Without prior words Pangu team surprised everyone by releasing untethered jailbreak for iOS 9.1 and probably the most frequently asked question has been whether it is possible to downgrade or upgrade to iOS 9.1, since it is not possible to jailbreak iOS 9.2.1.
Unfortunately, the short answer to that question is “No”.
And here’s a more detailed explanation. It is not possible 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 files quite a long time back. It also means that you can’t update from an older version of iOS. For example: You can’t upgrade from iOS 9 – iOS 9.0.2 or lower to iOS 9.1.
Apple has used this strategy to keep jailbreakers at bay. It not only releases new software updates that fixes the vulnerabilities used by the jailbreaks, it also stops signing the older iOS firmware file, which prevents users from downgrading back to the older iOS version that can be jailbroken.
Apple has been doing this for quite a while. The now defunct iPhone Dev team had explained how Apple had made it difficult to downgrade to older version of iOS:
And here’s a more detailed explanation. It is not possible 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 files quite a long time back. It also means that you can’t update from an older version of iOS. For example: You can’t upgrade from iOS 9 – iOS 9.0.2 or lower to iOS 9.1.
Apple has used this strategy to keep jailbreakers at bay. It not only releases new software updates that fixes the vulnerabilities used by the jailbreaks, it also stops signing the older iOS firmware file, which prevents users from downgrading back to the older iOS version that can be jailbroken.
Apple has been doing this for quite a while. The now defunct iPhone Dev team had explained how Apple had made it difficult to downgrade to older version of iOS:
Starting with the iOS5 beta, the role of the “APTicket” is changing — it’s being used much like the “BBTicket” has always been used. The LLB and iBoot stages of the boot sequence are being refined to depend on the authenticity of the APTicket, which is uniquely generated at each and every restore (in other words, it doesn’t depend merely on your ECID and firmware version…it changes every time you restore, based partly on a random number). This APTicket authentication will happen at every boot, not just at restore time. Because only Apple has the crypto keys to properly sign the per-restore APTicket, replayed APTickets are useless.But it is also important to note that one of the reasons Apple releases iOS software updates is to patch security vulnerabilities, which may not be directly related to jailbreaking. Few days back PanGu team had even recommended upgrading to iOS 9.2.1. So it is important to weigh the pros and cons of upgrading to the latest version. It may prevent you from jailbreaking your device, and hence allow you from customizing your iPhone or iPad, but upgrading to the latest version also ensures that your device is more secure.
This will only affect restores starting at iOS5 and onward, and Apple will be able to flip that switch off and on at will (by opening or closing the APTicket signing window for that firmware, like they do for the BBTicket).
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