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Bitcoin ABC (BCHA) Deep Reorganisation incident 29 March 2021

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Avatar for BigBlockIfTrue
Written by   70
1 year ago

On 29 March 2021, Bitcoin ABC experienced a deep reorg. The following is a reconstruction of events, based on blockchain data.

Due to its low price compared to other bitcoin forks, the BCHA network has a rather low hashrate compared to the overall double-SHA256 mining market. A large majority of hashrate is controlled by one single pool: ZULUPooL. ZULUPooL is a merge-mining pool, which mines BCHA simultaneously with Hathor.

Like many blocks, block 679554 was merged-mined by ZULUPooL. However, from block 679555 onward, it appears that ZULUPooL had trouble with their merged-mining setup, and switched to an emergency non-merged-mining mode where it would just mine BCHA without mining Hathor. In this emergency mode, no transactions were included in the BCHA blocks, other than the coinbase transaction.

After a string of 13 empty blocks, all mined by ZULUPooL, ZULUPooL mined a second block at height 679555. Apparently their normal mining setup worked again: this block was merged-mined with Hathor and contains 15 BCHA transactions. The new block 679555 then got three confirmations from Mining-Dutch and one from ViaBTC. ZULUPooL then shifted more hashrate from the old chain to the new chain, even though it also kept mining the old chain in the supposed emergency mode.

Mining on the old chain completely stopped after 457 post-split blocks, so eventually the new chain is going to overtake the old chain in both block height and proof-of-work.

There was some speculation that the split was caused by ABC's reorg protection mechanism escalating a block race into an accidental chain split, but this is clearly not what happened. Instead, miners collectively decided to abandon a dozen blocks (which later became hundreds), for reasons we can only speculate. A possible reason is a community dislike of long strings of empty blocks, which led to a similar decision to perform a deep reorg in November 2020.

In conclusion, the reorg protection features worked as planned: they rejected the new chain replacing the old chain. However, reorg protection is obviously not going to be helpful if in fact BCHA's community repeatedly desires and approves deep reorgs.

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Written by   70
1 year ago
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Comments

A deep reorg that was approved by every miner on the network? You sure as hell don't see that often.

$ 0.01
1 year ago

Nice investigation. Great to have a play-by-play report.

$ 0.01
1 year ago

These kinds of malicious or "accidentally malicious" attacks on ABC and BCH have been happening for months. Both chains are vulnerable and have been fighting them off or letting them harm the user experience. It is sad that it is happening. BCHA (may be rebranding to "eCash") has a plan I think will solve it (an avalanche-type code upgrade) sometime soon. Fingers crossed...

$ 0.00
1 year ago

BCH has had a deep reorg? That's new to me. Can you show me some chainsplits because my Electron cash has not shown a single one. Any chainsplits on BCH longer then 2 or 3 blocks? Cause natural orphans of 1 or 2 blocks happen once in a while.

$ 0.01
1 year ago

I did not say there was one. I was replying to the article above that suggests it happened. Something did happen, but I did not follow the details of what. That said, chainsplits only last long enough for me to notice them if two chains have ongoing mining support. Maybe you are more attentive to the day to day activities and still did not see anything?

$ 0.00
1 year ago

if in fact BCHA's community repeatedly desires and approves deep reorgs

That seems unlikely to be what's going on. If I am right, you probably know that. Maybe I am misunderstanding something about this seemingly (to me) disingenuous comment.

$ 0.00
1 year ago

At least the miners decided this (ZULUPooL, Mining-Dutch, ViaBTC). I don't understand why, it seems like a stupid decision. The community then went along with the decision.

$ 0.00
1 year ago