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This question already has an answer here:

Both my iPad and my android phone are configured to allow access via either a pin, or a fingerprint scan. However, immediately after a reboot, neither device allows a fingerprint for the first unlock; the pin is required instead. Both devices state "a pin is required to unlock after a reboot".

Why is this? What risk is associated with allowing a fingerprint unlock directly after a power on?

marked as duplicate by forest, Community Jan 7 at 5:12

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    I just found this duplicate. The answer explains why this is the case even with fingerprint readers. – forest Jan 7 at 3:34
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The PIN is used to derive an encryption key, whereas the fingerprint is used only for authentication. Because the encryption key is kept in memory during runtime, it is lost after a reboot and needs to be supplied again. It is supplied via the PIN or passphrase that you are required to enter.

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