Firmware Keys

VFDecrypt Keys are the keys which can decrypt the files that come with the firmware. Apple uses a public-private key encryption to ensure the safety of their files. Over time Apple has changed the way to encrypt firmware files, thus the way to decrypt files as well as the way to get the VFDecrypt Keys has also.

IMG2 was the file format used prior to iOS 2.0. For iOS 1.1.x, IMG2 files were encrypted with Key 0x837.

IMG3 encrypted files contain encrypted versions of the VFDecrypt Keys as part of the KBAG (key bag). These can be decrypted with the GID-key. For jailbroken iDevices the VFDecrypt keys can be retrieved with the devices hardware AES engine. The VFDecrypt key for the root filesystem image of an iDevice (~500 MB to 800MB in the case of iOS 5) requires either a decrypted Restore Ramdisk or Update Ramdisk. Once you have a decrypted Restore or Update Ramdisk, GenPass can be used to gather the keys for the root filesystem. For the root filesystem there is one key per device model, with no IV. You can mount this once it has been decrypted using your program of choice. (For example, 7-zip on Windows (after extracting the DMG on Windows, extract the biggest file with 7-Zip)

Firmware versions
This is a full and comprehensive list of all firmwares Apple Inc. has made available to the public in some way, be it the dev center or iTunes.