'Key' in the Bible

And to the angel of the church in Philadelphia say: These things says he who is holy, he who is true, he who has the key of David, opening the door so that it may be shut by no one, and shutting it so that it may be open to no one.

And at the sounding of the fifth angel I saw a star falling from heaven to the earth: and there was given to him the key of the great deep.

And I saw an angel coming down out of heaven, having the key of the great deep and a great chain in his hand.