Majority Control

Question

So I have heard that if you have control of the majority of the nodes you can then force in your own data. Like say for example your consensus is setup that you need 60% of agreement from nodes for a transaction to be verified. Lets say a hostile entity manages to get control of 61% of the nodes. How would they go about modifying the data on the chain for their own use? How would they use those nodes to validate invalid transactions?

Answer 1

First, it's not possible for a hostile majority to fake transactions from another participant, because they still do not have that participant's private key(s).

Second, it's not possible for a hostile majority to force other nodes to forget a transaction that was seen, because once aby node has seen that transaction, it is free to remember it.

What a hostile majority of validator nodes can do is: (a) prevent certain transactions from entering the blockchain, i.e. being confirmed, and (b) retroactively change the consensus regarding the authoritative fork on the chain.

But one might say that, philosophically, if the majority of your validator nodes are hostile, then you probably have bigger problems than your blockchain consensus breaking down. It means that the majority of business relationships on which you were relying have broken down as well.