So I wanted to jailbreak my iPhone and I was interested in it before but I never really took the time to. Research properly. Forgetting that I might jailbreak in the future, I updated my phone to iOS 9. I didn't really like iOS 9 and the fact that you can run a jailbreak on it.
I researched a little and found out that I can restore with an IPSW file and I was stoked. However, it seems that Apple only “signs” down to ios 8.4.1, an UN-jailbreak able version of iOS.
What does it mean if an IPSW file is not signed? And what would happen if I were to put an unsigned version of iOS on my phone?
I got the IPSW file from this website. I have tried researching pretty much everywhere but I cannot find an answer.
Best Answer
Easily spoken, you could say ipsw files are not complete. They're missing that so-called signature. It's like a contract; it has no value without the signature. Every time you install new firmware on the iPhone, iTunes calls to the Apple servers and ask for that approval signature. For numerous reasons, Apple doesn't give out those signatures for older firmware versions.
There's not much you can do. The only thing you can do is downgrading to 8.4.1 and hope one day somebody releases the jailbreak for it. But for now, you're stucked.
UPDATE: As of Oct 2015 Apple stopped signing 8.x. As of Oct 2016 Apple stopped signing 9.x. While this answer is not correct any more it still describes the general idea behind Apple firmware signing.