[bitcoin-dev] Getting around to fixing the timewarp attack.

[Gregory Maxwell <greg@xiph.org>]

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.