如果我可以更改节点的代码并正常运行，那么区块链系统（例如以太坊）如何？

Question

Lets take ethereum for instance(this question is general for all the public blockchains). Can i change the code (like sending transactions but not deducing account balance) and just run it as usual. Is there any underlying mechanism that prevents this? I mean how is the system secure if i could run a modified version of it?

Answer 1

As for your first question unless you fork and modify the ethereum code you can't make the changes you demand (send transactions without deducting balance). That pretty much answers your last question too, you can find all help you need to deploy code or get the origial source code here. Feel free to modify it.
https://github.com/ethereum

Also the working or Ethereum is not as simple as you think. You can't just write code (smart contracts) and deploy them in the Ethereum-Blockchain and expect them to run for free. You need ether for that which acts as a fuel for ether. To understand how security of Ethereum works you need to first understand the security of blockchain. I wouldn't go into the details but keep note that as you go further back into the chain it gets more and more difficult to modify the codes. Also Ethereum is quite secure, at least as much as you can expect from blockchain although there are security holes that seem to pop up every now and then, but after 50M hack back in 2016 and the following hard-fork Ethereum is pretty secure from average hackers.

So to sum it up, to modify the code you need to make a fork. And this fork is not at all related to the main Ethereum. SO you aren't running a modified version of Ethereum on the chain itself and hence it remains secure. Also you are quite limited when making smart contracts and they don't allow direct modification of the block-chain.

Answer 2

Say, you run an unmodified ethereum node, and you get 3 ethers from somewhere in your account. Now you rebuild your own modified node so that you send 2 ethers to me, but does not reduce 2 from your balance. What happens is that me and the rest of the world will verify and process the transaction that you signed and reduce 2 ethers from your balance. We don't run the modified version, right? You see what's going on? The next time you try to spend another 2 ethers, your node will think you still have 3 ethers and will sign and broadcast a transaction, but everyone else in the world will not accept your transaction, since they know you only have 1 ether. So you harm just to yourself.