Two years ago Apple was launching iOS 3 together with iPhone 3GS and was introducing a new system that was blocking the users to restore their devices to older iOS versions. Back then Dev Team found a method to overpass this restriction, by saving the SHSH and use it afterwards to be able to restore your device to any older iOS version, based on the saved SHSH.
We had lot of headaches with the SHSHs, and now it seems that Apple wants to save us from having problems with the whole SHSH fuss, by blocking the iOS downgradeprocedure. This means that once you will have iOS 5.1 installed on your device, you will not be able to return to iOS 5.0 at all.
Dev Team explained last night what the new restrictions imposed by Apple involves, comparing the new system with the one already existent for baseband downgrade. At the moment it’s impossible to downgrade the baseband, unless you have an iPhone 4 and install a beta version of iOS that comes with baseband you can return to a lower value. That’s what Apple will do with iOS 5, implementing a system that will generate a unique restore code for each iDevice, a code which will be checked every time you power on your device, and which, for now, is impossible to crack or duplicate.
The unique restore code is generated randomly at every restore and is not related to ECID SHSH, so without the Apple encryption code it will be difficult to “guess” the code or fake the iDevice’s authentication with every power on.
This new system will be introduced by Apple in the same time with iOS 5 and the interesting part is that Apple could let you return to a previous firmware version. This possibility may be provided by signing the restore codes (similar to SHSHs) for the old devices. Generally, after launching a new version of IOS Apple still singes the SHSS to the old version for a few days, so something similar could happen in iOS 5.
The good news for those owning a device compatible with iOS 5, except for iPad 2, is that limera1n exploit will still allow a tethered jailbreak for iOS 5 and that will be able to downgrade to iOS 4, if you previously saved the SHSH, but older versions of iTunes will be required in order to perform these actions. Dev Team claims that Apple will launch new versions of iTuens which will contain methods to block the restore to iOS versions, so we recommend you to keep iTunes 10.3 installation kit on a safe place on your computer, as you might need it later.
Finally, Dev Team claims that the new system will block the downgrade starting with iOS 5 GM which will be launched in a few months. I believe that lot of us expected Apple to introduce such restrictions, but just like SHSH method appear two years ago, new restore methods will be found, and Dev Team said they already found some ways to combat the restrictions.
In conclusion, starting with iOS 5 we will have to use new ways to restore older iOS firmware versions.
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