Promising the impossible?

A couple of weeks ago in the aftermath of the Charlie Hebdo massacre, the Prime Minister announced that, if the Conservatives won the forthcoming election, he would immediately introduce legislation giving the intelligence agencies power to break into and read any electronic communication between suspected terrorists. Problem is, he didn't say - and didn't really seem to understand that there was a need to say - how this was to be done.  I am surprised to find that I have not posted before about cryptography, which is somewhat of a hobby of mine, but the fact is that the majority, if not all of such communications will be encrypted and probably in such a way that they literally cannot be decrypted.  How such a situation has come about is a long story, and we'll go through the history a step at a time in a series of posts under the title "It's a secret".  So watch out for those.