What is an SHSH Blob?

It is something that is mentioned a lot on iDB throughout our jailbreaking tutorials and other jailbreak-related news. “Make sure you save your SHSH Blobs before you update,” is a pretty common saying around these parts. We’ve been asked several times lately what it is, so I figured I’d post it.
An SHSH Blob is basically what Apple uses to verify the version of iOS you are running. That’s why we tell you to save your SHSH Blobs before upgrading your firmware on your iDevice. Otherwise you might not be able to downgrade to a previous version in the event you needed to.

That is unless you managed to save your SHSH Blobs with a program like TinyUmbrella. Doing so will allow you to trick iTunes, into thinking you are running an older version.
“Cody, why can’t these SHSH Blobs just be downloaded like everything else?”  Your SHSH Blob is device specific. It uses the ECID (Exclusive Chip ID) of your device, along with a challenge key, as part of the authentication process with Apple.
Because that key is static (doesn’t change), and you’re using the same ECID, it is possible for you to save your own SHSH blob for later use. It can then be used to downgrade your iDevice if you accidentally update your firmware and there’s no jailbreak available.
Now make sure you save you SHSH blobs. If you don’t know how, this tutorial will show you how to.