All about SHSH
 

This is a portion of another post I made, just thought it would be beneficial to have it on its own as there are still many questions and misconceptions about what SHSH are and how to get them.

About SHSH

A good explanation of SHSH is here: Caching Apple's Signature Server - Jay Freeman (saurik)
Basically it is a certificate of sorts that apple gives your device when trying to restore to a certain firmware. Cydia can store these for you and allow you to use them to downgrade when you need to.There are 3 ways to obtain SHSH for your device:
1: When jailbroken open cydia and press the “make my life easier” that pops up when you first open it.
2: Edit your hosts files as detailed in that link above from Saurik.
3: Use tiny umbrella, a guide on how to use tiny umbrella to save your SHSH and use them to downgrade can be found here: Apple, iPhone & iPad Forums | *****i.com
4: You can use iFaith or redsnow to extract the SHSH from the device itself for the firmware you are currently on. This is one way you can save SHSH even after Apple has stopped signing a firmware.

Once Apple stops signing a certain firmware you cannot save your SHSH for that firmware anymore. So even if not jailbroken tiny umbrella can be used to save your SHSH for the current firmware so that when a jailbreak does come out you will always be able to restore to that firmware even if a new firmware comes out.

Some facts about SHSH:
1: You do not need to be jailbroken to save SHSH.
2: You do not need to be on a certain firmware to save SHSH for that firmware. You can be on 3.1.3 and just as easily save SHSH for 4.1 with tiny umbrella.
3: Once one of these 3 methods is used to save SHSH cydia will automatically save SHSH for all future firmwares when possible. So even if you think you do not have SHSH it is always recommended to download tiny umbrella and check just to be sure.
4: SHSH are device specific. They cannot be transferred from one device to another.

What devices need SHSH:

iPhone 4S/4/3GS/3G
iPod Touch 4G/3G/2G
iPad 1/2/3
Exceptions to this rule apply to iPhone 3G and iPod Touch 2G. They only require SHSH for firmware 4.0 and up. For lower than this putting the iphone in dfu and restoring should suffice. Also there are ways around it if you have a iPhone 3G jailbroken by redsn0w/pwnage tool/sn0wbreeze then they can be restored to a custom 4.0 firmware without the need for SHSH.

Again a more in depth guide about How to downgrade/upgrade is detailed here:
http://"http://forum.gsmhosting.com/...tore-1474330/"

A guide to see if you can jailbreak or not can be found here: