I think it's safe to conclude that no fighter plane radios were encrypted, due to requiring extremely bulky equipment at the time. Communication between enemy fighters was theoretically possible, since all you need to do is tune in to the enemy's frequency, but most planes could only use a very limited set of preset frequencies. Of course this does mean planes with radio operators and ground stations could tune in and capture intelligence such as the enemy's heading and squadron sizes, attempt to jam the frequency, or (in the case of navigation radars) even misdirect the enemy's target.
Even extremely simple countermeasures like frequency hopping were not possible, since these require computing technology that was simply unavailable at the time. Instead, fighters employed techniques such as radio silence, codenames and frequently-changed callsigns, which last to this day.
More specifically, in the Spitfire's case, one radio it used was the TR1133 which could only have 4 preset frequencies operated via push buttons. Fighter pilots probably had more on their plate to worry about than attempt to find the enemy's frequency.
Additional reading here: http://forum.axishistory.com/viewtopic.php?t=188945