||"Kepa/Cephas" are biblical names for "Peter."The Hebrew word connotes a hollow gravel, barely a stone.
"Peter/Petros" means "rock"-- an amalgum larger than a stone,but connoting a sizeable piece of rock.
The "Rock/Petras" upon which the Church is built connotes a massive Boulder.
It is reasonable to understand, then, that the "Keys to the Kingdom" points to this understanding:
A rather unremarkable stone can become a rock,
when remembering that the Boulder is Boss upon the holy, invisible Mountain of Elohim.