Quote from: Grinder on May 20, 2011, 07:29:57 AM

Quote from: kjj on May 19, 2011, 10:34:46 PM

With more than 50% of the hashing power, you can hide a double spend for a few minutes, and only from funds that you have legitimately. You can not counterfeit anything.

With more than 50% you can just define every transaction that is not in one of your blocks is invalid, by not validating the blocks found by others. There's no need to double spend or counterfeit.

Sorry, you are just completely wrong about this. Validity of a transaction is not determined by being in a block or not. A transaction that isn't in a block merely hasn't been accepted (yet).

Say you had 51% of the total mining capacity.

What are the odds of getting the next block, and being able to prevent transactions that you don't approve of for ~10 minutes? 51%
And the odds of getting the block after that and continuing the blockade out to ~20 minutes? 26%
Add if you need ~30 minutes? 13%
40 minutes? 7%
50 minutes? 3%
An hour? 2%
Two hours? 0.031%