First time auth on new hardware would certainly ask for at least a PIN but more likely your iCloud password. For the encryption keys to transfer I think you would also have to allow the request from some other device which currently held them.

The Secure Element would need to be upgraded to support "multi-tenancy".

That's all assuming the feature works by building on the existing infrastructure.

Ideally between your face and your iCloud password you could bring your "profile" to new hardware without having to touch an existing device. After the first time, just your face is enough.

Meanwhile the front door lock on my house built a couple of years ago only has 10,000 different keys. They probably sold more locks than that in the last few months.

A four-digits pin code has 1:10000 chances to get into someone else's phone (on first try).

Read the GP comment. This is talking about walking up to any random phone and having your own environment synced to it seamlessly. For that to work, the phone would have to know it’s you with basically perfect precision. So 1 in a million is not nearly good enough for that use case.

But you can only enter a pin on a phone you have physically with you. With this proposed auto-sync, every new phone is potentially a gateway to anyone else's phone.

Not that it's much better, but I thought iOS now required 6 digits going forward?