SHSH is a necessity for every Apple device to brush firmware. Its full English name is: Signature HaSH blobs, that is, Chinese signature HaSH, which is a signature certificate generated by Apple verification server according to device information.

SHSH degradation principle: iTunes (Apple's software) sends the information of this device to the verification server, requesting to restore SHSH. If the restored version does not turn off verification, the server will send SHSH and iTunes can continue; However, if verification of this version is turned off, iTunes will report an error directly and cannot be downgraded \ upgraded.

Attachment: Apple will close the verification of the old version soon after releasing the new firmware. Therefore, it is important to maintain SHSH.