Downgrading iOS / Upgrading to version not the most recent [closed]

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How can I tell what the latest version of iOS that can still be "restored" to an iphone is?

Background: As a developer, it is important to be able to test multiple operatings systems (for those one off, or OS specific bugs), but as a part of their battle against the Jailbreak community, Apple limits what versions of the operating system can be installed on a device. Is there somewhere this logic is published, and or location that tracks what specific dates that various operating systems become invalid to restore?

Example: I have a problem reported on iOS 5.x for 3GS devices, and it is not repeatable on iOS 6.x on the same device. I have a 3GS device with 4.x on it that I would like to use to see if this is a one off user issue or a legitimate problem, so I download 5.1.1 (www.downloadios.net) and try to install (by holding the option key while clicking on the restore button and selecting the ipsw installer that I downloaded) but iTunes gives me -

How can I know what versions are eligible?

Answer 1

It looks like the short answer is as Aamir said:

Apple stops verifying old updates usually soon after new iOS version is released.

So going backward isn't really possible. Except, if you had the foresight to use a program like Tiny Umbrella which allows you to get the SHSH signatures (and later spoof them) for the devices you own. In my experience it could only download the device signatures for your device for the current most recent OS (and perhaps one version back if it is close enough to release), so you'll have to make sure to get the SHSH signatures for all your devices each time a new version of iOS is posted -- You can then go back to any of the versions you have archived for the devices you have by downloading the appropriate IPSW from a site like this one and use Tiny Umbrella to spoof the apple TSS server while installing using the option key through iTunes.

Answer 2

Apple uses SHSH blobs as a signature for all iOS updates and restores.

and Apple stops verifying old updates usually soon after new iOS version is released.