Also, as I understand it, on certain radio systems, each radio must have a unique radio-ID which would be assigned by and detected by the radio system's control-computer. Radios with unrecognized or unapproved IDs, or radios that are considered stolen, or radios that are malfunctioning, can be remotely disabled by the communications center. Some models that require a unique radio ID also indicate their GPS location on the communication-center's monitoring station.
So, even if the encryption key was known, the individual radio would still need administrative and operational approval to join and remain on the system. That is why in some places, law enforcement agencies "loan/assign" radios to approved news reporters, since the news reporters cannot listen without an approved radio.
Hope this helps,
Last edited by QDP2012; 02-09-2013 at 11:18 PM..