King Pelias has obtained the throne of Thessaly by killing Jason’s father, Jason is born and secretly sent to exile.
Pelias, consults the Oracle and is warned to beware of a man with one sandal.
Later at a series of games that Pelias holds Jason turns up to claim his place as the rightful king. He is wearing only one sandal, after an encounter he Hera, Pelias seeing this sends him on the quest for the Golden Fleece, which Jason accepts and completes, returning to claim his place as King. The story then gets more complicated at this point!
This seems like a good analogy for getting and encrypted key and using it to send a message (in this case the quest for the fleece) and then also for the return message, when Jason comes back to claim the throne.
Not quite a 100% analogy but I am sure it could be made to fit?