Technology

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.