Versions above iPhone OS 3.0 require the iBEC, iBSS, and LLB to be fully signed with an SHSH for the ECID of that device. IPhone OS 1.x and 2.x do not use SHSH signatures, and can therefore be downgraded to at any time, even on devices that do use SHSH signatures, such as iPhone 3G. Older devices ( iPhone and iPod touch) do not use SHSH signatures, so installation of any firmware on these devices is possible. SHSH blobs are unique to each device by ECID. 4.3.3), while TinyUmbrella gets SHSHs from Apple's servers (whatever firmwares Apple is currently signing). iFaith dumps the SHSHs from your device's storage (whatever's installed on your device, e.g. This is the case for iFaith, but not for TinyUmbrella. Users often misunderstand this system and think that the SHSH firmware version they back up depends on the firmware version they have installed on their device. It is not necessary that the device is jailbroken to do the backup. With the tools mentioned below it is possible to backup the signature. Therefore it is recommended to save the signature for your device as long as Apple issues it. But if you have saved signatures for an older iOS version, you may be able to use a replay attack to restore that version. Apple only issues signatures for the currently-available iOS version, which disallows installing older iOS versions.
This signature is needed to restore a specific iOS version it is generated by Apple based on hardware keys of the device and the hash of the firmware.
SHSH often also refers to backup files with this signature ("SHSH blobs"). SHSH, or "Signed Hash" refers to the signature (currently RSA) on boot images or APTickets that the device bootloader will verify before allowing execution of an image.
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