Boss key for Forest Temple?

[Kryptenx]

Damn I must be stupid, but where is the boss key for the Forest Temple? I have everything else done but the door is locked and I have no key.

 

[Kodae]

It's in one of the first rooms you went into. The one with the water and all the wood planks to walk on. There's a chest behind gates. You have to boomerang the windmills in a certain order (look at the floor) and it opens.