From: Rusty Russell <rusty@rustcorp.com.au>
To: Peter Todd <pete@petertodd.org>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
	"Christian Decker" <decker.christian@gmail.com>
In-Reply-To: <87muwhvozr.fsf@rustcorp.com.au>
References: <87po25lmzs.fsf@rustcorp.com.au>
	<201805100227.42217.luke@dashjr.org>
	<87vabnq9ui.fsf@rustcorp.com.au>
	<CADZtCShwOV+GuJ5__GMi9hd2_X=BztASPBihDXakU3Mjb39wcQ@mail.gmail.com>
	<87zi0tisft.fsf@rustcorp.com.au>
	<20180521035658.vfo4wx6ifum2s2o5@petertodd.org>
	<87muwhvozr.fsf@rustcorp.com.au>
Date: Thu, 31 May 2018 12:17:58 +0930
Message-ID: <87h8mov8v5.fsf@rustcorp.com.au>
MIME-Version: 1.0
Content-Type: text/plain
Cc: Matt Corallo <matt@chaincode.com>
Subject: Re: [bitcoin-dev] Making OP_TRUE standard?
Precedence: list

Rusty Russell <rusty@rustcorp.com.au> writes:
> AFAICT the optimal DoS is where:
>
> 1.  Attacker sends a 100,000 vbyte tx @1sat/vbyte.
> 2.  Replaces it with a 108 vbyte tx @2sat/vbyte which spends one of
>     those inputs.
> 3.  Replaces that spent input in the 100k tx and does it again.
>
> It takes 3.5 seconds to propagate to 50% of network[1] (probably much worse
> given 100k txs), so they can only do this about 86 times per block.
>
> That means they send 86 * (100000 + 108) = 8609288 vbytes for a cost of
> 86 * 2 * 108 + 100000 / 2 = 68576 satoshi (assuming 50% chance 100k tx
> gets mined).

This 50% chance assumption is wrong; it's almost 0% for a low enough
fee.  Thus the cost is only 18576, making the cost for the transactions
463x lower than just sending 1sat/vbyte txs under optimal conditions.
That's a bit ouch.[1]

I think a better solution is to address the DoS potential directly:
if a replacement doesn't meet #3 or #4, but *does* increase the feerate
by at least minrelayfee, processing should be delayed by 30-60 seconds.

That means that eventually you will RBF a larger tx, but it'll take
much longer.  Should be easy to implement, too, since similar timers
will be needed for dandelion.

Cheers,
Rusty.
[1] Christian grabbed some more detailed propagation stats for me: larger
    txs do propagate slower, but only by a factor of 2.5 or so.