Apple uses a Cryptographic Verification System called . When iOS boots, the SEP and iBoot bootloader check a digital signature inside the IPSW against Apple's signing server. If the signature is missing or invalid—even by one bit—the device refuses to boot. Modern devices (A12+ chips) have a hardware fuse called the Nonce . This means you cannot simply flash a modified IPSW; you need a bootrom exploit (rare) or a jailbreak that bypasses signature checks after boot.
If you modify a GSM-only IPSW and flash it to a CDMA device (or vice versa), the baseband bootloader fails. The device boots to the "Connect to iTunes" screen permanently. Recovery: Requires factory restore. modify ipsw file
An IPSW is essentially a renamed .zip archive. If you change the extension to .zip , you can unpack it to find several critical components: Apple uses a Cryptographic Verification System called
Instructions that tell the device how to install the software. Why Modify an IPSW? Modern devices (A12+ chips) have a hardware fuse
If you have a device that can accept a custom or modified IPSW (such as a legacy device in Pwned DFU mode), you can attempt to flash it: