Apple releases newer iPhone iOS to fix minor bugs and introduce new features. As soon as you update to newest iOS firmware you will loose carrier unlock and jailbreak and in worst scenario the newest iOS build might be full of bugs. In such cases you might feel a need to downgrade your firmware to your previous version. You can only retain carrier unlock and jailbreak if you have taken backup or archived SHSH Blobs to Cydia server or locally.
What are SHSH Blobs? Why should I save SHSH Blobs?
Apple actually stops signing older firmware files as soon as it releases a new firmware in order to stop you from downgrading to previous iPhone iOS firmware. You WILL loose unlock as well as jailbreak as soon Apple rolls-out a newer iOS firmware, fixing any existing exploit. In order to Jailbreak your iPhone iOS again you MUST grab your SHSH Blobs on Cydia Server.
With Firmware Umbrella you can easily backup your SHSH blobs so that you can downgrade and rerun the jailbreak on your iPhone, iPod Touch or iPad incase you unintentionally upgrade to the future firmware releases by Apple. It is always advisable to grab or take back up of SHSH Blobs [ECID SHSH] on current installed iPhone iOS firmware.
Steps to save SHSH Blobs (ECID SHSH) files on Mac OS
Step 1 : Download Firmware Umbrella (TinyUmbrella) from here to your Desktop.
Step 2 : Double click Umbrella.dmg file to mount it and drag the Umbrella app from the mounted dmg into your Applications folder. Double click the Umbrella app to launch it.
Step 3 : Enter username and password. TinyUmbrella will then detect the type of device and its firmware.
Step 4 : Check the Advanced Options box. Make sure that the ECID for your device is coming correctly and then select the correct firmware version, and select ‘Cydia’ as Location.
Step 5 : Press the Save My SHSH button. It is important to note that you can only obtain SHSH BLOBS for the latest iOS firmware version released by Apple.
Once completed you will be notified that it was successful. “You have saved your SHSH locally and the request was sent to CYDIA. This means that CYDIA DOES have your SHSH. Do NOT bug semaphore about the Cydia home page showing this version.”
Note: Sometimes due to server overload SHSH blob are not saved through Cydia. In such case you must save and take backup of SHSH BLOBS locally. In order to do that, just change the Location dropdown to Apple (instead Cydia, as in Step 4) and click the Save My SHSH button again.
If the file that gets saved is anywhere from 60-65k bytes then the file should be just fine.
Download TinyUmbrella to Save SHSH Blobs on Mac
via - iclarified