SHSH blob is a term for a small piece of data that is part of Apple's digital signature protocol for iOS restores and updates, designed to control the iOS versions that users can install on their iOS devices (iPhones, iPads, iPod touches, and Apple TVs), generally only allowing the newest iOS version to be installable. 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.