Or, even easier, the FBI (or any other anti-BTC agency) simply has to take control of the 4 or 5 biggest BTC mining pools.
That will give them control of more than 51% of BTCs hash power, with 4 or 5 relatively cheap arrests.
The same approach could also be used on pretty much any crypto-currency, but in NXTs case, you'll have to find and take down a lot more people to gain control of enough NXT to make a 51% attack effective.
They can realistically 51% attack Bitcoin with less than 51% of bitcoin's processing power... all they pretty much have to do is take control of one of the larger pools. Page 10 shows how easy it would be to do a double spend with a certain percentage of the network hash rate on a transaction with a certain number of confirmations:
https://bitcoil.co.il/Doublespend.pdfWith 30% of the network, you have a 60% chance to successfully double spend a transaction with 1 confirmation.
With 42% of the network, you have a 58.4% chance to successfully double spend a transaction with 6 confirmations (when a Bitcoin transaction is considered secure).
It is much more likely than anyone around here would like to admit. If one of the larger pools are compromised the attackers could double spend the pools' withdrawals with decent success.
