Re: perhaps itself encrypted with a key known only to law enforcement
My dear Streaky, PGP is very much a thing, You should google it.
I think we're at cross purposes here. "A weakness added by technical means" is wordplay and not helpful to this discussion.
Clearly you are upset at the concept of law enforcement having access to comms that you feel should be encrypted for ever until the end of time. That's not unreasonable, but I'm not interested in legislative or emotional arguments. Yes, people will leave a messaging platform that does this. I already made that point in my first post.
I'll restate my point for clarity. Encrypted communication between two devices could be "backdoored" for law-enforcement without making it easier for a third-party who snoops on the traffic to decrypt. The argument levelled against "backdooring" is that it opens the door for everyone, not just law enforcement, and I am saying that is simply not the case here.
As I'm clearly playing devils advocate, here's how I would construct the system.
Law enforcement generate a keypair and send the public key to Whatsapp, and keep the private key in safe. WhatsApp generate a keypair, and use the public key as I've described. They encrypt the private key with law-enforcement's public key, print it out and put it in a safe, then delete the "plaintext" private key. Or, if you prefer, store parts of the printout in multiple safes in multiple jurisdictions, including bank vaults.
Now to decrypt any communications you need the private key of law enforcement (in their safe), the encrypted comms (on WhatsApps servers) and access to the safes in WhatsApp's offices, which they're only going to open with a court order. It's safe from NSA hacking, it's safe from NSA and Law enforcement acting together, it's safe from WhatsApp acting on their own.
Of course no system is impenetrable, but if you think this system (if implemented as described) is vulnerable then please tell me how you would do it, either as an over-zealous government, a corrupt law-enforcement official or a third party. Facts please, not hyperbole.