To determine the new difficulty, it is supposed to compare the timestamps of 
block (2016n - 1) with block (2016n - 2017). However, an off-by-one bug makes 
it compares with block (2016n - 2016) instead.

A naive but perfect fix is to require every block (2016x) to have a timestamp 
not smaller than that of its parent block. However, a chain-split would happen 
even without any attack, unless super-majority of miners are enforcing the new 
rules. This also involves mandatory upgrade of pool software (cf. pool software 
upgrade is not mandatory for segwit). The best way is to do it with something 
like BIP34, which also requires new pool software. 

We could have a weaker version of this, to require the timestamp of block 
(2016x) not smaller than its parent block by t-seconds, with 0 <= t <= 
infinity. With a bigger t, the fix is less effective but also less likely to 
cause intentional/unintentional split. Status quo is t = infinity.

Reducing the value of t is a softfork. The aim is to find a t which is 
small-enough-to-prohibit-time-wrap-attack but also big-enough-to-avoid-split. 
With t=86400 (one day), a time-wrap attacker may bring down the difficulty by 
about 1/14 = 7.1% per round. Unless new blocks were coming incredibly slow, the 
attacker needs to manipulate the MTP for at least 24 hours, or try to rewrite 
24 hours of history. Such scale of 51% attack is already above the 100-block 
coinbase maturity safety theshold and we are facing a much bigger problem.

With t=86400, a non-majority, opportunistic attacker may split the chain only 
if we have no new block for at least 24 - 2 = 22 hours (2-hours is the protocol 
limit for using a future timestamp) at the exact moment of retarget. That means 
no retarget is possible in the next 2016 blocks. Doing a time-wrap attack at 
this point is not quite interesting as the coin is probably already worthless. 
Again, this is a much bigger problem than the potential chain spilt. People 
will yell for a difficulty (and time wrap fix, maybe) hardfork to resuscitate 
the chain.


> On 21 Aug 2018, at 4:14 AM, Gregory Maxwell via bitcoin-dev 
> <> wrote:
> Since 2012 (IIRC) we've known that Bitcoin's non-overlapping
> difficulty calculation was vulnerable to gaming with inaccurate
> timestamps to massively increase the rate of block production beyond
> the system's intentional design. It can be fixed with a soft-fork that
> further constraints block timestamps, and a couple of proposals have
> been floated along these lines.
> I put a demonstration of timewarp early in the testnet3 chain to also
> let people test mitigations against that.  It pegs the difficulty way
> down and then churned out blocks at the maximum rate that the median
> time protocol rule allows.
> I, and I assume others, haven't put a big priority into fixing this
> vulnerability because it requires a majority hashrate and could easily
> be blocked if someone started using it.
> But there haven't been too many other network consensus rules going on
> right now, and I believe at least several of the proposals suggested
> are fully compatible with existing behaviour and only trigger in the
> presence of exceptional circumstances-- e.g. a timewarp attack.  So
> the risk of deploying these mitigations would be minimal.
> Before I dust off my old fix and perhaps prematurely cause fixation on
> a particular approach, I thought it would be useful to ask the list if
> anyone else was aware of a favourite backwards compatible timewarp fix
> proposal they wanted to point out.
> Cheers.
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> bitcoin-dev mailing list

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