Subject: Re: [bitcoin-dev] SPV Mining reveals a problematic incentive issue.

From: Eric Lombrozo <[email-masked]>
Date: Mon, 13 Jul 2015 10:49:40 -0700

My other email client messed up. I apologize for the blank message.

Anyhow...

Even though the cost of mining bad blocks is high enough to deter most
deliberate attacks, if we're not properly validating blocks, this
deterrence does not stop bugs nor version issues and it opens up attack
vectors like someone hacking into a mining pool server.

It is imperative we continue to look for ways to make secure validation
cheaper. I would make this #1 priority. Not only is it crucial to the
integrity of our security model - it is crucial for scalability and
decentralization as well.

- Eric Lombrozo
On Jul 13, 2015 11:05 AM, "Peter Todd" <[email-masked]> wrote:

> On Sat, Jul 11, 2015 at 11:24:48AM +0200, Jorge Timón wrote:
> > All miners should validate transactions precisely because of the latest
> > attack you've described. Full miners can gain a lot from this attack to
> > leverage their full validation against spv miners who blindly spend
> energy
> > hashing on top of something that may be worthless crap. SPV mining makes
> no
> > sense, but some miners claim they're doind it for very short periods of
> > time, which shouldn't be as bad as doing it all the time.
> >
> > I think it would be more rational for them to keep mining on top of the
> old
> > block until they've fully validated the new block (which shouldn't take
> so
> > long anyway), even if this slightly increases the orphan rate.
>
> You're missing something really critical about what F2Pool/AntPool were
> (are?) doing: They're finding out about new blocks not by getting block
> headers from just anywhere, but by connecting to other pools' via
> stratum anonymously and determining what block hash they're telling the
> hashers at the pool to work on. (e.g. what prevblockhash is in the block
> header of shares being generated)
>
> If other pools try to fake this information they're immediately and
> directly losing money, because they're telling their own hashers to make
> invalid blocks. This of course has a high chance of being detected, and
> can easily be FUDed into "STOP MINING AT FOO POOL!" reardless of what
> the ivory tower game theory might say. The only hope the pools have is
> to somehow identify which connections correspond to other pools with
> high reliability and target just those connections - good luck on that.
>
>
> Anyway, all this concern about SPV mining is misguided: relying purely
> on SPV w/ low #'s of confirmations just isn't very smart. What SPV can
> do - at least while the inflation subsidy is still high - is give
> reasonable protection against your third-party-run trusted full nodes
> from lying to you, simply because doing so has well-defined costs in
> terms of energy to create fake blocks. Targetting enough people at once
> to make a fake block a worthwhile investment is difficult, particularly
> when you take into account how timing works in the defenders favor - the
> attacker probably only has a small % of hashing power, so they're going
> to wait a long time to find their fake block. Between that and a trusted
> third party-run full node you're probably reasonably safe, for now.
>
> --
> 'peter'[:-1]@petertodd.org
> 0000000000000000086007e31decd6eb80e07f77271ef50c69e1e6342161f4e5
>
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