Odysseus

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 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.

FAQ
Odysseus is a tool/method, which was developed by xerub to downgrade devices using winocm’s kloader. It works by booting decrypted and patched bootloader files and ramdisk to put the device in restore mode. After device is in restore mode, the restore process is the same as in iTunes. Odysseus was initially designed to preserve your current baseband, but it was observed that some baseband/iOS combinations work fine, some don’t. In case the baseband does not work, you will have no service on your phone. An option to create custom ipsws in conjunction with OTA buildmanifests was added to odysseus at a later time. This can, essentially, handle OTA/baseband downgrades, but the process is quite convoluted. In OdysseusOTA, tihmstar added a feature that allows creating custom ipsw, which fetches ota blobs instead of normal blobs. Those can be used for downgrading, but still require the kDFU procedure, because normal ota blobs are signing a different ramdisk. With ota blobs it is possible to downgrade the baseband, which is otherwise not possible. With odysseus you need to backup the installed baseband, perform the downgrade and restore saved baseband. Some the baseband/iOS combination work fine, some don’t. In case the baseband does not work, you will have no service on your phone. Technically odysseusOTA can do everything what odysseus can, but it only supports ota downgrades, so if you want to downgrade with saved shsh blobs to versions where no ota blobs are signed, please use odysseus.

Links

 * Official page
 * GitHub