system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does the iPhone require a passcode after restart if the fingerprint scanner is secure?
gpt
As was made famous by the FBI/Apple confrontation from last year, recent iPhones do a pretty decent job of encrypting all content stored in the phone.  The way the encryption works is that the *master key*—the big, 256-bit secret number that's required to decode the phone's contents—is never stored in the phone's permanent memory, but rather reconstructed when it's needed.  Reconstructing the master key requires two pieces of information:

* The phone's secret unique ID, stored in the processor when it is built;
* The passcode selected by the user.

The fingerprint scanner cannot be used for this, because it's an *inexact* scanner that doesn't produce the same result twice.  It's like taking two photos of the same person—you can recognize that it's the same person in the two photos, but the photos won't be identical.  But reconstructing the master key requires an exact match—something that's possible with a passcode but not with a fingerprint scanner.

So the phone uses the fingerprint scanner to reduce the frequency at which you'd need to reenter your passcode.  The phone can only use your fingerprint to let you in when the master key is available in temporary memory.  Restarting the phone loses the content of the temporary memory, so you must reenter the passcode.