Great, thanks! I guess 'longest chain' hides some complexity, since it seems to imply height only. This totally makes sense though.

I know you can't fake the difficulty of a solved block, but I was suggesting that you could solve 2016 (low-difficulty-target) blocks in like 1 day, which would make the next block's difficulty target very high. So if 'longest chain' meant something like: 'height plus difficulty of the next block', then there'd be room for an attack. That's why I wanted clarification of what 'longest chain' really meant. But given what you said, this is impossible.

Thanks for the guidance.

The hardcoded checkpoints make sense, but could you give more detail about the difficulty being an important part of choosing the main chain? What's the formula? Also, couldn't the difficulty be faked?

If I recall, the difficulty is adjusted every two weeks. The attacker could time the attack to happen right at the changeover, and make it appear that the previous two weeks of their solo effort ramped up the difficulty significantly.

Thanks!

Apologies if this has already been brought up, or is otherwise a stupid question.

Could a rogue miner decide to branch off of the blockchain from a very early block (when the difficulty was very low - and keep it low), and solo mine a chain of huge length which would exceed the current chain's length and thus become the longest?