It would cost a lot more than $40 million to majority-attack Bitcoin. I estimate $3-4 billion or more. Mostly data center costs. Let's assume an attacker is ve

It would cost a lot more than $40 million to majority-attack Bitcoin. I estimate $3-4 billion or more. Mostly data center costs. Let's assume an attacker is very good and needs only 6 months to design his own ASICs and build data centers to host the farm. The network is at 4 Phash/s today, so 6 months from now we should be around 4 * 2^6 = ~250 Phash/s (the network has been doubling in size every month for the past 10 months). The best ASICs, 28nm KncMiner, are approximately 100 ~Ghash/s and 100 Watt each. So an attacker would have to build 250 Phash/s of these to clearly outperform the network: that is 2.5 million chips at 250 megawatt total. And to plan for a potential delay of 30 days in his plan, an attacker would have to build not 250 Phash/s but 500 Phash/s of ASICs to attack the network. 5 million chips. 500 megawatt data center. For comparison, Facebook spent $210 million on their 28 megawatt Prineville data center. So a 500 megawatt data center would probably cost $3-4 billion. Even the well-funded NSA couldn't get their comparatively punny Utah data center to run correctly and it has been delayed by more than 1 year: http://www.pcworld.com/article/2052960/nsa-data-center-suffers-meltdowns-is-delayed-by-a-year-report-says.html http://www.pcworld.com/article/2052960/nsa-data-center-suffe... Is China seeing Bitcoin as a threat? No. And even if it was, is it big enough for them to be willing to spend $3-4 billions to destroy it? I don't think so.