Where do you find keys in dustforce?

Question

Half the levels are locked when you start the game. In order to unlock them, you have to find keys. But I've completed every level around the whole map, and still can't find any keys.

I've found keys lying around, but those are just pushed into doors automatically, so they aren't used to unlock levels.

Answer 1

According to this (emphasis mine):

Interestingly enough more than half the levels present within the Dustforce universe are initially locked. To unlock these levels players are required to earn keys. How do you earn keys? Well it’s simple enough – all Dustforce asks is that you complete each level to perfection … and look good while doing it. At the end of each level Completion and Finesse grades are rewarded (ranging F through S): Completion refers to how much dirt has been collected, while Finesse refers to the player’s ability to keep their combo intact while traversing the level. To receive a key, the coveted SS grade is required; meaning that players must not only collect all the trash in the level, but do so without getting struck by enemies in a timely manner to keep their combos intact.