[Cryptography] Bitcoin, fork you very much

Natanael natanael.l at gmail.com
Thu Dec 21 14:49:08 EST 2017


Den 21 dec. 2017 20:33 skrev "John Levine" <johnl at iecc.com>:

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.


Blocks are considered valid if;

* The syntax is correct for the header and for all transactions. The header
includes a Merkle tree hash of the transactions.
* If all values are within the correct limits, such as that the time of one
block may not be too far back or into the future compared to the prior one.
* If the miner's "coinbase transaction" is valid, that first transaction in
the block in which the miner claims the reward and fees to his own address.
He can't claim a larger payout than the available reward + fees in the
transactions bundled in the block.
* If all transactions only try to claim valid unspent transaction outputs
from previous transactions, and if they follow all the scripting rules
correctly (different address formats enforce different scripts), and if
outputs are not larger than the inputs.

And critically, where your idea fails:

* If the proof of work is valid, meaning that the integer representation of
SHA256(SHA256(block header)) is less than the current mining difficulty
target value.

Changing the coinbase transaction to steal the coins changes the Merkle
tree hash in the header and thus invalidates the proof of work, because the
header hash changes too - randomly. With a very very tiny probability to be
valid PoW.

(Note that the above criteria are not complete, there are more factors
involved. But they're sufficient to describe the concept.)

It's relatively much easier to just attempt to isolate your people from any
cryptocurrency nodes.
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