I'm a student studying Electrical and Computer Engineering and a ham radio operator (i.e. my knowledge of encryption is basic). More and more, police and local government radio has become encrypted, this obviously prevents the average police scanner user from tapping in, but what about a better resourced organization?
A lot of systems use either 56-bit DES or a proprietary 32-bit motorola system to encrypt the audio stream which is then transmitted over a digital protocol. From what I can dig up, the systems use a fixed key, but can be over-the-air rekeyed daily, (based on my experience I doubt most organizations use the feature).
As someone familiar with modern encryption the 32 and 56 bit keys seems far short of the 128-512 bit systems available today. IIRC, DES was broken in the late 90s by the EFF.
So perhaps there's some "size doesn't matter (as much)" reason. Otherwise, is it that these systems are vulnerable but not worth anyone's time (Many police agencies still transmit analog voice and don't have a scanner related crime problem).
Here are some more specific references: http://www.batlabs.com/encrypt.html http://wiki.radioreference.com/index.php/Motorola_Encryption_Topics http://www.freepatentsonline.com/3639690.pdf http://www.freepatentsonline.com/4167700.pdf