You are referring to a 51% attack, but you are confused as to what it can achieve.
If a miner with a majority of hash power joins the network, then they can build a chain with more proof of work than the rest of network, meaning they can dictate which transactions are and are not included, and can double spend any of their own transactions.
What they cannot do is "manipulate current transactions", as you put it, and change any receiving addresses to their own. Regardless of how much hash power they possessed, they still need to mine valid transactions. They cannot change a transaction's output address to their own without possession of the necessary private key(s). If they chose to start mining invalid transactions which sent everyone's coins to themselves, then the rest of the network would simply ignore these invalid transactions and invalid blocks and continue on as if nothing was happening.
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