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Date: Tue, 13 Sep 2011 11:24:35 -0500
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To: Gavin Andresen <gavinandresen@gmail.com>
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Cc: bitcoin-development@lists.sourceforge.net
Subject: Re: [Bitcoin-development] Difficulty adjustment / time issues
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Gavin Andresen wrote:
> Background:
>
> Timejacking:
>    http://culubas.blogspot.com/2011/05/timejacking-bitcoin_802.html
>
> And a recent related exploit launched against the low-difficulty
> alternative chains:
>    https://bitcointalk.org/index.php?topic=43692.msg521772#msg521772
>
>
> Seems to me there are two fundamental problems:
>
> 1) Bitcoin should be overlapping the ranges of block timestamps that
> it uses to calculate difficulty adjustments.
>
> 2) Bitcoin's "what time is it" code is kind of a hack.
>
>
> Fixing (1) would mean a potential block-chain split; before
> considering doing that I'd like to consider second-best solutions.
>
> Fixing (2) is easier; incorporating a ntp library and/or simply
> removing the bitcoin mining code from the client but requiring pools
> and miners to have accurate-to-within-a-minute system clocks (or their
> blocks will be "discouraged") seems reasonable to me. If you want to
> produce blocks that the rest of the network will accept, run ntp on
> your system.
>
> I THINK that fixing (2) will make (1) a non-issue-- if miners can't
> mess around with block times very much then it will be very difficult
> for them to manipulate the difficulty for their benefit.
>
The first thing I always do when I grab the source for my colo server is 
patch util.cpp so that GetAdjustedTime() returns GetTime() with no 
adjustment.  But I'm the kind of guy that buys special GPS receivers 
because stratum 2 isn't low enough and occasionally checks ebay for 
caesium fountains.

NTP has been around for long enough now that there is no reason for the 
client to screw with the clock.  If the client sees different times on 
the network, it should issue a warning, and if it is off too far, it 
should give an error and fail to run (and/or peers should reject it).

But that doesn't solve the whole problem, because the block timestamp 
checking is based on the assumption that the node is looking at the 
bitcoin clock rather than the, ahem, real clock.  If we change the idea 
of network time to NTP, we will then need to write (and test!) new block 
timestamp rules to account for the new assumptions.

I'm not sure that just fixing item 2 is going to stop the attacks found 
by ArtForz, et al.  Some of the attacks Art pointed out are particularly 
bad because they change the incentive structure of the system, at least 
in the short term.  We need to flip that back around ASAP.

Also, this is going to cause problems for at least one pool operator.