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

From: Emin Gün Sirer <[email-masked]>
Date: Mon, 28 Dec 2015 14:30:14 -0500

On Mon, Dec 28, 2015 at 2:12 PM, Peter Todd via bitcoin-dev <
[email-masked]> wrote:

> 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.
>

This is not quite right: we know that selfish mining is a guaranteed win
at 34%. We do not know when exactly it begins to pay off. The more
consolidated and centralized the other mining pools, the less of a threat
it is below 34%; the more decentralized, the more likely it is to pay off
at lower thresholds.

Far more concerning is network propagation effects between large and
> small miners.

On a related note, the Bitcoin-NG paper took a big step towards moving
these kinds of concerns out of the realm of gut-feelings and wavy hands
into science. In particular, it introduced metrics for fairness (i.e.
differential
rate in orphans experienced by small and large miners), hash power
efficiency, as well as consensus delay.

> 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.

Indeed, there is a slight, quantifiable benefit to larger pools. Which is
why
we need to be diligent about not letting pools get too big.

> 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)
>

I can see why they don't want to go public with this: it means that they
are less profitable than other pools.

It still looks to me like Ittay's discovery is doing exactly the right
thing:
this pool will need to be more careful when signing up new people,
curbing its otherwise steady march towards the 51% boundary.

- egs

- egs