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Date: Thu, 14 May 2015 01:46:04 +0200
Message-ID: <CABm2gDoOGcdUeo_xgPp7w7AFs3Fza5VdeqSyV-T9MNhuN3RMeA@mail.gmail.com>
From: =?UTF-8?B?Sm9yZ2UgVGltw7Nu?= <jtimon@jtimon.cc>
To: Alex Mizrahi <alex.mizrahi@gmail.com>
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Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] Long-term mining incentives
Precedence: list

On Wed, May 13, 2015 at 12:31 PM, Alex Mizrahi <alex.mizrahi@gmail.com> wrote:
> But this matters if a new node has access to the globally strongest chain.
> If attacker is able to block connections to legitimate nodes, a new node
> will happily accept attacker's chain.

If you get isolated from the network you may not get the longest valid
chain. I don't think any other consensus mechanism deals with this
better than Bitcoin.

> So PoW, by itself, doesn't give strong security guarantees. This problem is
> so fundamental people avoid talking about it.
>
> In practice, Bitcoin already embraces "weak subjectivity" e.g. in form of
> checkpoints embedded into the source code. So it's hard to take PoW purists
> seriously.

Checkpoints are NOT part of the consensus rules, they're just an
optimization that can be removed.
Try keeping the genesis block as your only checkpoint and rebuild: it
will work. You can also define your own checkpoints, there's no need
for everyone to use the same ones.
In a future with committed utxo the optimization could be bigger, but
still, we shouldn't rely on checkpoints for consensus, they're just an
optimization and you should only trust checkpoints that are buried in
the chain. Trusting a committed utxo checkpoint from 2 years ago
doesn't seem very risky. If the code is not already done (not really
sure if it was done as part of auto-prune), we should be prepared for
reorgs that invalidate checkpoints.
So, no, Bitcoin does NOT rely on that "weak subjectivity" thing.