Odysseus is a tool by xerub to downgrade/upgrade/restore specific 32-bit devices to specific firmwares. This method only works if you are jailbroken, have valid SHSH Blobs and a valid APTicket.
How it works
It uses a tool included in winocm's [Archived 2015-08-10 at the Wayback Machine] ios-kexec-utils called kloader. kloader is a CLI that you can run on an iOS device which loads an (decrypted) image. It allows you to bootstrap a pwned iBSS and then initiate a restore using idevicerestore with a custom IPSW, which allows you to downgrade/upgrade/restore to the firmware of your choice. There are firmware bundles that allow you to restore each device to each iOS. If the bundle for the device and iOS you want to get does not exist, the downgrade/upgrade cannot happen. Each bundle is made by grabbing the Keys/IVs from the original firmware. They are posted here.
This only works on some jailbroken 32-bit devices with an untether that has tfp0 enabled. The latest versions of TaiG, Pangu9, Pangu8, Pangu, evasi0n7, and evasi0n all have tfp0 activated. Also, this will *NOT* change your baseband. If you go too far up or down with iOS version, it may be that the version of iOS you restored to does not understand the baseband left behind by the prior version of iOS. If that happens, you will not get past activation and you cannot re-jailbreak the device. As a consequence, the device will remain in activation limbo and you'll have to restore to the currently signed version of iOS.
|iPad 2 (iPad2,1)||iPad (3rd generation) (iPad3,1)||iPad (3rd generation) (iPad3,3)|
|iPhone 4 (iPhone3,1)||iPhone 4 (iPhone3,3)||iPhone 4S (IPhone 4,1)||iPhone 5 (iPhone5,2)||iPhone 5c (iPhone5,4)|