Explained: SHSH Blobs; How To Downgrade iOS

Disclaimer: I originally posted the following as an unnamed edit of @WYSE’s forum post.

Why am I concerned about SHSH blobs?
Firmwares need to be remotely signed by Apple at the moment of installation. Apple issues signatures only for its latest firmware. This means that if they had their way, nobody will be able to downgrade to an older firmware, and all upgrades have to be to the latest firmware.

What are SHSH blobs?
But hey, they don’t have their way. Once your device is running a firmware, you have what is called an SHSH blob, specific to this firmware and this device. This blob is essentially Apple’s signature of approval for this firmware to run on this device. If you save this blob, you can return to this firmware on this device without seeking Apple’s signature again (which they won’t give anyway).

How do I save them?
There are three methods; first we will talk about the one that works with both non-jailbroken and jailbroken devices. The second method is a lot easier, though, so if your device is already jailbroken, you can skip ahead. There is also a third method, just in case your device is currently running a firmware that is no longer being signed by Apple.

Method 1
Method 2
Method 3

So how do I downgrade or install a firmware that’s not the latest?
You need to tell iTunes where to look for the signature when you’re restoring to a firmware that is not the latest. iTunes will by default look to Apple for the signature (which Apple will not be providing for a firmware that’s not the latest). Your saved blobs are precisely what iTunes is looking for. To redirect iTunes to your saved blobs, you can either run TinyUmbrella’s TSS server, or edit your computer’s hosts file. Do either one of these before attempting to restore. For more information, you can also read this guide.

Things to note:
You will not have SHSH blobs for firmwares you’ve never been on, or if you didn’t save them while you were on the firmware. That’s why you should always save your SHSH blob every time you get on a new firmware. Apple typically stops signing older firmwares within two weeks after releasing a new firmware.