If you meant you want a (not a paper) to restore an unsigned IPSW, check:

If you are attempting to restore to a version no longer signed by Apple, follow these general steps. Success is highly dependent on having saved SHSH blobs from when that version was still signed.

I notice you're asking for a "paper" related to an that is free . This suggests you might be looking for academic research or technical documentation on tools that bypass Apple's signing mechanism to restore older iOS versions.

Historically, this has been the go-to open-source tool for advanced users to restore unsigned firmware using saved SHSH blobs. Its compatibility varies significantly based on the device's processor (SEP/Baseband compatibility).

: As with any tool that modifies device firmware, there are risks involved, such as the potential for bricking the device if the process is not followed correctly or if there's an interruption. Users should proceed with caution and ensure they have a backup of their data.