IPSW

What does signed IPSW mean?

When an IPSW is signed that means that you can restore it in itunes and the file that is called shsh bloobs are avalible. When apple realeses a new ios they always stop signing the older version of ios because they dont want people to downgrade and Jailbreak and they always want people on the latest ios.

Here you can check if the ios is still signed or not and you can download the ios too: https://sh.st/KWC0X