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	<CBBB62CD-2E30-4C9F-962E-3F340B29EDA7@xbt.hk>
Date: Fri, 20 May 2016 10:05:36 -0500
Message-ID: <CAAEDBiE08h=+8ntQ=mMyA0jaxj2H_6r2k0u4GdOhEkFNYEAhYQ@mail.gmail.com>
From: Matthew Roberts <matthew@roberts.pm>
To: Johnson Lau <jl2012@xbt.hk>
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Cc: bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] BIP: OP_PRANDOM
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Good point, to be honest. Maybe there's a better way to combine the block
hashes like taking the first N bits from each block hash to produce a
single number but the direction that this is going in doesn't seem ideal.

I just asked a friend about this problem and he mentioned using the hash of
the proof of work hash as part of the number so you have to throw away a
valid POW if it doesn't give you the hash you want. I suppose its possible
to make it infinitely expensive to manipulate the number but I can't think
of anything better than that for now.

I need to sleep on this for now but let me know if anyone has any better
ideas.


On Fri, May 20, 2016 at 6:34 AM, Johnson Lau <jl2012@xbt.hk> wrote:

> Using the hash of multiple blocks does not make it any safer. The miner of
> the last block always determines the results, by knowing the hashes of all
> previous blocks.
>
>
> == Security
>
> Pay-to-script-hash can be used to protect the details of contracts that
> use OP_PRANDOM from the prying eyes of miners. However, since there is also
> a non-zero risk that a participant in a contract may attempt to bribe a
> miner the inclusion of multiple block hashes as a source of randomness is a
> must. Every miner would effectively need to be bribed to ensure control
> over the results of the random numbers, which is already very unlikely. The
> risk approaches zero as N goes up.
>
>
>

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<div dir=3D"ltr"><div>Good point, to be honest. Maybe there&#39;s a better =
way to combine the block hashes like taking the first N bits from each bloc=
k hash to produce a single number but the direction that this is going in d=
oesn&#39;t seem ideal. <br><br></div><div>I just asked a friend about this =
problem and he mentioned using the hash of the proof of work hash as part o=
f the number so you have to throw away a valid POW if it doesn&#39;t give y=
ou the hash you want. I suppose its possible to make it infinitely expensiv=
e to manipulate the number but I can&#39;t think of anything better than th=
at for now.<br><br></div><div>I need to sleep on this for now but let me kn=
ow if anyone has any better ideas.<br></div><div><br><br></div></div><div c=
lass=3D"gmail_extra"><br><div class=3D"gmail_quote">On Fri, May 20, 2016 at=
 6:34 AM, Johnson Lau <span dir=3D"ltr">&lt;<a href=3D"mailto:jl2012@xbt.hk=
" target=3D"_blank">jl2012@xbt.hk</a>&gt;</span> wrote:<br><blockquote clas=
s=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;pad=
ding-left:1ex"><div style=3D"word-wrap:break-word"><div>Using the hash of m=
ultiple blocks does not make it any safer. The miner of the last block alwa=
ys determines the results, by knowing the hashes of all previous blocks.</d=
iv><span class=3D""><div><br></div><div><blockquote type=3D"cite"><div dir=
=3D"ltr"><p style=3D"margin-bottom:0in;line-height:100%"><br>
</p><p style=3D"margin-bottom:0in;line-height:100%">=3D=3D Security</p><p s=
tyle=3D"margin-bottom:0in;line-height:100%">Pay-to-script-hash
can be used to protect the details of contracts that use OP_PRANDOM
from the prying eyes of miners. However, since there is also a
non-zero risk that a participant in a contract may attempt to bribe a
miner the inclusion of multiple block hashes as a source of
randomness is a must. Every miner would effectively need to be bribed
to ensure control over the results of the random numbers, which is
already very unlikely. The risk approaches zero as N goes up.</p></div></bl=
ockquote></div><br></span></div></blockquote></div><br></div>

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