Updating and restoring ipod touch software
The only exception to this is that i Pad 2 owners with any saved i OS 4 firmware generation SHSH blobs can downgrade to to any firmware that they also have saved SHSH blobs for.
Downgrading devices with the A5, A5X and A6 chip on the i OS 6 firmware generation is not possible at this time.
In order to enforce this limitation SHSH blobs were introduced, which is a hash signature system.
Although this is getting technical, an SHSH is created by an SHSH formula with 3 or 4 TSS keys (say with me).
Once you have your SHSH blobs signed, as stated previously, if you upgrade to a new firmware version and decide you don’t like it you can downgrade back to the firmware version which you have saved SHSH blobs for.
There is unfortunately some restrictions that need to be kept in mind though.
If you are Jailbroken then most of the times your SHSH blobs are already saved on Cydia’s server automatically and you shouldn’t have to back them up manually.
The good thing is there is indeed still a way to downgrade your i Phone, i Pod Touch, i Pad or even Apple TV, but it relies on you saving SHSH blobs.
For the most part, older i OS devices with an A4 chip can be downgraded to any firmware version as long as you have saved SHSH blobs and APTickets.
This includes the i Phone 2G, i Phone 3G, i Phone 3GS, i Phone 4, i Pad 1G, i Pod Touch 1G, i Pod Touch 2G, i Pod Touch 3G and the i Pod Touch 4G.
Whenever you upgrade or downgrade your i Phone, i Pod Touch or i Pad, new SHSH blobs have to be generated because one of the parts to the SHSH formula is the firmware version.
In order to prevent firmware restores Apple only signs one firmware version at a time, which is obviously the latest firmware version available.