Date: Tue, 6 Oct 2015 16:20:31 +1000
From: Anthony Towns <aj@erisian.com.au>
To: bitcoin-dev@lists.linuxfoundation.org
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User-Agent: Mutt/1.5.24 (2015-08-30)
Subject: Re: [bitcoin-dev] Let's deploy BIP65 CHECKLOCKTIMEVERIFY!
Precedence: list

On Mon, Oct 05, 2015 at 06:46:28PM +0200, Mike Hearn via bitcoin-dev wrote:
> The example is this: find someone that accepts 1-block confirmed
> transactions in return for something valuable. There are plenty of them out
> there. Once the soft fork starts, send a P2SH transaction that defines a
> new output controlled by OP_CLTV. It will be incorporated into the UTXO set
> by all miners because it's opaque (p2sh).
> 
> Now send a transaction that pays the merchant, and make it spend your
> OP_CLTV output with an invalid script. New nodes will reject it as a rule
> violator. Old nodes won't.

Old nodes running bitcoind will see it as OP_NOP2, and will reject it
unless they've manually disabled SCRIPT_VERIFY_DISCOURAGE_UPGRADABLE_NOPS,
which (aiui) has been available since bitcoin 0.10 [0], but not backported
to 0.8 or 0.9.

[0] https://github.com/bitcoin/bitcoin/commit/0391423

That covers about 4700/5880 nodes going by bitnodes.21.co; but I can't
tell how many miners it covers.

Further, AIUI, nodes running 0.8 or 0.9 will still apply IsStandard()
checks to scripts attempting to spend p2sh outputs [1], so will also
fail to either mine or relay your OP_NOP2 payment.

[1] https://github.com/bitcoin/bitcoin/commit/6259937

> So at some point an old miner will create a
> block containing your invalid transaction, the merchant will think they got
> paid, they'll give you the stuff and the fraud is done.

My understanding is that this isn't supposed to be a problem because you
won't be able to find an old miner that will do that; released versions
of bitcoin already block it by default.

Sure, someone could disable those checks and not pay attention to a soft
fork that will cause their blocks to be orphaned, but I'm not seeing why
that's any different a threat compared to someone deliberately mining
invalid blocks to do 1-confirmation doublespends against merchants not
running a full node.

At least, that's my understanding, and I'm not an expert, so corrections
appreciated.

Cheers,
aj