Many users regret update iOS on your iPhone or iPad, due to the fact that the update sometimes causes problems of slowness, crashes or incompatibility with some applications. The natural desire is to want to go back to the old version, but when trying to reinstall, iTunes shows an error warning, allowing only to install the latest version of the system.
This is because Apple has implemented a signature system in the iOS firmware to allow only install versions that it authorizes. In this article we will explain this better.
From the beginning, Apple used a trick to discourage the so-called iPhone downgrade, which is the installation of a previous system version of the one that is already installed. It activated the need for a digital signature the moment iTunes connects to the server to validate the update. If the firmware that version has the active subscription, the installation is done, otherwise the process is interrupted.
When Apple stops signing a version, it can no longer be installed on the iPhone or iPad. Usually when a new update is released, the previous version remains signed for a few days, as a precaution in case something goes wrong. But it is normal for the company to stop signing the firmware old after a while, to not allow the downgrade.
But why does she do that?Well, one of the likely reasons was to hinder jailbreak, which was much more popular and necessary than it is today. The other reason would be to facilitate the user experience, as doing downgrade it is not something for beginners, since the backup of a higher version is incompatible with previous versions and this could cause loss of data, photos, etc.
How do I know if the subscription is still valid?There are some sites that list the versions that are signed or unsigned, to make life easier for the user. One of them is ipsw.me.
In the example in the image above, the website indicates that it is still possible to install iOS 11.2 (in green) on an iPhone 6s Plus even if it is already running iOS 11.2.2. However, if the user tries to install version 11.0.3 (which is in red, unsigned), he will not be able to finish the process.
Therefore, as a rule, you cannot return the system version of your iPhone or iPad to an old one that is no longer signed by Apple.