The IPSW file’s created by PwnageTool (apple firmware files) when unzipped, contain completely unencrypted DMG files which can easily be mounted in the Finder for a pristine factory fresh iPhone root filesystem, ready for use in the open source toolchain to make wonderful jailbroken iPhone apps. You can jailbreak your iphone using PwnageTool, and use the same firmware file to build your toolchain so you can start writing apps for it!