[gizmo]

I agree this isn't a backdoor. Facebook is already a Trusted Third Party with the responsibility to deliver an honest closed source client binary, as are Apple and Google for delivering the binary unaltered to the end users.

This backdoor cannot be exploited by third parties, only by Facebook themselves, who already have much easier ways to intercept or manipulate communication. So although I don't think Whatsapp makes the right trade-off here (people get a new phone only once every few years, so why optimize for that edge case?), I'm not concerned about the privacy implications either.

I suspect other commenters here are confused about the nature of the Signal protocol, and who you have to trust for the system to be secure. If you used to believe that Facebook is 100% unable to intercept or tamper with Whatsapp communication, then this would be upsetting. But since they're a trusted party already, this changes nothing.