There’s a lot of confusion about the SHSH blobs, and once saved ( see tutorial here and here ), how can we use them to downgrade our iOS devices. The problem is, that iOS devices have to communicate with Apple’s servers before allowing the installation of any version of firmware (restore or update ) .With the release of Firmware 3.1.3, Apple’s server has stopped certifying all previous firmware and this makes it impossible to restore your device to a previous release. The certification is done through 3 files called IBSS, IBEC and ECID that, once signed, they are a kind of “green light” to install the firmware through iTunes.