[h=3]What is SHSH?[/h]The important detail of the signing process is then to know exactly what is signed. Over the years, this has changed. The system that I had described a few years ago in a previous article, "Caching Apple's Signature Server", involves "personalizing" the files that are used as part of the iOS operating system software installation process (which is known as "restoring").This personalization is done by adding data to each file that is specific to the device on which the software will be installed, and then having the resulting "personalized" file be signed again by Apple. (I say signed "again", as the original files distributed by Apple are already signed, but they do not contain this device-specific information and are entirely generic.)The device-specific information that was used for this process is called, depending on where you find it, the "unique chip ID" or the "ECID" (an acronym that no one is sure of the meaning behind, but probably means something like "exclusive chip ID"). This ECID is a small block of data represented as a number that is unique to every iOS device that Apple has shipped.This ECID is then sent to Apple, along with the list of files that are being signed. Apple then returns a "blob" for each file, which is a block of data that replaces the existing signature on the file with both the personalization information (which is the ECID), sometimes some random data seemingly just "for good measure", as well as a new signature that signs the entire file.The actual signature inside of this blob is the SHSH, or signature hash (maybe "signed hash"; again, no one is really that certain, but we largely agree on "signature"). As the ECID of a device never changes, if you can then save the SHSH of a personalized file, you can always use it later to install that file, even if Apple is no longer willing to sign it: we thereby like saving these.