That sounds like selfish mining, and the magic number is 25%. That's the
minimal pool size.
Today the threshold is 0% with good connectivity.

If I misunderstood your point, please elaborate.

Ittay



On Tue, Nov 5, 2013 at 12:05 PM, Peter Todd <pete@petertodd.org> wrote:

> On Tue, Nov 05, 2013 at 11:56:53AM -0500, Ittay wrote:
> > Hello,
> >
> > Please see below our BIP for raising the selfish mining threshold.
> > Looking forward to your comments.
>
> <snip>
>
> > 2. No new vulnerabilities introduced:
> > Currently the choice among equal-length chains is done arbitrarily,
> > depending on network topology. This arbitrariness is a source of
> > vulnerability. We replace it with explicit randomness, which is at the
> > control of the protocol. The change does not introduce executions that
> were
> > not possible with the old protocol.
>
> Credit goes to Gregory Maxwell for pointing this out, but the random
> choice solution does in fact introduce a vulnerability in that it
> creates incentives for pools over a certain size to withhold blocks
> rather than immediately broadcasting all blocks found.
>
> The problem is that when the pool eventually choses to reveal the block
> they mined, 50% of the hashing power switches, thus splitting the
> network. Like the original attack this can be to their benefit. For
> pools over a certain size this strategy is profitable even without
> investing in a low-latency network; Maxwell or someone else can chime in
> with the details for deriving that threshold.
>
> I won't get a chance to for a few hours, but someone should do the
> analysis on a deterministic switching scheme.
>
> --
> 'peter'[:-1]@petertodd.org
> 0000000000000005e25ca9b9fe62bdd6e8a2b4527ad61753dd2113c268bec707
>