The Key of David
. . . and the government shall be upon his shoulders . . .
"Kepa/Cephas" are other names for "Peter."  
The Hebrew word form connotes a hollow gravel, barely a stone.
"Peter/Petros" means "rock"-- an amalgum larger than a stone, but connoting a piece of rock.
The "Rock/Petras" upon which the Church is built connotes a massive Boulder.
It's reasonable to understand that the Keys to the Kingdom consists of this understanding:
that a rather unremarkable stone can become a rock,
keeping in mind, always, that the Boulder is Boss upon the Mountain of Elohim.
Adam Kadmon