I'm not sure if the implementation of the kata is just that bad ( because it is, really ), or if the idea is actually that bad.

You could try to redo the kata with a better, more complete, description ( and some markup ), another example of multiple keys with multiple nestings, and random tests. The 5 kyu estimated rank is a bit optimistic, but that'll sort itself out. I would actually like to be able to submit a solution. But you are going to need random tests. And you might get the issue "this has been done". But it's worth a shot IMO.

Did you read available documentation on creating your first kata?

No random tests.