natters,
bmerr here. In Matt 16:18-19, Jesus uses the words "kingdom" and "church" interchangeably. It would have been a bit unreasonable for Jesus to speak of building one thing, and then promise to give Peter the keys to something else.
Since the kingdom, or church, came on Pentecost of Acts 2, one might go there to see what the keys given to Peter were.
Of course, Peter's sermon is recorded for us in Acts 2. This is the first gospel sermon ever preached. It is at this point that the kingdom of God was opened up under the authority of Jesus Christ. Those who believed were told to "Repent, and be baptized every one of you in the name of Jesus Christ for the remission of sins, and ye shall receive the gift of the Holy Ghost."
I'd say that the keys to the kingdom included faith, repentance, and baptism for the remission of sins. These are the terms, or conditions, of entrance to the church, or kingdom, of Christ.
Also, as you mentioned, apostolic authority was given to bind or loose on earth what had been bound or loosed in heaven. The KJV kind of reads backwards in Matt 16:19, from what I've read.
In Christ,
bmerr