What about apps that keep the encryption keys in the user phones and can't decrypt the contents of messages? Do they intend to ban those apps?
My idea was that rather than ban it outright and make it like most other internet traffic (decrypted on a server), they could mandate some kind of cacheing of the _encrypted_ data on the server. Then, once they'd lawfully executed a warrant to grab the suspects phone, they can decrypt at their leisure. Apps using per-message keys will probably be a no-no.
Myself - I live in the UK - I hope Apple, Google and Facebook all say a big "fuck you" and cut this country off from their services, but I have a feeling the 'compromise' is going to be something like the paragraph above.
The world is run by technology companies now. Not governments. Governments haven't caught on to this yet, at least officially.
But the bottom line about law is that the law is whatever Google, Apple, Facebook, Microsoft, and Amazon says it is.
That's not the most comforting thought in the world, but that's how it is. And it's sort of okay for now because all of those companies are currently run by idealists. God help us when they are not.
For right now though, there isn't a chance in hell that Cameron's ideas will have any traction. He's just saying things that will win him some support from a certain segment of the population.