The Key to the City of St. Canard is an honorary symbol of St. Canard. The city eagerly gives it to heroes, but only the ones with a positive image. It might be there are actually two keys.
At least twice has the Key to the City of St. Canard been handed to a hero: once to Gizmoduck and once to Darkwing Duck. Gizmoduck received a golden key roughly the span of his shoulders with a round head, three thicker areas, and a gate-shaped foot. The same model key was given to Darkwing at another time, but that key is of usuable size and, indeed, has the ability to open all the jail cells in the St. Canard Police Station. Whether this means there are two keys or the large one was replaced by the small one is unclear.
Gizmoduck is given the Key to the City of St. Canard as a way of welcoming Duckburg's hero to St. Canard. Darkwing Duck doesn't get the Key to the City until he improves his image from a hero in the shadows to a hero handing out flowers. Negaduck, irritated by Darkwing's new sugary angle, steals the key from him and uses it to free all criminals locked up in jail. He has them ransack the city, then hands the key back to Darkwing to frame him for the chaos. Darkwing has to flee from the angry citizens and in the process bumps into the mayor, to whom he returns the key before continuing his escape.