Quote from: riggasconi on January 23, 2011, 04:09:28 PM

Satoshi Nakamoto writes in his white paper that it is not needed to control all of the bitcoin connections, but just a majority of them. Am I missing something ?

You are confusing "control 50+% of generating power" with "control connections."

Lets say you control 51% of the generating power.

You can:

Spend bitcoins once. Then wait for them to be confirmed by the rest of the network as many times as the merchant requires, while secretly working on another version of the block chain where you did NOT spend them. Your secret block chain should be longer than the network's, since you control 51% of the generating power.

So you announce your secret block chain, and instead of sending those coins to a merchant you include a transaction where you send them to yourself. YEAH! you just ripped off the merchant! Wahoo!

You cannot rip off two merchants with the same bitcoins-- one or the other of the transactions will be seen as valid.

And you cannot "unspend" the transaction to the merchant-- if you don't spend it SOMEWHERE, the merchant's bitcoin node will re-announce it to the network and all the other nodes will consider those bitcoins "spent, just waiting to be included in the next generated block."

If you run the numbers again with the realistic double-spend scenario, you'll see crime doesn't pay. There is no way you can rent enough hashing power to commit a profitable double-spend attack.

If you can steal the hashing power (maybe you're a bot farmer), then if you run the numbers you'll find it is more profitable to just generate blocks and sell the bitcoins rather than try to somehow get stuff trying to double-spend.