Restoring an unsigned IPSW is technically impossible for the average user because Apple must provide a digital signature (SHSH blob) to authorize the installation. Once Apple "unsigns" a firmware version, their servers will no longer provide this authorization, causing standard tools like iTunes and Finder to block the process.
The following table summarizes the features and compatibility of the free unsigned IPSW restore tools: unsigned ipsw restore tool free
Why do people use unsigned IPSW restore tools? Restoring an unsigned IPSW is technically impossible for
We have tested and verified the following free tools. They range from beginner-friendly GUI apps to complex command-line utilities. Saved SHSH blobs for older devices (iPhone 4s and earlier)