[Cryptography] Bitcoin, fork you very much

[Ray Dillinger]

On 12/21/2017 08:20 AM, John Levine wrote:
> Let's say I'm with the Chinese government and decide that I am tired
> of people evading currency controls and money laundering with Bitcoin.
> So we adjust the Great Firewall of China to block port 8333.  We also
> add some MITM proxies that take newly mined blocks from the Chinese
> side, rewrite them to put the newly mined btc into government-approved
> wallets, fill up the blocks with transactions from outside China, and
> send them along.

This would not work.

Modified blocks would not have a hash proof-of-work.

That is, if you put the original hash in a modified block, people
will hash the block, say, "hey that doesn't match", and reject it.

If you hash the modified block, people will hash the block, look
at the hash, say "hey, that hash isn't below the work threshold",
and reject it.

The alternative of course is to modify the block, do some really
stupid amount of hashing on a computer made of unbelievium, find a
nonce to stick into the block that makes the modified block hash
low enough, and send it on.  That would be Hard.

				Bear

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