I think there’s a fundamental that everyone misses. If you get to the raw message that a successful decryption of an alleged terrorist communication exposes, does it provide any value in practice? Probably a question only our friends in the security services, based on their operational experience, could answer authoritively.
If I were passing over a nefarious intent in a message, I’d imagine it would go along the lines (as one example) like:
“The baby giraffe is walking down the tarmac path in the rain”.
Which, in the grand scheme of things, is about as much use as a cow on stilts.
I suspect the true actionable value is in the time, source and destination metadata, not in the content – and the definitive network of connections between people that surfaces from any analysis of these over time. I suspect the content – outside the most stupid of terrorists – is of no actionable value, but where the associated metadata is already available today with no further work needed.
I’d invite the Home Secretary and her advisors to form policy based on what GCHQ or her technical staff advise, in terms of how many cases they could solve faster or at all in their experience, if her wildest decryption dreams could be enacted. I suspect the ROI of banning or subverting end to end encryption would be, in practice, show it to be a completely useless endeavour.