Why does Touch ID stop working after restarting the iPhone?

Now that Touch ID has established its presence After the arrival of the iPhone 6 and the new generation of iPads, you may be wondering some questions about this Apple fingerprint reader.

If you are the owner of any of the devices equipped with Touch ID, surely you have realized that after a reboot, you need to enter the lock code that we have established to be able to use it again. What is this about?

Basically This is a security measure, annoying for us but useful to prevent unauthorized access to the mathematical information that represents our footprints. On the Apple support website for the iPhone 5s we have more information on this topic that also applies to the iPhone 6, iPhone 6 Plus, iPad Air 2 and iPad Mini 3:

Touch ID does not store any images of your fingerprints; it only stores a mathematical representation of it. It is impossible to reverse engineer the image of your real footprint from this mathematical representation. iPhone 5s also incorporates a new advanced security architecture called Secure Enclave within the A7 chip, which was developed to protect data from codes and fingerprints. Fingerprint data is encrypted and protected using a key that is only available to the Secure Enclave. This data is only used by Secure Enclave to verify that your fingerprint matches the recorded fingerprint data. Secure Enclave is isolated from the rest of the A7 processor and the rest of iOS. Therefore, neither iOS nor any app access your fingerprint data, nor is it stored on Apple's servers, nor is it backed up in iCloud or anywhere else. Only Touch ID uses them, and they cannot be used to make associations with other fingerprint databases.

It seems that after a reboot, Secure Enclave hangs automatically and needs to be unlocked manually after a restart, something that involves entering the four-digit code that we enter when configuring Touch ID on our device.