Quote from: riggasconi on January 23, 2011, 03:35:48 PM

A technical question: isn't owning the majority of the CPU power enough to impose a malicious chain, regardless of the size and age of the network, and the consequent difficulty ?

No. Rewriting old blocks requires you to generate them again. So if you want to go back 6 blocks, you have to do the work required to generate them with the current difficulty and continue to compete against legitimate generators.

Quote from: riggasconi on January 23, 2011, 04:01:13 PM

I'm sorry but I don't understand your argument. The goal of the attacker is to harvest goods/services and have 0 BTCs at the end of the attack, but be plenty of goods/services. The attacker will therefore not suffer from the FRN/BTC ratio plummeting because of panic triggered when the the community realizes to have been hacked, which is after the attack is completed. I hope you agree with me.

That's much more difficult. A future version of Bitcoin will probably let the second recipient identify this attack immediately, since it is easy to see. A more likely attack is one where the second spend is back to the attacker.