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Author Topic: stealth 51% attack  (Read 1880 times)
Costia
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January 30, 2012, 09:20:26 PM
 #21

the double spend is not inserting 2 transactions in the same block that double spend - this kind of block will be rejected
the idea is that you spend the money on the real chain, and then release your own longer chain that doesn't have this transaction in it. so you end up with having the money in your wallet although you spent it on the real chain.
thats why the 6 confirmations - there are chainsplits happening when 2 miners find a new block at about the same time - but this will get sorted out when the next block is found and will choose which split is now the longest
http://blockchain.info/orphaned-blocks

to be able to do this at will and for a long period of time you will need 51%
you can cause shorter splits with less
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hazek
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January 30, 2012, 09:50:09 PM
 #22

Hmm looks like I really didn't understand what the 51% or more hashing power attack was suppose to look like or do.

I reread this:
Quote
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.

And am now even more confused than before. If I understand this right, then this shouldn't be under the "Probably not a problem" category and am actually surprised we haven't had this type of an attack yet. I still don't understand why the need for 50% or more, why say 30% wouldn't be enough..

My personality type: INTJ - please forgive my weaknesses (Not naturally in tune with others feelings; may be insensitive at times, tend to respond to conflict with logic and reason, tend to believe I'm always right)

If however you enjoyed my post: 15j781DjuJeVsZgYbDVt2NZsGrWKRWFHpp
Costia
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January 30, 2012, 09:57:41 PM
 #23

because the chances of making it last N blocks is (percent/100)^N
most people wait for 6 confirmations - 0.3^6=0.000729 = 0.07% -> less than 1 out of 1000
and then after 6 blocks it will get reversed (chance to make it stick for 7 blocks is ~1 out of 5000)
also you will loose the reward for those blocks, so on average you will loose 6blocks*50BTC*1,000(due to the cahnce of 0.07%) = 300K BTC in legitimate block rewards for a chance to double spend
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