Government disrupting reception of bitcoins. Is it possible? I was thinking of a very specific scenario: Suppose we have a new organization, let's call it "WikiLeaks2", which receives donnations mostly in the form of Bitcoins. Suppose a national government tries to shut it down by disrupting its reception of funds, so they set up a number of miners with the code modified so they check each transaction against a list of black-listed Bitcoin addresses (among them, that of WikiLeaks2) and automatically reject a transaction coming to/from those addresses.

Let's assume this government has successfully set up moles deep within the organization, so any new reception address is known by its agents after a few minutes. Would they need to control 51% of mining power to make it impossible to send funds to such an organization, or would disruption be possible with a fewer percentage?

Which begs the question: who validates the acceptance/rejection of a transaction, done by the miners, is correct? I.e. who validates the validators?
Would they need to control 51% of mining power to make it impossible to send funds to such an organization



If you have less than half of the mining power:
You control what transactions go into blocks you mine. You can definitely slow down payments to WikiLeaks2. However, you can't prevent payments from going out forever. Other miners will look at the WikiLeaks2 transactions, see nothing wrong with them, and include them into their blocks. You cannot do a 51% attack because the honest nodes, working together, will simply overpower you.
If you have more than half of the mining power: (51%)
You can now prevent everyone else from getting blocks. This means that you can prevent transactions you don't like from getting confirmations forever.



  Which begs the question: who validates the acceptance/rejection of a transaction, done by the miners, is correct? I.e. who validates the validators?


Certain network rules are never voted about - For example, transactions must be signed, a block must have a valid timestamp, you can't spend more bitcoins than you have, that sort of thing.

What bitcoin miners do vote about is what should be on this global, shared, ordered list of transactions.