Why does the Vatican’s flag have keys on it and why is it colored yellow and white?

The flag of Vatican City is one of very few square flags in use.
It consists of two vertical rectangles, yellow on the left, white on the right, so as to symbolise the colors of Saint-Peter’s keys (gold and silver).
The Papal symbol is placed at the center of the white rectangle.
It represents the Papal Tiara, along with Saint Peter’s crossed keys and a red cord connecting the keys.