Unable to downgrade to iOS 8.1.2

Question

Im trying to downgrade my iPhone 5c to ios 8.1.2 but every time I try it it gives me an error (with no error code) "The iPhone "~~~" could not be updated because the firmware file is not compatible" Ive downloaded / tried to install on my phone: GSM, CDMA, and Global none of these work it takes about 3 minutes to download the IPSW's (it predicts to be 18-20 hours long) Product code: A1456 (CDMA) still unable to update. Thanks in advance! (ive tried multiple computers)

Answer 1

Yes I know, it's rather an exception than a rule.
From the IOS Security Whitepaper:
The authorization server checks the presented list of measurements against versions for which installation is permitted, and if it finds a match, adds the ECID to the mea- surement and signs the result. The server passes a complete set of signed data to the device as part of the upgrade process. Adding the ECID “personalizes” the authorization for the requesting device. By authorizing and signing only for known measurements, the server ensures that the update takes place exactly as provided by Apple.
The boot-time chain-of-trust evaluation verifies that the signature comes from Apple and that the measurement of the item loaded from disk, combined with the device’s ECID, matches what was covered by the signature.
These steps ensure that the authorization is for a specific device and that an old iOS version from one device can’t be copied to another. The nonce prevents an attacker from saving the server’s response and using it to tamper with a device or otherwise alter the system software.
Another paragraph added:
The startup process described above helps ensure that only Apple-signed code
can be installed on a device. To prevent devices from being downgraded to older versions that lack the latest security updates, iOS uses a process called System Software Authorization. If downgrades were possible, an attacker who gains possession of a device could install an older version of iOS and exploit a vulnerability that’s been fixed in the newer version.