Hey Greg, I'm relatively new to Digital also but what I do know is that encryption is sometimes used on particular channels to prevent eavesdropping on confidential comms. It basically scrambles the modulation (voice) of any transmission to a handset that isn't on the encrypted net thus making the sound, sound a little bit like 'Donald Duck'. It's impossible to understand an encrypted conversation and I believe impossible to decode.
What I have heard on the grape-vine however is that most Shopwatch/ City Watch don't use the encrypted option on there handsets although this may be subject to area and not necasariky true as I'm yet to buy a capable handheld and find out myself.
In my particular area there is a major shopping complex where the control room uses DMR for all Security however the individual shops all have radios on an analogue channel (probably cause the handhelds are cheaper). So, if say a store security guard is calling the control room to request CCTV, I can hear that conversation but when the control room guides the complex security in, they use a DMR channel which I can't hear.
If your in the U.K and want to listen to the Police say, that to my understanding, and illegal in any case, is impossible so if your spending hours searching on the net for ways how, your better off investing your time on something else.
Hopefully the above helps, remember I'm only a beginner too but I hope I've managed to answer some of your questions.