It is said that in the case of an attacker having a sustained 51% power, the attack still cannot change the block reward. Why?

My naive understanding is that, since consensus is controlled by the attacker, this suffices to push through a protocol change that changes the reward.

The main branch is the only one that can still confirm transactions, so clients would need to update their software to stay on this (malicious) main branch.

2 Answers 2


If the attacker tries to change the block reward, other users will see the new blocks as invalid. This will result in a fork with only the attacker in the new chain. Other nodes will not even have the choice to pick the longer chain since the new chain to them will be invalid.

  • Since the attacker controls most nodes, would clients retry different nodes until they find a valid (not compromised) node? Then they could join the 'old' network.
    – mafu
    Jun 1, 2018 at 10:26
  • 1
    @mafu I believe that when a node sends you invalid blocks, you disconnect from him after a couple of tries and ban him (at least temporarily). However the clients would not have to 'rejoin' the old network since they never left. For them there only exists one chain. Also I believe you got a couple things mixed. If the attacker has the majority of the hashpower it does not mean he controls many (or most) nodes. That is a different scenario.
    – Mike D
    Jun 1, 2018 at 10:57

Even you have 51% of hashpower you can not change the software on other computers to accept your blocks.

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