When iTunes restores or updates an iOS firmware, Apple has added many checkpoints before the iOS version is installed and on-device consolidation begins. This protocol is part of iPhone 3GS and later devices. 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). 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.
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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. This process is controlled by the TATSU ("TSS") Signing Server (gs.) where updates and restores can only be completed by iTunes if the version of iOS is being signed. Apple's public name for this process is System Software Authorization (formerly called System Software Personalization before iOS 7). 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. Unofficial term referring to digital signatures Apple generates and usesĪ 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.
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