There are alot of guides how to save ECID SHSH or SHSH blobs, but only few of them explaining What Is ECID SHSH, or what is SHSH blobs.
“ECID” stands for Exclusive Chip ID. This is a unique chip identification that is different for every iDevice (iPhone, iPad, iPod Touch). So, each iDevice (iPhone, iPad, iPod Touch) will have its own ECID and couldn’t be the same. Since the releases of iPhone 3GS (MC model) Apple start signing the firmware device. You can see the process while restoring or updating iPhone firmware through iTunes, it says “Verifying iPhone restore with Apple”.
What does it mean?
iTunes sends ECID (Exclusive Chip ID) to Apple servers (known as TSS server) whenever user attempts to restore or upgrade the firmware (OS software) on any iDevices to generate unique signature based on ECID. The Apple server then determines whether or not the firmware you are requesting to restore to is the current firmware available. If it is, then Apple’s server takes the ECID and sends back a digital signature approving the restore to iTunes, and the restore process begins. This signature is called the SHSH (an abbreviation for Signature HaSH) and is also referred to as a blob, as a blob of data is received from Apple.
If your request is to restore to firmware that is not current, Apple denies the request by not sending a digital signature back to iTunes, and the restore process comes to a halt with an error “The iPhone ‚ÄúiPhone‚Äù could not be restored. This device isn‚Äôt eligible for the requested build.”
The whole verification process is explained here: Verification process while restoring iPhone firmware through iTunes.
It is widely thought that this a new security feature, implemented to stop jailbreaking of future firmwares.
Let’s take a sample:
Now Assume that Apple releases a newer version of OS (e.g 3.1.3). Their TSS server immediately stop singing the older versions (older than 3.1.3) and only signs 3.1.3. If you accidentally update your iPhone to this newer version (3.1.3) you can’t downgrade to any older version (3.1.2) because iTunes can‚Äôt get SHSH from Apple TSS servers. Even if you are going to restore the same firmware version (restore 3.1.2 version to your iPhone) without upgrading to 3.1.3, iTunes will refuse to restore it, because iTunes only sign the 3.1.3 firmware. But the point is that if you previously saved the SHSH for 3.1.3 and give it to iTunes, iTunes will downgrade your device.
That’s why save SHSH blobs is so important.
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