Short Thoughts on Difficulty Calc

A short take

I found this text in EIP 2 of the Ethereum github repo. It’s from the justification section of the EIP. It explains some of the choices in the difficulty calculation code found here:

The difficulty adjustment change conclusively solves a problem that the Ethereum protocol saw two months ago where an excessive number of miners were mining blocks that contain a timestamp equal to parent_timestamp + 1; this skewed the block time distribution, and so the current block time algorithm, which targets a median of 13 seconds, continued to target the same median but the mean started increasing. If 51% of miners had started mining blocks in this way, the mean would have increased to infinity. The proposed new formula is roughly based on targeting the mean; one can prove that with the formula in use an average block time longer than 24 seconds is mathematically impossible in the long term.

The use of (block_timestamp - parent_timestamp) // 10 as the main input variable rather than the time difference directly serves to maintain the coarse-grained nature of the algorithm, preventing an excessive incentive to set the timestamp difference to exactly 1 in order to create a block that has slightly higher difficulty and that will thus be guaranteed to beat out any possible forks. The cap of -99 simply serves to ensure that the difficulty does not fall extremely far if two blocks happen to be very far apart in time due to a client security bug or other black-swan issue.

I put is here only so I can reference it from other articles in my Byzantium Thoughts series.

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