Quote from: bcearl on July 02, 2011, 07:11:06 PM

Quote from: Meni Rosenfeld on July 02, 2011, 06:14:49 PM

Quote from: theymos on July 01, 2011, 11:01:59 PM

Quote from: Meni Rosenfeld on July 01, 2011, 12:05:09 PM

Reversing 1000 future blocks takes a week with 52%.

This is certainly wrong, and your other numbers are probably wrong, too. If you control 52% of the network, you must use one of your blocks to negate a legitimate block 48% of the time. So 48% of the network is producing legitimate blocks, 48% of the network is negating those blocks, and only 4% is left producing new blocks. The network would only produce 5.76 blocks per day.

My numbers are not wrong, they just relate to a different attack from what you have in mind. I was talking about building up an alternative branch starting from the current block, without releasing blocks during construction. After about a week, the honest network will have on average 1000 blocks while the attacker will have on average 1083 blocks, and with high probability he will have more blocks.

What does an attacker get from having the longer blockchain? Waste of energy?

The Bitcoin protocol specifies that the longest branch is the valid one. Thus an attacker having the longer chain can decide which transactions are to be included in it.

And I thought we already said that this allows disrupting Bitcoin commerce, for:
1. Damaging Bitcoin for political reasons.
2. Profiting from shorting.
3. Other stuff.