iPhone 5s/6/6 Plus users (and the newer breed of iPad users, too), have you ever wondered why you need to enter your pin after rebooting your device, why wont the Touch ID just work?
Here's the answer from Apple Support Blog itself.
"Touch ID doesn’t store any images of your fingerprint. It stores only a
mathematical representation of your fingerprint. It isn’t possible for
your actual fingerprint image to be reverse-engineered from this
mathematical representation. iPhone 5s also includes a new advanced
security architecture called the Secure Enclave within the A7 chip,
which was developed to protect passcode and fingerprint data.
Fingerprint data is encrypted and protected with a key available only to
the Secure Enclave. Fingerprint data is used only by the Secure Enclave
to verify that your fingerprint matches the enrolled fingerprint data.
The Secure Enclave is walled off from the rest of A7 and the rest of
iOS. Therefore, your fingerprint data is never accessed by iOS or other
apps, never stored on Apple servers, and never backed up to iCloud or
anywhere else. Only Touch ID uses it, and it can’t be used to match
against other fingerprint databases."