The Key of David as in Rev 3:7 "The words of the holy one, the true one who has the key of David, who opens and no one shall shut, who shuts and no one opens." is Jesus - Jesus is pictured as the one who holds the "key of David" which is earlier referred to as "the keys of Death and hades" (Rev 1:18). This "key of David" is mentioned in Is 22:22 where Eliakim replaces Shebna as the leading minister of David's kingdom.
In Isaiah the "key" of the kingdom is a symbol of the authority of David given to the minister/steward who acts with the authority of the king. "He shall open and none shall shut; he shall shut, and none shall open"
Jesus makes further reference to this when HE calls himself "the one who opens and no one shall shut, who shuts and no one opens."
Jesus holds the Key as He is the King in the line of David.
As we see, Jesus as the King, gives this authority to Peter, making Peter the steward/prime minister of God's Kingdom I will give you the keys of the kingdom of heaven, and whatever you bind on earth shall be bound in heaven, and whatever you loose on earth shall be loosed in heaven" Mt 16:19 -- Peter is the steward of the authority of Jesus, God who is the true holder of the Keys as we read in Rev 3:7
Not 'even' Peter held the 'key of David'... I have read what Peter penned and it is shocking how much of what he penned has evolved into traditions of whomever got a whim.
Jesus holds the Key as He is the King in the line of David. Stumbling all over this ... 'line of David' means just that 'blood line'... without that knowledge one will worship just about anything or anybody.