Actually, no. Current estimates rate the cost of a 51% attack at approximatly 4.656.671.184$ for hardware only which is not a sum which even large countries wi

Actually, no. Current estimates rate the cost of a 51% attack at approximatly 4.656.671.184$ for hardware only which is not a sum which even large countries will have easy (and certainly not unnoticed) access to. This estimate is based on the current hash rate at 14.975.580.960 GHash/s and the fastest mining hardware available which is the AntMiner S5+ at ~2300$ with a hashrate of 7.722 GHash/s. That means you'll need about ~1.939.340 AntMiners which will consume 3436W each. At an electricty cost of 0.05ct/kwh (this is china, US is at about ~0.20ct/kwh) this gives you an additional cost of ~7.996.185$ per day not factoring in any labor/location costs and most notably no cooling costs which will be high with this kind of high performance asic. This estimate only holds true though if you buy every bit of hardware yourself, in reality its probably more practical to simple coerce big mining pool owners to work for you, but thats another problem :) edit: relevant xkcd: https://xkcd.com/538/ https://xkcd.com/538/