Spain are the only international side to defend the international tournament since its inception in 1960, having lifted the trophy in 2008 and 2012.
The Porto goalkeeper will contend with Manchester United's David de Gea for the No. 1 jersey at next summer's competition and remains hopeful of his country's chances of success.
"It would be extraordinary to lift a third trophy," he said. "It is difficult but not impossible."
Casillas, 34, believes hosts France are well equipped to end the tournament as victors but remains wary of the talents of other top nations.
"France I like," he said. "You have to respect Italy. Germany are world champions! Croatia can be a surprise."