How do I Open the Red-Locked Doors?

Question

If you climb to the very top of Dustforce's hub world, you find a row of locked doors, and a scientist with the following message:

I wouldn't go in there if I were you.

How do I open these doors? I don't want to know what's behind them, so please don't post it in your answer.

Answer 1

You need to get a SS on a gold door level to get a red key.

An SS on a silver door gets you a gold key.

Spoiler withheld by request, but iirc there are 16 red door locks here ;)