From the Talmud:
In rabbinic literature, giving keys symbolized the granting of authority. Rabbi Akiva stated that God would give the angels Michael and Gabriel the keys to open the 40,000 gates to hell. Another Jewish belief held that God held four keys in his hand: the keys to rain, conception, resuscitation of the dead, and crops.
(Fascinating Bible Facts, David Howard, Jr., Ph.D. and Gary Burge, Ph.D.)
And the fifth angel sounded, and I saw a star fall from heaven unto the earth and to him was given the key of the bottomless pit. And he opened the bottomless pit; and there arose a smoke out of the pit, as the smoke of a great furnace; and the sun and the air were darkened by reason of the smoke of the pit. And there came out of the smoke locusts upon the earth: and unto them was given power, as the scorpions of the earth have power. -- Revelation 9:1-3
And I saw an angel come down from heaven, having the key of the bottomless pit and a great chain in his hand. And he laid hold on the dragon, that old serpent, which is the Devil, and Satan, and bound him a thousand years.--Revelation 20:1,2