[Cryptography] ghash.io hits 50% of the Bitcoin compute power

[Nemo]

Jon Callas <jon at callas.org> writes:

> John Levine <johnl at iecc.com> writes:
>
>> I'm running Bitcoin Core 0.9.1 on my laptop, which I spin up every
>> once in a while to update my copy of the blockchain.  I have no idea
>> what it would do if it started getting lots of messages saying whoa,
>> there was a fork 100,000 blocks ago and here's the new version of the
>> blockchain.
>
> It would do the same thing that every other piece of software that has
> to do complex things and error detection on hard-to-simulate,
> won't-happen-in-the-real-world conditions does: fail massively in some
> spectacular way.

Before answering questions about the Bitcoin software, may I suggest
learning something about it?

The Bitcoin software has a concept called a "checkpoint lockin":

  https://en.bitcoin.it/wiki/Checkpoint_Lockin

The 0.9.1 software includes a checkpoint at block 279,000:

  https://github.com/bitcoin/bitcoin/blob/0.9.1/src/checkpoints.cpp#L53

Whether checkpoints are a good idea -- in particular, whether they imply
that Bitcoin is not nearly as decentralized as advertised -- is a
legitimate matter for debate. But on a purely technical basis, the
current block number is 306,006, so the correct answer to John's
question is "nothing would happen because his software would reject any
attempt to rewrite that much history".

As for "fail massively in some spectacular way", there is less than zero
evidence to support that claim even for shorter-term blockchain
forks. First, spinning up a test Bitcoin network is trivial, and the
developers have done quite a lot of simulation of potential failure
modes, including forks. Second, the live network experienced a real fork
in 2013 (search for "blockchain fork"), during which all versions of the
software kept chugging along fine; they just disagreed about which
version of the blockchain represented reality...

 - Nemo
   https://self-evident.org/