But "on update" isn't really the issue. If the code can be swapped underneath it, how does it know an "update" took place? Again you're in the situation where all of that process would have to be managed by hardware, when what is really happening is that the enclave is a computer running signed software that can be replaced.
Sure, but the signed firmware could be written to delete any stored keys when it accepts an update and the phone's passphrase has not been provided. That's assuming that it manages it's own update process, has the firmware securely stored within it's own die, etc. It's entirely possible ... but only Apple really knows.
I would not actually be shocked if they originally did wipe out stored info on firmware update, but had some issues with people updating their phone and losing everything, so they ifdef'd that particular bit out in the name of usability.
But... the firmware is stored in external storage. How does it even detect that an update was performed? You're saying that if the hardware had affirmative control over all the secure storage (e.g. the RPMB partitions on eMMC, etc...), then it could have been written to do this blanking.
Well, yeah. But then you'd have a system that couldn't be updated in the field without destroying the customer data (i.e. you'd have a secure boot implementation that couldn't receive bug fixes at all).
It's a chicken and egg problem. You're handling the problem of the "iPhone" not being a 100% snoop-and-tamper-proof device by positing the existence of an "interior" snoop-and-tamper-proof device. But that doesn't work, because it's turtles all the way down.
Ultimately you have to get to a situation where there is a piece of hardware (hardware on a single chip, even) making this determination in a way that has to be 100% right from the instant the devices go out the door. And that's not impossible, but it's asking too much, sorry. We're never going to get that.
It's certainly possible to design such a system with external firmware and still allow for secure updates.
The enclave would store (in secured storage) a hash of the last used firmware. Hardware would have a hash update capability, but this destroys all other stored information (i.e., keys) if used when the enclave is not currently in an unlocked state.
On boot, hardware verifies firmware signature as usual but also compares the firmware hash (already calculated for the signature check) to the stored value. If there is a mismatch, update the stored hash. Since the enclave is currently locked, the hardware clears the keys.
Since it's in hardware, you're correct that it would have to be 100% right, but that's quite feasible for a simple update mechanism (indeed, the most complicated bits are reused pieces from the signature check which already has this requirement).
Have it store the firmware itself encrypted with the UID. It never leaves the secure enclave so only the secure enclave itself could "sign" updates. You could still allow for recovery by providing a method to reset the UID.
