'Key' in the Bible

"To the angel of the church in Philadelphia write the following: "This is the solemn pronouncement of the Holy One, the True One, who holds the key of David, who opens doors no one can shut, and shuts doors no one can open:

Then the fifth angel blew his trumpet, and I saw a star that had fallen from the sky to the earth, and he was given the key to the shaft of the abyss.

Then I saw an angel descending from heaven, holding in his hand the key to the abyss and a huge chain.