I am not a cryptographer, but the standard objection to this is that the NSA key will leak, either generally or be stolen by a Russian/Chinese agent.
And in implementation, how many keys are we talking about? USA, UK, France, Germany, USA, Australia... Every country's law enforcement will demand a key, and how long will that remain secure?
The best argument is simply that you cannot ban e2e encryption because there’s thousands of people who are able to implement it all over the world. Banning E2E just means that everybody who cares about privacy (including the “bad guys” and privacy conscious users) will switch to a banned implementation, and everybody else will have their privacy put at risk for no reason at all.
Devil's advocate: the answer to that is that perfect is the enemy of good. Most "bad guys" are pretty dumb and won't bother using actually secure communication channels especially if messengers keep advertising that they do end-to-end encryption. And even for those who do care enough, most of them aren't all that tech-savvy and will make mistakes.
All that to say that a ban doesn't have to be 100% effective to make a meaningful difference.
> especially if messengers keep advertising that they do end-to-end encryption
That's probably a crime in UK. It is a crime in plenty of countries.
Anyway, the most impactful an anti-e2e law can be is to force people into getting some functional thing from free-droid, instead of naively getting it from the play store. The bar of intelligence required for that is still pretty low.
Firstly, you can just rotate the key if that happens. It's one software update away.
Secondly, protecting keys isn't that hard. That's what HSMs are for. Not only have no secret keys ever leaked from the NSA as far as I know, not even when insiders turned against them and leaked as much as they could, but this isn't a noteworthy achievement either.
Indeed, if the Chinese demand a key under that scheme it is hard to see how the data will be kept secure against the Chinese spy agencies. And they will demand, the system is there and obviously available.
Plus, who would be stupid enough to use that protocol? It is sending bright flashing messages saying "we're reading your emails, mate!". Only people who were legally compelled to use WhatsApp would be reachable, everyone else would more to some other system.
