I went into this elsewhere. Every update to any iOS device (well, any since iPhone 3GS) requires a signature unique to that device. Since iOS 5, it also includ

I went into this elsewhere. Every update to any iOS device (well, any since iPhone 3GS) requires a signature unique to that device. Since iOS 5, it also includes a nonce generated on device at the time of upgrade, so you can't even replay the signature, it needs to be signed at the time you install the new version. This is why you can't downgrade to earlier versions of iOS after Apple stops signing them. Anyway, I hope it's clear now why that was wrong.