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You can't 'just add any transaction', the transaction has to follow the rules of the protocol (such as respecting prior owners of coins, respecting the total number of coins in circulation, etc) otherwise the block will be ignored (no matter how much work it has attached).

The general technical audience tends to greatly overestimate the amount of power that a 51% attack has. A 51% attack can only do two basic things:

1. prevent certain transactions from making it onto the chain (somewhat expensive) 2. re-write history so that previously confirmed transactions are no longer on the chain (quite expensive - increasingly expensive the further back in time you rewind)

You can't change the rules of the system, you can't arbitrarily steal funds from users, you can't change the inflation rate, etc.

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