'Key' in the Bible

'And to the messenger of the assembly in Philadelphia write: These things saith he who is holy, he who is true, he who is having the key of David, he who is opening and no one doth shut, and he shutteth and no one doth open!

And the fifth messenger did sound, and I saw a star out of the heaven having fallen to the earth, and there was given to it the key of the pit of the abyss,

And I saw a messenger coming down out of the heaven, having the key of the abyss, and a great chain over his hand,