What exactly is the 51% Attack?
How hard would it be to accomplish this attack?
How easy is it to overcome the attack should it actually occur?
4I gave you +1 but your question body is almost exactly the same as the question title. If you don't have anything else to write, maybe you could tell us what research you've done on the topic so far and where you've tried to look? The question body is usually meant to have more than just a repeat of the title!– Nike DattaniMay 5, 2021 at 0:05
The 51% attack is used by an attacker to take over the blockchain. Usually, the community of a blockchain has to reach consensus, which means that once someone mints a new block, other nodes validate it. The majority has to agree that the new block is valid and thus the blockchain has grown one block. All other nodes accept the new block and all it's content (transactions) as valid and "follow" it, as in using is as a starting point to calculate the next block.
For a proof of work blockchain like Bitcoin, this would mean to have more hashing power than anyone else on the blockchain. This would even allow to you overwrite existing blocks. To regain control, the "honest minority" had to increase their hashing power.
For a proof of stake blockchain like cardano, an attacker would require more than half of all staked ada. An attacker would have to buy 22.5 billion ada of the currently circulating supply of ~33bn ada or control enough pools to which this amount of ada had to be delegated to. Buying ada would drive the price up, depending on how fast he wants to buy and how much he agrees to pay per ada and would be very difficult, same goes for running enough pools that attract enough delegators as well.
The attacker could then dictate the content of the minted blocks. To regain control in a scenario where the adversary bought the ada, the "honest minority" would have to buy his coins, which the attacker would have to agree to. In theory, this could be very difficult. However, it is almost impossible for an attacker to obtain that many ada in the first place. If the attacker controls enough pools thus has enough control over more than half of all staked ada, the honest owners of the coins could simply switch to honest pools.
As for all 51%: with a growing number of honest users participating in a healthy network, an attacker will have a harder time to take it over, until it becomes basically impossible.
1I'd like to add, that the attacker only needs control of the majority of ada; this means he doesn't have to buy it, but could simply offer other incentives for delegators. (think of exchanges or big pools).– user306May 5, 2021 at 18:34
Thank you! I added this to my answer as well. He has to control the staked ada, it does not has to be his own. May 5, 2021 at 21:58
The 51% number would imply the attacker controlled 51% of the STAKED ADA. Not necessarily 51% of all ADA. Of course the closer we get to 100% of all ADA staked the closer those two numbers get to each other.