60 | +This rule enables CPU mining on testnet but has led to block storms<ref>https://blog.lopp.net/the-block-storms-of-bitcoins-testnet/</ref> which rule #2 below addresses.
61 |
62 | -Block storms happen organically whenever the 20-minute exception is applied to a difficulty period’s last block, causing the block to be mined at a difficulty of 1. The difficulty adjustment rules then limit the subsequent period’s difficulty to a value between 1 (the minimum) and 4. Blocks will be generated rapidly in the subsequent low-difficulty periods while the difficulty climbs back to an adequate range. An arbitrarily large number of blocks can be generated quickly by repeatedly using the 20-minute exception on every last block of difficulty periods. The block storm is then bounded only by miner hash rate, the need for last blocks to have a timestamp 20 minutes after the second to last block, the Median-Time-Past nTime rule, and the requirement that blocks can't be more than 2 hours in the future. Overall a sustained attack would eventually be limited to a maximum cadence of six blocks per second.
63 | +=== 2. Block Storm Fix ===
64 |
65 | -A block storm does not require a time warp attack, but one can be used to amplify<ref>A perpetual block storm attack with entire difficulty periods being authored in less than 3.5 days that resets the difficulty to the minimum in the last block of every difficulty period would adjust to a new actual difficulty of 4 every period. An attacker that additionally leverages a time warp attack would start their attack by holding back timestamps until the latest block’s timestamp is at least two weeks in the past, and then limiting their block rate to six blocks per second, incrementing the timestamp on every sixth block. Only on the last block they would use the current time, which both resets the difficulty to one per the 20-minute exception and would result in a difficulty adjustment keeping the difficulty at the minimum due to the elapsed time exceeding the target. This would allow lower the difficulty for all blocks to difficulty 1 instead of difficulty 4</ref> it.
It seems to me that some of the description of block storms was dropped in the rewrite. Perhaps it could be moved to a footnote or rationale, for readers that are not already familiar with the concept of block storms rather than requiring them to read Lopp’s article.
Yeah, that would be good. As far as I can remember this was requested by reviewers of the original bip.
