On Thu, Mar 29, 2018 at 05:50:30PM -0700, Jim Posen via bitcoin-dev wrote:
> Taken a step further though, I'm really interested in treating the checkpoints
> as commitments to chain work [...]

In that case, shouldn't the checkpoints just be every 2016 blocks and
include the corresponding bits value for that set of blocks?

That way every node commits to (approximately) how much work their entire
chain has by sending something like 10kB of data (currently), and you
could verify the deltas in each node's chain's target by downloading the
2016 headers between those checkpoints (~80kB with the proposed compact
encoding?) and checking the timestamps and proof of work match both the
old target and the new target from adjacent checkpoints.

(That probably still works fine even if there's a hardfork that allows
difficulty to adjust more frequently: a bits value at block n*2016 will
still enforce *some* lower limit on how much work blocks n*2016+{1..2016}
will have to contribute; so will still allow you to estimate how much work
will have been done, it may just be less precise than the estimate you could
generate now)

Cheers,
aj