On 25/04/14 20:17, Mike Hearn wrote:
>     Proving that you can convince the economic majority that the
> 
>     interpretation of existing blocks is in any way up for grabs would set a
>     dangerous precedent, and shake some people's faith in Bitcoin's overall
>     robustness and security (well, mine anyway.)
> 
> 
> Hmm, then I think your faith needs to be shaken. Bitcoin  is money, and
> money is a purely artificial social construct. The interpretation of
> what a bitcoin means, or what a dollar means, has always been and always
> will be a human decision taken in order to achieve some socially useful
> goal. 

My argument does not concern what a bitcoin means, just what data in the
blockchain means. People are free to value an individual bitcoin however
they like. But it's useful if we all agree on a standard that defines
who owns them - ie. the protocol as described in Satoshi's whitepaper. I
recognise that your ability to provide a valid scriptSig for /any
existing UTXO in the blockchain/ as proof of your ownership of the
corresponding bitcoin. You want to pick and choose which UTXO (well,
coinbase; same diff) you consider valid and spendable /after they've
become part of the blockchain/, regardless of the fact they're buried
under PoW.

As an illustration, consider Counterparty - an altcoin whose TXns are
embedded as unvalidated data in the bitcoin blockchain. A lot of people
imagine that an XCP transaction buried under 100 blocks and a BTC
transaction buried under the same 100 blocks are equally secure. You
tell me: are they? It's the same PoW chain after all.

Hell no they're not! The way Counterparty interprets that data in the
blockchain is anything but stable or well documented. On more than one
occasion they've gone "whoops, found a bug that caused some transactions
to occur that we don't consider valid - we'll just fix that." A suddenly
the reference client doesn't consider the XCP in your wallet valid
anymore - they just magically disappear - because the parent of the TXn
that paid you was actually invalid. Nobody rewrote history via PoW; they
simply tweaked their interpretation of the existing history.

When you have a *bitcoin* TXn buried under 100 blocks you can be damn
sure that money is yours - but only because the rules for interpreting
data in the blockchain are publicly documented and (hopefully)
immutable. If they're mutable then the PoW alone gives me no confidence
that the money is really mine, and we're left with a much less useful
system. This should be more sacred than the 21m limit.