It seems that bitcoin doesn't have limits in the difficulty adjustment, but I am pretty sure Monero does. If your black box checks these limits, that sets up a

It seems that bitcoin doesn't have limits in the difficulty adjustment, but I am pretty sure Monero does. If your black box checks these limits, that sets up a hard limit on how low you can get the difficulty. Which means that (over sufficiently short times) you'd still need a significant fraction of the publicly available hashing power to create such a forged chain. As an alternative, you could set a limit of 'at least N blocks, and at least X accumulated difficulty'. This way any forks with reduced hashing power wouldn't reach the accumulated difficulty requirement. Edit: I was wrong, bitcoin has a maximum difficulty factor of 4. So you could decrease the difficulty by 4 every 2 weeks.