A direct reference back to the keys of a davidic kingdom,
Isaiah 22 (from memory) which are the symbol of an office, as steward, in charge whilst the king was away.
The jews were obsessive about their own history, even more obsessive about the old testament and deriving meanings from it, so this meaning of keys will clearly have occurred to them, not least because Jesus was heralded as a davidic king, and the messiah would come from that line.
Trouble is, protestants don't like the obvious conclusion from that argument, even though Luther and Calvin believed it!
Isaiah 22:22
And the key of the house of David will I lay upon his shoulder; so he shall open, and none shall shut; and he shall shut, and none shall open.
Isaiah 9:6
For unto us a child is born, unto us a son is given: and the government shall be upon his shoulder: and his name shall be called Wonderful, Counsellor, The mighty God, The everlasting Father, The Prince of Peace.
Revelation 3:7
And to the angel of the church in Philadelphia write; These things saith he that is holy, he that is true, he that hath the key of David, he that openeth, and no man shutteth; and shutteth, and no man openeth;