If an evil computer network wanted to attack Bitcoin, would they need to have more computing power than the present Bitcoin network and then attack a current block, or would they need even more to go back and recalculate all the checksums in the block chain since day one?
I am thinking that as history continues the work required grows and therefore the need for more computing power makes an attack of this sort less feasible.
Changing the blockchain at a point earlier than the most recent checkpoint is impossible, at least with the main client. It won't be accepted by those clients.
One can fork the chain at any point, but the most current block is the easiest as it does not require to backtrack and solve the previous blocks. Unless you have some specific transactions you want to invalidate, you would start from the current block.
While rewriting a lot of already existing blocks is not feasible, having 51% of hashing power would be sufficient to kill Bitcoin project, if that is the intention. Because it opens many different ways to attack Bitcoin network, the most frightening is the ability to execute double spend.
