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From: Andy Parkins <andyparkins@gmail.com>
To: Jorge =?iso-8859-1?q?Tim=F3n?= <timon.elviejo@gmail.com>
Date: Wed, 23 Nov 2011 12:54:41 +0000
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Cc: bitcoin-development@lists.sourceforge.net
Subject: Re: [Bitcoin-development] Addressing rapid changes in mining power
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On 2011 November 23 Wednesday, Jorge Tim=F3n wrote:
> With the current system, the timestamp can also be cheated, but miners
> have no direct incentive to do it. With your system, they increase
> their probability of mining a block by putting a false timestamp.
> Also, where's the network clock you're talking about? Isn't it the
> timestamps in the blockchain?

(1) The "probability of mining a block" is old-think.  The probability of=20
mining a block is 100% in my system.  Instead, it becomes "the probability =
of=20
your block being the hardest" and that requires actual hashing power=20
regardless of the timestamp you write on the block.  I could write that my=
=20
block was generated next year; but I can't fake the hashing power it needs =
to=20
generate one year's worth of hashes.

If chain difficulty were summed correctly (sum(log(difficulty)), I guess),=
=20
then time makes not the slightest difference anyway.  You can issue blocks =
at=20
any time with any difficulty, and the "hardest" chain always wins.  The blo=
ck=20
period can be anything, and it is only the block reward that makes it=20
necessary to pick a particular period for block issuing (even that could be=
=20
worked around I guess with a variable reward, but why bother?).

(2) For the network clock; see util.cpp:GetAdjustedTime().

(3) Current clients do have an incentive: more time.  The more time they ge=
t,=20
the more hashes they can try.  The current client already checks the=20
timestamp:

  main.cpp:CBlock::CheckBlock()

    // Check timestamp
    if (GetBlockTime() > GetAdjustedTime() + 2 * 60 * 60)
        return error("CheckBlock() : block timestamp too far in the future"=
);

My suggestion only requires that the two hour window be reduced; and a lowe=
r=20
limit to be added.  Also: while the miners have an incentive to lie about t=
he=20
time, the nodes they broadcast to have an incentive to reject mistimed bloc=
ks,=20
so you won't gain much by lying to your peers since your block won't be=20
accepted -- the incentive is therefore removed.

Note: my system also prevents an attack that is possible with current bitco=
in:=20
recalculating the entire chain.  Let's say Visa want to take over bitcoin. =
=20
They buy enough computing power to significantly beat the current bitcoin=20
network; then they start recalculating the entire block chain; since early=
=20
blocks were low difficulty, it's not that hard to do.  Once they overtake t=
he=20
real chain, they have effectively undone all previous transactions.  (I'm n=
ot=20
suggesting this is likely; and it's actually mitigated by the hard-coded bl=
ock=20
hashes).  The point is that blocks are only generatable for the time when t=
he=20
rest of the network is willing to add them to the chain.


Andy

=2D-=20
Dr Andy Parkins
andyparkins@gmail.com

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