It saves SHSH signatures for your device received from the Apple server and let to downgrade OS indefinitely. Apple stops signing older firmwares once a new one is out.įirmwareUmbrella sends the same exact request that iTunes sends Apple when requesting the signatures for your device to be restored. If the signature does not match, the device raises an error and the restore process stops. If the signature matches then the restore process can continue. Your device checks the information and verifies the signature (making sure it really came from Apple because it CANNOT be forged as the encryption is very high). iTunes packages up this valuable information and sends it to your device. Apple takes the firmware version files and combines them with your ECID and generates a hash that ONLY APPLE can generate. It is signed with a unique id (ECID) that only your device has. See, the firmware is now signed for the below devices.
The way they stop you is basically by a simple response that iTunes receives when you try to restore the firmware of your choosing. Once you ‘upgrade’ they don’t let you downgrade. When you try to restore an older firmware on the devices listed below, Apple won’t let you do that.
