Well, it depends on how much more computing power a hypothetical nation-state has at its reserve compared to the rest of the world. I wish I could find the pape

Well, it depends on how much more computing power a hypothetical nation-state has at its reserve compared to the rest of the world. I wish I could find the paper analyzing the possibilities of attacking with less than 51% power, 33% IIRC but in lieu of that, check this out: http://bitcoin.stackexchange.com/questions/1037/what-can-xxxcoin-do-to-stop-the-impending-51-attack http://bitcoin.stackexchange.com/questions/1037/what-can-xxx... And Quantitative Analysis of the Full Bitcoin Transaction Graph https://eprint.iacr.org/2012/584.pdf https://eprint.iacr.org/2012/584.pdf which analyzes how lots of Bitcoin accounts are seemingly controlled by the same group (so they are closer to a 51% attack than would otherwise appear). And there is always the possibility of 0days eg the Replay Attack in Ethereum https://medium.com/@timonrapp/how-to-deal-with-the-ethereum-replay-attack-3fd44074a6d8 https://medium.com/@timonrapp/how-to-deal-with-the-ethereum-...