Board Thread:Theories and Speculation/@comment-34277398-20180522015234/@comment-734582-20180522121930

Quadraticequation wrote: So they never actually specified this? Nobody knew Obito existed, so the only person they knew to be powerful enough to summon Kurama, was Madara. Doesn't mean that Obito is as powerful as Madara, or that Madara left something specific for him behind. If you can tame a Biju with the sharingan, you can force them into a summoning contract.

I think it's fairly obvious that "powerful enough" entails both aspects: Powerful enough to put a Biju under genjutsu, and then powerful enough to summon them.