The signature check is burned into the —the very first code that runs when you turn on your iPhone. It is a hardware-level lock. You cannot trick the iPhone into thinking an unsigned IPSW is signed unless you have a bootrom exploit (a "checkm8" level vulnerability), which only exists for iPhones up to the iPhone X.
On the surface, an "unsigned IPSW" sounds like a technical footnote. In reality, it’s one of the most powerful weapons in Apple’s ecosystem control arsenal. Let’s pull back the curtain on what signing means, why Apple enforces it, and what happens when an IPSW loses its signature. First, a quick primer. An IPSW (iPhone/iPod/iPad Software) is the firmware file that contains the entire operating system for your iOS device. It’s essentially a zipped archive of the system image. unsigned ipsws
For modern iPhones (XS and newer), the signing check is mathematically unbreakable via software. In the early 2010s, advanced users used to save SHSH blobs . These were digital receipts of a valid signature issued by Apple. You could save the blob for iOS 6 while Apple was signing it, then replay that blob to the server months later to downgrade. The signature check is burned into the —the