The state actor in your scenario can't steal anyone's money. They can only create transactions for addresses they control with a private key.

If you listed everything that would need to happen to cause a 51% attack to be successful, along with the odds that this could happen, you'd see that it has zero chance to succeed.

(I'm using some quick back-of-the-napkin odds to calculate the feasibility that this is even possible...)

- Build large mining network under control of state actor that matches the hashing power of the entire network (5% chance this is possible)

- Bad state actor buys something with bitcoin. (100% possible)

- Bad state actor receives the good within 3 blocks or 30 minutes. (5% chance they receive this in time)

- Bad state actor starts building bad blocks with alternate transaction redirecting the BTC into their own wallet. (1% chance)

- Bad state actor solves 3 (3 blocks back to the original transaction) + 2 blocks (to make this a longer blockchain than the good blockchain) faster than the rest of the network. (3.5% chance if they have over 50% of the hashing power)

- Bad state actor did all of this without the rest of the bitcoin network noticing and routing around the state actor (10% chance)

So you have something like 5% X 100% X 5% X 1% X 3.5% X 10% => 0.00000875% chance that this is possible.

Granted, I'm taking a guess at the odds of most of the numbers, but I don't think I'm that far off. The bitcoin network is currently operating at over 3 million TH/s. [1].

To set up another network with that same hashing power would cost at least $2 billion dollars, if not more.

You could buy 250,000 AntMiner S9s (if you could even purchase that many without anyone knowing and to avoid availability problems) for about $500 million.

Then you'd have to set up a place that could hold that many miners. Okay so get a warehouse wired up and properly cooled for another $100 million.

Then electricity is about 1350 watts per miner, requiring a $200 power supply. So add another $50 million just for power supplies....but let's say you wire it up custom, so I'll knock that down to $25 million.

We need 337.5 million watts of power to run our miners. At 10 cents per KW/h, the electric bill will be around $24.3 million a month--just to run the miners. Round that up a bit to power the rest of the electricity, run the UPS and backups, the computers for the staff, and let's say the monthly electricity cost would be about $35 million a month...which is $420 million a year.

Back to the staff, you'd probably need 500 people to maintain such a big operation, minimum. If each state actor employee received about $100k/year in salary, the personnel costs are about $50 million/year.

Now multiply all this by two because you would never set up an operation like this without building in redundancy. This comes to about $2.2 billion for a year of operations.

$2.2 billion to have a 0.00000875% chance at a double spend or to reject some transactions from making it to the blockchain. You'd be insane to approve this project.

But let's say the bad state actor is able to pull off this feat. Then the bitcoin miners do a hard fork and just route around the bad state actor the next day. Everything picks up again where it left off on Monday.

- [1] https://blockchain.info/charts/hash-rate