iPhone - What are SHSH Blobs and how to downgrade your iOS firmware

Summary
An SHSH Blob is a signature file which is verified against Apple Server to verify the 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 becuase, you are trying load a 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 its the current version.


This applies to all apple based devices
iPhone, iPad, iPod Touch etc...


An SHSH Blob CANNOT be shared across multiple devices
I.e. you can't use a friends! This is because the ECID (Exclusive Chip ID - Unique ID for the device) is saved alongside it and must match.


Why and when to save these SHSH Blobs?
You should save these SHSH Blobs everytime your device is updated to a new firmware version. This will allow you to downgrade the firmware for the device regardless of Apple unsigning the firmware or not. So keep a collection, and you can downgrade to whichever you wish.


Why should I downgrade?
- Your generally unhappy with the new firmware
- The new jailbreak has problems, and you would like to revert to a previous firmware where your previous jailbreak was running smoothly.
- The new firmware only has a Tethered jailbreak available.

Teathered vs. Unteathered?
This simply means, weather you need a computer to start your phone. If a release of redsn0w offers a Teathered solution, then !everytime! your phone turns off (out of battery, new app install etc.) you will need to plug it into your computer and use redsn0w to boot the phone! The batteries are pretty decent, so you probably don't even care or it's no big deal.


I don't have my SHSH Blobs for a previous firmware version, but I still want to downgrade...
A utility called "An SHSH Blobs Extractor" is being worked on by iH8sn0w... This allows you to extract blobs for a previous firmware version from the device. Stay tuned though, it's not out yet!


How do I save an SHSH Blob for my device
NOTE: Do you not need a jail broken device to save your SHSH Blob.

You have a number of options here, but the way that works for jailbroken and non-jailbroken devices is by using TinyUmbrella (For Windows and MAC).

TinyUmbrella Download
TinyUmbrella-5.11.01.pkg (MAC)
TinyUmbrella-5.11.01.exe (Win)
View the TinyUmbrella blog to get the latest version here

1. Open TinyUmbrella
2. Click "Start TSS Server"
3. Save SHSH
4. The SHSH files for your device will now be stored on your local system.

NOTE:
If your device is jailbroken, you can store these on the Cydia server. This will allow you to revert back to a previous firmware without worrying about keeping them safe. Also, if you buy an Apple device from ebay, the seller may have already done this, so its good to check Cydia if this is the case!


Downgrading your device with SHSH Blobs
Once you have the SHSH Blobs for the firmware you would like to downgrade to, then you can use iFaith to achieve this! More information here