Your device's bootloader checks this blob. If it matches, the installation proceeds. How to Save SHSH Blobs
If a new update makes your phone slow or you dislike the features, you can only go back to an older version if you have saved the blobs for that specific version while it was still being signed.
Starting with iOS 5, Apple introduced a —a random number generated for each restore request. This means you can't just "replay" an old blob; the blob must match the specific nonce your device is currently expecting. Advanced tools (like FutureRestore ) are often required to manage nonces and successfully use your saved blobs for a downgrade. Engineering Security - School of Computer Science
Your device sends its ECID and the firmware version to Apple's servers.
An (Signature HaSH) is a unique digital signature that Apple uses to verify the firmware version you are trying to install on your device. Every time you restore or update your iPhone via iTunes or Finder, the software sends a request to Apple’s servers. Apple then "signs" this request with a blob specific to your device's unique ID (ECID) and the specific iOS version.
Popular community tools like TSS Saver (online) or Blobsaver (desktop application) can automatically fetch and store these for you.
You cannot save blobs for a version of iOS that Apple is no longer signing. You must be proactive.
Apple typically only "signs" the most recent version of iOS (and sometimes the one immediately preceding it). This is known as the . Once Apple stops signing an older version, it becomes impossible for a standard user to downgrade to it. SHSH Blobs are essential for: