> there would have to be some process the result of which is the ability to decrypt any encrypted disk. Fiddling with the internals doesn't change the nature of it.

Yes, that process is called allowing authorized users to access the data. The owner is an authorized user, and there's a process for him/her to access the data on the phone. A cop with a valid warrant is just as legally authorized to access that data as the user is.

> But under your system the foreign government can keep a copy of everyone's device until they sufficiently infiltrate the US government and then decrypt all the years of data they've been collecting

Then make it more time consuming and cumbersome break the key escrow than it is to just break the user's password. There's all sort of things you can do: airgap all of the keys; move them to a different airgapped system after a couple of years; archive them to an encrypted tape after a few more years; generate multiple keys for each time period, allow the device manufacturer to choose one at random, then store the private keys at different locations. Imagine how happy China will be if they spent 15 years trying to break into the systems at FBI Washington only to discover that the key they were after is stored on an encrypted backup tape at FBI Boston.

At this point I have to stop and ask how much protection are we devoting to the task? Technically, no security system stops a determined adversary - it just slows them down. The idea is to either make the enemy have to expend more time and resources to get their data than they are actually willing to expend, or slow them down long enough to be caught. I used to work for the military in a secure facility a little over a decade ago. We kept our sensitive documents locked in safes. Safes are rated based on how long it would take trained safecracker to break into them. The safes we used were rated at around 45 minutes each - and they were behind a thick vault door rated at about an hour. Why was this acceptable? Because our sensitive documents were all behind that door and split between multiple safes, security walked by the facility every half hour and it took less than two minutes for a large group of armed guards to get there.

While we want to slow down our adversary, at same time an authorized user needs to be able to get to the data when they need it. The use case scenario for the device owner is different for a cop with a warrant. It's acceptable to have a lengthy, somewhat cumbersome process for the police with more security in place, since a cop doesn't need to gain access to a user's phone multiple times a day - they'll generally never have to access the contents, and if they do it's probably only going to be once. An iPhone user isn't going to tolerate getting cryptographically secure sign-offs stating that they are authorized to access the device from multiple secured facilities every time they turn on the device. A cop looking into a serious criminal investigation will.

Now I'd ask who it is that we're designing the system to protect against? How determined and well funded are they? Am I designing a system to protect the user from someone willing to devote a nation state's resources to breaking it, or am I designing a system that will protect a user from data theft by criminals? Apple's existing encryption system won't stop them from getting your data if they really want it - especially when you are physically in China. Walking around with a cell phone in your pocket is itself a huge security vulnerability. If we redesign the iPhone to be a computing device that's indefinitely secure against Chinese intelligence services, it ceases to be a cell phone and instead becomes a standalone computer sitting under armed guard in the basement of the Pentagon.

If you're travelling through China with sensitive data that the government wants, perhaps you should reevaluate storing it on your cell phone.