What are SHSH Blobs and why do you need to save them?
SHSH Blob is a signature file which is verified against Apple Server to verify the iPhone is running the latest version of iOS. If for some reason, when you are try to restore to a previous version, Apple will not allow you to restore it because, you are trying load an older version of iOS. To make this possible, we need to send a request to a different server (Local or designated) which sends a SHSH blob (which was saved by you) back to iTunes faking that it’s the current version.
And it is important to save your SHSH if you want to stay untethered on iOS 4.2.1
Below are the steps to follow:
Step 1: Download Tiny Umbrella for Windows ,
Download Tiny Umbrella for Mac
Step 2: Open Tiny Umbrella and connect your device first.
Step 3: Now click on Save SHSH and it will save your SHSH from Cydia. You have to sit back and relax.
Now you will be done and finally saved your SHSH. The same is for Mac users also.