... and every nation state on the planet would turn their computers to cracking that key.
Are GCHQ that dense? Yes, yes they are.
No, they are not. The obvious thing to do would be to automatically generate and use a new keypair every time you intrude into a messaging or chat service. The public key is sent to the app, and you decrypt with the private key. Provided you decrypt and store the intercepted material as you get it, you won't need that keypair again.
I have no idea how the crypto on WhatsApp and other services works - but I'd be very surprised if it wasn't that way anyway. Maybe they go one step further and use the public key crypto to share symmetric session keys with the other parties - if that is still done nowadays, to reduce processing requirements (it's years since I looked at crypto) - but the scheme would be essentially the same.
Your nation states wouldn't try to crack any keys. They would try to work out how to intrude without detection - i.e. how to exploit the back door.
Biting the hand that feeds IT
