On Mon, Jan 29, 2018 at 10:40 PM, Tier Nolan via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:

Much of Bitcoin operates on the assumption that a majority of miners are honest. If 50%+ of miners set their timestamp reasonably accurately (say within 10 mins), then the actual timestamp will move forward at the same rate as real time.

Thank you for replying. I agree that under the 50%+ assumption, timestamps are reasonably accurately, but I fail to see a reason to make this assumption.

I'm comfortable with the 50%+ assumption regarding ledger manipulation (double-spending, deletion of transactions etc.). I'm much less comfortable with it regarding timestamps manipulation.

Consider the following situation:

(1) miners are selfish,

(2) miners have a financial incentive to be dishonest.

(1) is a common state on how miners function nowadays. (2) is the case that interests us when coming to do this analysis.

In the case of ledger manipulation, the 50%+ assumption is not because we assume that miners are good-hearted (this violates (1)). It is there due to an assumption that the financial damage to a miner would be bigger than the gain in (2). This happens since a ledge manipulation may cause miners to lose block rewards, and certainly will devaluate Bitcoin, an asset which they possess.

In the case of timestamps manipulation, I don't see any financial damage caused to miners. Timestamps manipulation (besides the 2016*n blocks) won't harm the function of Bitcoin, and may even go undetected (it seems to me that the main blockchain explorers don't track it). I don't see a justification for the 50%+ assumption here.

Dishonest miners could set their timestamp as low as possible, but the median would move foward if more than half of the timestamps move forward.

If we want to be pedantic, the best lower bound for a block timestamp is the timestamp of the block that closes the adjustment interval in which it resides.

If you are assuming that the miners are majority dishonest, then they can set the limit to anything as long as they don't move it more than 2 hours into the future.

The miners could set their timestamps so that they increase 1 week fake time every 2 weeks real time and reject any blocks more than 2 hours ahead of their fake time. The difficulty would settle so that one block occurs every 20 mins.

Possible improvement:
-----------------------------
We may consider exchanging average with standard deviation in the difficulty adjustment formula. It both better mirrors changes in the hash power along the interval, and disables the option to manipulate timestamps without affecting the difficulty.

I'm aware that this change requires a hardfork, and won't happen any time soon. But does it make sense to add it to a potential future hard fork?

For check locktime, the median of the last 11 blocks is used as an improved indicator of what the actual real time is. Again, it assumes that a majority of the miners are honest.

_______________________________________________
bitcoin-dev mailing list
bitcoin-dev@lists.linuxfoundation.org
https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev

_______________________________________________
bitcoin-dev mailing list
bitcoin-dev@lists.linuxfoundation.org
https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev