[bitcoin-dev] We need to fix the block withholding attack

Multipool Admin admin at multipool.us
Mon Dec 28 19:33:11 UTC 2015


On Mon, Dec 28, 2015 at 11:12 AM, Peter Todd <pete at petertodd.org> wrote:

> On Sat, Dec 26, 2015 at 12:12:13AM -0800, Multipool Admin wrote:
> > Any attempt to 'fix' this problem, would most likely require changes to
> all
> > mining software, why not just make mining more decentralized in general?
> >
> > For example, allow anyone to submit proofs of work to Bitcoind that are
> > some fraction of the network difficulty and receive payment for them if
> > they're valid.  This would also encourage the proliferation of full nodes
> > since anyone could solo mine again.  Then, the next coinbase transaction
> > could be split among, say, the top 100 proofs of work.
>
> That's certainly be a good place to be, but the design of Bitcoin
> currently makes achieving that goal fundementally difficult.
>

Agreed, however I don't think it would be impossible or even really that
difficult, and would be a great way to increase decentralization while
simultaneously fixing other issues with mining.

Proofs of work would be valid if they're built on top of the current block
hash, and we could require (difficulty/N) proofs of work that are >=
(difficulty/N) to assemble a valid block.  Same as mining shares work.

The block assembler who finds the final diff/N 'share' could get a small
bonus as an incentive to complete the block as quickly as possible.  Or
alternatively, a checksum could be computed of all the current diff/N
shares in the mempool and that way only the final share would need to be
broadcasted to the entire network, and clients with the correct checksum
could assemble the block themselves without having to download the entire
block.  This would drastically decrease data usage on the network.

> Eligius already does their miner payouts like this.
> >
> > If you want to fix an issue with mining, fix the selfish mining issue
> first
> > as it's a much larger and more dangerous potential issue.
>
> Do you specifically mean selfish mining as defined in Emin Gün
> Sirer/Ittay Eyal's paper? Keep in mind that attack is only a significant
> issue in a scenario - one malicious miner with >30% hashing power -
> where you're already very close to the margins anyway; the difference
> between a 50% attack threshold and a 30% attack threshold isn't very
> significant.
>

Yes, that's what I'm talking about.


> Far more concerning is network propagation effects between large and
> small miners. For that class of issues, if you are in an environemnt
> where selfish mining is possible - a fairly flat, easily DoS/sybil
> attacked network topology - the profitability difference between small
> and large miners even *without* attacks going on is a hugely worrying
> problem. OTOH, if you're blocksize is small enough that propagation time
> is negligable to profitability, then selfish mining attacks with <30%
> hashing power aren't much of a concern - they'll be naturally defeated
> by anti-DoS/anti-sybil measures.
>

The possibility that a previously 'good' actor with 30% of the hashpower
going 'rogue' becomes more and more of a concern as the block subsidy
decreases.


> > I don't believe it was ever clearly established whether Eligius suffered
> a
> > block withholding attack or was just the victim of a miner with (what
> was,
> > at the time) a large amount of faulty hardware, however, from the
> > Bitcointalk threads at the time I believe it was assumed to be the
> latter.
>
> I think the latter was assumed as well, although ruling out of the
> former is impossible.
>
> Note though that Eligius is *not* the only pool to have had problems
> with block withholding, though AFAIK Eligius is the only one who has
> gone on record so far. (as I said in my original post, I'm relaying
> information given to me under condition of confidentiality)
>

What is the incentive not to go on record about this?

--Adam
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