OK guys, let's settle this and move on. That should have been done early on but confusion reigns. Digital is one thing, encryption is clearly another, apples and oranges all over again.
Systems like Tetra are simply digital, not encrypted. The reason why they can't be decoded is the encoding/decoding hardware is proprietary meaning scanner manufacturers would have to obtain a license for EACH one and use different hardware for each one, none are compatible. That would complicate things to the point that you couldn't afford to buy a scanner so equipped. Market economics are always the deciding factor.
As far as encryption goes, it's ILLEGAL to decode and monitor so no manufacturer in his right mind would produce a scanner so capable. It's been done with software and successfully prosecuted which put a quick end to the whole idea of cracking the codes. No arguments please, I had a front row seat and there were a few from Monitoring Times sitting in the gallery when it all went down. If somebody has a back issue oh, around 1999 you read how it all started and abruptly ended rather mysteriously. They never published the particulars, just slammed down the lid so don't expect a follow up story.
"P25 is not designed in such a way to enable operational encryption between systems and users in the way we are used on the internet, where encryption is seamless and accessible. It will take a leap of technology, cooperation and planning to make widespread full time encryption possible."
Er, yes and no. Public Key Encryption is what's used on the Internet much to the annoyance of the feds but that's another long and boring story. Ocean County NJ law enforcement already has an interop system and yes, it is encrypted. New Jersey has several and they're not so it all depends on the intended use. You seem to have the wrong concept of interoperability, they don't talk to each other directly, they use a common system. There's nothing insurmountable standing in the way of building wide area systems, linked systems or even a nationwide network, it's been done long ago by Amateur Radio only we're not allowed encryption. BTW two of ours are worldwide, imagine that.