And right here’s a extra targeted rationalization. It isn't imaginable to downgrade from iOS 9.2.1 or iOS 9.2 to iOS 9.1 as Apple has stopped signing the iOS 9.1 firmware information somewhat a very long time again. It additionally signifies that you'll be able to’t update from an older model of iOS. For instance: You can’t improve from iOS 9 – iOS 9.0.2 or decrease to iOS 9.1.
Apple has used this technique to stay jailbreakers at bay. It now not most effective releases new device updates that fixes the vulnerabilities utilized by the jailbreaks, it additionally stops signing the older iOS firmware document, which prevents customers from downgrading again to the older iOS model that may be jailbroken.
Apple has been doing this for rather a whilst. The now defunct iPhone Dev staff had explained how Apple had made it tricky to downgrade to older model of iOS:
Starting with the iOS5 beta, the position of the “APTicket” is converting — it’s getting used similar to the “BBTicket” has all the time been used. The LLB and iBoot levels of the boot series are being subtle to rely at the authenticity of the APTicket, that is uniquely generated at each repair (in different phrases, it doesn’t rely simply in your ECID and firmware model…it adjustments each and every time you repair, primarily based in part on a random quantity). This APTicket authentication will occur at each and every boot, now not simply at repair time. Because most effective Apple has the crypto keys to correctly signal the consistent with-repair APTicket, replayed APTickets are unnecessary.But additionally it is necessary to notice that one of the most purposes Apple releases iOS device updates is to patch safety vulnerabilities, which is probably not in an instant similar to jailbreaking. Few days again PanGu workforce had even advisable upgrading to iOS 9.2.1. So it is crucial to weigh the professionals and cons of upgrading to the up to date model. It would possibly save you you from jailbreaking your software, and therefore permit you from customizing your iPhone or iPad, however upgrading to the up to date model additionally guarantees that your software is extra safe.
This will most effective have an effect on restores beginning at iOS5 and onward, and Apple will probably be in a position to turn that transfer on and off at will (through beginning or final the APTicket signing window for that firmware, like they do for the BBTicket).