this post was submitted on 09 Nov 2024
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Why does rebooting it improve the safety or security of the phone?
When you first boot up a device, most data on that device is encrypted. This is the Before First Unlock (BFU) state. In order to access any of that data, someone must enter the passcode. The Secure Enclave uses it to recreate the decryption keys that allow the device to access that encrypted data. Biometrics like Face ID and Touch ID won’t work: they can’t be used to recreate the encryption keys.
Once you unlock the device by entering the passcode the device generates the encryption keys and uses them to access the data. It keeps those keys in memory. If it didn’t, you’d have to enter your passcode over and over again in order to keep using your device. This is After First Unlock (AFU) state.
When you’re in AFU state and you lock your device, it doesn’t throw away the encryption keys. It just doesn’t permit you to access your device. This is when you can use biometrics to unlock it.
In some jurisdictions a judge can legally force someone to enter biometrics, but can’t force them give up their passcode. This legal distinction in the USA is that giving a passcode is “testimonial” because it requires giving over the contents of your mind, and forcing suspects to do that is not legal in the USA. Biometrics aren’t testimonial, and so someone can be forced to use them, similar to how arrested people are forced to give fingerprints.
Of course, in practical terms this is a meaningless distinction because both biometrics and a passcode can grant access to nearly all data on a device. So one interesting thing about BFU vs AFU is that BFU makes this legal hair-splitting moot: biometrics don’t work in BFU state.
But that’s not what the 404 Media articles are about. It’s more about the forensic tools that can sometimes extract data even from a locked device. A device in AFU state has lots of opportunities for attack compared to BFU. The encryption keys exist, some data is already decrypted in memory, the lightning port is active, it will connect to Wi-Fi networks, and so on. This constitutes a lot of attack surface that hackers could potentially exploit to pull data off the device. In BFU state, there’s very little data available and almost no attack surface. Automatically returning a device to BFU state improves resistance to hacking.
Also, in the BFU state, iPhones at least, won’t allow any data connections through USB
It’s more complicated than that. It’s called USB restricted mode. The lightning port is always willing to do a minimal subset of the protocols that it supports in order to do smart charging. By default most of the protocols it supports are disabled in BFU state. In AFU state it gets more complex than that. Accessories that you’ve previously connected can connect for one hour after the device is locked. This helps keep USB restricted mode from being really annoying if you briefly disconnect and reconnect an accessory.
USB restricted mode can be disabled by a user option (Settings > [Touch / Face] ID & Passcode > Allow Access When Locked > Accessories) or by a configuration profile. Disabling it allows accessories to connect at any time, and generally lowers the security of your device. But in some cases that’s necessary, for instance when you use an accessibility accessory to use your device.
If USB restricted mode is a concern for you, you should consider Lockdown Mode (Settings > Privacy & Security > Lockdown Mode). This changes several settings on your device to make it much more resilient to attack.
Very informative. Thank you.
You mentioned the lightning port. Is there any difference with the newer phones with USB-C when it comes to these functions?
I’m glad you find this informative. It’s a topic that’s important to me both personally and professionally, and there’s a lot of wrong information out there. But the best and most reliable info is in the Apple Platform Security Guide, such as Activating data connections securely and Direct memory access protections for Mac computers.
In this topic I don’t think there’s any important difference between USB-C and lightning. Both form factors support a bunch of USB protocols as well as some Apple-only protocols, and both have USB restricted mode.