Date: Mon, 7 Nov 2011 16:02:43 +0100
From: Pieter Wuille <pieter.wuille@gmail.com>
To: Gavin Andresen <gavinandresen@gmail.com>
Message-ID: <20111107150240.GA26096@ulyssis.org>
References: <CABsx9T2XLj4gZVPYodteaVCm0chR1n4WLUoSqB6+NnmWCDqHKQ@mail.gmail.com>
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Cc: Bitcoin Dev <bitcoin-development@lists.sourceforge.net>
Subject: Re: [Bitcoin-development] Difficulty adjustment / time issues
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On Tue, Sep 13, 2011 at 11:06:37AM -0400, 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

Here is an idea for an alternative (simple but hacky) solution:
* Keep all network rules as they are now.
* The timestamp value of mutliple-of-2016-blocks is set equal to
  the highest timestamp that occurred in the previous 11 blocks,
  instead of the current time. This will always obey the previous
  rules (it's always at least the median of the past 11 blocks,
  and never more in the future than them).

Initially, roll out software that only uses this new rule for
block creation, but doesn't enforce it. When enough miners have
upgraded, choose a point in the future where it becomes mandatory
(causing a block chain split only for those creating blocks using
old software).

If i understand the problem correctly, this will prevent an attacker
from introducing a time lapse in between the 2015-block windows.
One problem i do see, is that it prevents X-Roll-Time for miners.
Maybe a short interval (1 minute? 10 minutes?) instead of a fixed
value could be allowed for the multiple-of-2016 blocks.

Comments?

-- 
Pieter