How much would 51% attack cost?

Question

I was thinking about situation where for example NSA would want to gain control over significant portion of Ethereum network.

1. How much would it cost to for example create mining pool that would be able to perform 51% attack?
2. Could they use some existing infrastructure? For example https://en.wikipedia.org/wiki/Titan_(supercomputer) - I picked this one as it was top result for supercomputer gpu on google..

If you look at Utah data center NSA was OK with spending 1.5 billion dollars on some kind of data center that we don't know that much about.

Answer 1

While undoubtedly expensive, an assault on 51% Hash power using new mining rigs can't be ruled out as long as PoW is the bulwark in the consensus system.

I would suggest a less expensive and more likely scenario would be an alliance of mining pools that further pool their resources. In BTC's history, there were periods during which certain mining pools exercised more than 50% and wisely didn't harm the network.

When PoS is a thing, we'll need to adjust to a new understanding strengths and limitations of the consensus system.