Does anyone have a moment to explain this one to me? Seems to me you'd of course have to store your own key on the device. And if Apple is storing it themselves... that's news to me and pretty concerning.
In a no-trust model once you sign in on a new device you wouldn't see all of your old messages. Because all of your old messages are decrypted and sent to the new device this is evidence that a new signing key was added to the communication chain.
Apple could also do this to decrypt your messages to give to a government agency or for whatever purposes they want without notifying you.
https://techcrunch.com/2014/02/27/apple-explains-exactly-how...
Page 99 here. https://manuals.info.apple.com/MANUALS/1000/MA1902/en_US/app...
Which describes that the private keys are generated and held on device.
Now I think this model *may change when you enable icloud messaging, in which case an encrypted messaging key may be stored in your icloud account. So you may opt to have apple store it, but in an encrypted manner that they can't undo. This part is a bit speculative on my part though, so grain of salt.