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51% is not necessary. A case study in censorship though the pools has proven this: https://steemit.com/ethereum/@dhumphrey/f2pool-manipulates-usd1-2-million-o

• dlubarov 9y ago
  51% is necessary to create viable forks with arbitrary lengths. What that mining pool did was much less harmful: they just censored other parties' transactions during an ICO. Unless a coordinated majority is censoring your transactions, you can't really be censored, your transactions will just be slightly delayed sometimes. Which is normally very harmless. The only problem here was a poorly run ICO, where prices were dependent on the order of transactions. That wouldn't be the case with e.g. a reverse Dutch auction similar to the process for Treasury bills.