SHSH blob - Wikipedia, the free encyclopedia
this is why IF he, or anyone else, did not save shsh blobs while on the desired firmware, and while Apple is still signing, downgrading will NEVER happen
SHSH blobs is a Hash signature system (Signature HaSH blobs) made by Apple Inc. to control manual software downgrades on iPhones, iPads, and iPod touches (a typical prelude to Jailbreaking). An SHSH is created by an SHSH formula (CLI Application) with 3 or 4 TSS keys- the device (e.g. iPhone 4 CDMA), the firmware version being signed (e.g. 4.2.8) and the device's ECID- a unique ID contained within every device. The SHSH is a Plist, built with blobs, with each blob intended for a different part of the software (like kernel and Apple logo). These blobs control which firmware is restorable and which is not. When Apple wishes to restrict users' ability to "downgrade" their devices to a prior firmware version, Apple can simply refuse to generate this hash during the downgrade attempt, and the downgrade will not be successful (or at the very least, will require significant technical intervention).
furthermore, this is not exclusively for jailbreakers. anyone can do this