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Hehe, so sad D&T is not registered on this forum. Could anyone ask him the following:


Alice wants to attack the blockchain.
She owns private keys of 400 accounts totalling to 75% of the stake.
She is planning to rewrite the history from block 5'000.
Legit chain is at block 5'300 (less than 720).
Cumulative difficulty at block 5'000 is 8'000'000.
Cumulative difficulty at block 5'300 is 9'000'000.
How many SHA256 operations in average it's necessary to do to find a branch where cumulative difficulty at block 5'300 is at least 9'000'001?
Hint: Blocks from 5'000 to 5'300 were forged by 100% of the stake..