suggesting.

to do what the OP suggested you won't even need 50%, since this is random even with 40% you can be ahead of the main chainblock for a few hours/days

to do what the OP suggested you won't even need 50%, since this is random even with 40% you can be ahead of the main chainblock for a few hours/days

With 51% hashing power you can hash well past the "official chain" till the point that the "good guys" regaining the longest chain is improbable.

What? What do you mean ahead for a few hours/days? I thought the 51% attack was carried out by getting lucky with the 51% of hashing power or more to get at the min 6 consecutive blocks found by your miner so that you can insert a fraudulent doubles spent transaction into the block chain and fool someone it is legitimate before you run out of luck and some other miner finds a block invalidating your transaction?

I thought all the amount of hashing power you have raises the odds of finding 6 consecutive blocks high enough to make it worthwhile to even attempt such an attack and that 51% was deemed where these odds get high enough?

How does that happen if the blocks you found are fraudulent and will get ignored once the "good guys" find a block?

Attacker has a lot of computing power
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
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
It's much more difficult to change historical blocks, and it becomes exponentially more difficult the further back you go. As above, changing historical blocks only allows you to exclude and change the ordering of transactions. It's impossible to change blocks created before the last checkpoint.
Since this attack doesn't permit all that much power over the network, it is expected that no one will attempt it. A profit-seeking person will always gain more by just following the rules, and even someone trying to destroy the system will probably find other attacks more attractive. However, if this attack is successfully executed, it will be difficult or impossible to "untangle" the mess created -- any changes the attacker makes might become permanent.