How to protect from 51% Attack?

Question

What is the best way to reduce the risk of this attack?

Answer 1

How to protect from 51% Attack?

The nature of the system means that this attack cannot be prevented. Think of it this way, if you have a perfectly decentralized system in which the participants have control over the network (not some centralized authority), then the users get to vote on changes. The way to vote in blockchain is with your mining hashpower. If a majority (>50%) of the network votes on a change, then the change goes into effect (theoretically). So, how could you prevent this unless you centralize the network?

Now, in actuality, an attacker would likely need much more than 51% because not only do they have to outpace the network, they have to do so with every block after the one they want to modify, because what if a new block is mined by someone else while they are trying to outpace the network? They would need much more hashpower to have a good chance of successfully pulling it off.

Prevention

The real answer is you can't really prevent it, since it is a decentralized network, but if you are designing a new blockchain, the answer is to make it as decentralized as possible. Here are some considerations:

1. Commoditization of the mining hardware (commoditizing ASICs). Note this goes against some conventional thinking, that hashing algorithms should be ASIC resistant, but there is a good article that explains why that is a bad idea: ASICs and Decentralization FAQs Users who have to pay a lot, or find it difficult to get hashpower will likely not mine and it will be left to a few large players with the resources to do so. This results in more centralization of mining.

2. Avoid forking an existing coin with much larger hashpower. Users of the original coin now own coins on your new chain and are incentivized to attack it if they have a much larger portion of hashpower they can switch over to the new coin. If you do fork an existing coin, consider changing the hashing algorithm so miners of the original coin would have to invest more capital in order to attack.