Well, this looks good for iPhone users, at least.

Small point of contention:

The Signal messaging service, for example, includes the ability to transfer an account to a new device in a way that avoids the need for manual key re-verification, but if Alice signs in to a new device without using this mechanism, the participants in all of her conversations are warned that their “safety number with Alice has changed,” even if they had never manually verified that safety number with Alice.

I think getting notified that someone has switched to a different device, even if you had not previously verified their keys, is potentially valuable. I’m not sure why Apple says this as if it has no value.

Signal is also not a great comparison, because the entire system has a single point of truth, the user’s current phone. Everything else, such as the desktop and iPad app, run through the phone as the primary source of verification.

A better comparison would be with Matrix, and all things considered, Matrix has needed a massive overhaul in terms of device verification for a long time. If Apple implements this, it’ll probably blow Matrix out of the water in terms of usability and lack of frustration.

Of course I will never be able to tell, because like signal, you require a mobile phone to actually use iMessage… and the phone must be an Apple one, of course. Unless somebody hands me an iPhone for free, I probably won’t ever check in on this.

Update: the “automatic verification” Apple proposes looks like it works, from a thousand foot view, the same way unverified Signal contacts do.

Alice is notified about any self-verification failures with a notification, a badged Settings app icon, and an explanation of the error in the Contact Key Verification pane of Apple ID settings.

Except Signal does it by default… and it looks like iOS requires opt-in.

When a user enables iMessage Contact Key Verification…

Opt-in on both sides of every conversation for maximum functionality.

If Alice’s device detects a validation error, and both she and Bob have enabled iMessage Contact Key Verification, Alice is notified about the error directly in the Messages conversation transcript.