A SHSH blob (based on the acronyms for signed hash and binary large object; also called ECID SHSH, referring to the device's ECID, a unique identification number embedded in its hardware) is an unofficial term referring to the digital signatures that Apple generates and uses to personalize IPSW firmware files for each iOS device. They are part of Apple's protocol designed to ensure that trusted software is installed on the device, generally only allowing the newest iOS version to be installable. Apple's public name for this process is System Software Authorization (formerly called System Software Personalization before iOS 7). This process is controlled by the TATSU ("TSS") Signing Server (gs.apple.com) where updates and restores can only be completed by iTunes if the version of iOS is being signed. Developers interested in iOS jailbreaking have made tools for working around this signature system in order to install jailbreakable older iOS versions that are no longer being signed by Apple. Technical details SHSH blobs are created by a hashing formula that has multiple keys, including the device type, the iOS version being signed, and the device's ECID. When Apple wishes to restrict users' ability to restore their devices to a particular iOS version, Apple can refuse to generate this hash during the restore attempt, and the restore will not be successful (or at least will require bypassing the intended function of the system). This protocol is part of iPhone 3GS and later devices. TATSU Signing Server When iTunes restores or updates an iOS firmware, Apple has added many checkpoints before the iOS version is installed and on-device consolidation begins. At the first "Verifying iPhone software" iTunes communicates with "gs.apple.com" to verify that the IPSW file provided is still being signed. The TATSU server will give back a list of versions being signed. If the version is not being signed, then iBEC and iBoot will decline the image, giving an error of "error 3194" or "declined to authorize the image" iTunes will communicate with iBoot throughout the process of an update or restore ensuring the firmware has not been modified to a Custom Firmware ("CFW"). iTunes will not update or restore a device when it suspects the file has been modified. This is a chain process, before installing the firmware, the installed iBoot has to verify the to-be-installed iBoot, and so on. You cannot install unsigned iOS versions, unless 1) you possess SHSH2