iH8sn0w from the jailbreak community, the main behind the popular tool sn0wbreeze, has something to add on custom IPSW’s.
He recently tweeted that Apple has eliminated the option to restore iOS 7 custom IPSWs for iOS 7, when the iTunes 11.1 was released. This means that custom restores would require alternative methods.
More specifically, the new iTunes update is taking into account each img3’s hash (partial) and recalculating them instead of relying on the ones that are offered by BuildManifest.plist). This is going to produce images that are accompanied by a totally different hash and they would easily get rejected due to Apple’s TSS. It has also been revealed that the iREB present on s5I8900x devices (like iPod Touch 1G, iPhone 2G and iPhone 3G) are also broken by iTunes 11.1.
However, the good news is that iH8sn0w is developing a tool that allows custom firmware restore without having to resort to iTunes.