Firmware Keys are keys which decrypt bootloaders, ramdisks, and root filesystem of iOS firmware, if those components are encrypted. Apple uses encryption to make it harder to analyze and modify them. Over time Apple changed the way they encrypt firmware files, hence the way to decrypt them and get decryption keys changed as well.
With the release of the iPhone came the IMG2 file format. They were used on all known iPhone OS 1.x firmwares. For the 1.1.x series, they were encrypted with the 0x837 key. The discovery of the 0x837 key led to the ability to decrypt any 1.x firmware.
Following IMG2 came the IMG3 file format. They were introduced with iPhone OS 2.0 beta 4, and have been in use ever since. In order to maintain their integrity, they use multiple layers of encryption. Apple took encryption seriously with IMG3 by utilizing AES (based on the Rinjndael key schedule). In terms of the pre-iPhone OS 3 VFDecrypt key, it is stored as plain-text in the "__restore" segment of the ASR image within the ramdisks.
The ramdisk keys can only be retrieved with the processor specific GID Key. The GID key is currently unretrievable and can only be utilized through the built-in AES engine. To complicate things even more, the engine is only accessible through a special bootrom or iBoot exploit (jailbreaks typically expose it with /dev/aes_0). This makes usage of the key nearly impossible.
However, once you have access to the AES engine, the entire system falls apart. You are able to upload an encrypted ramdisk and grab the decryption keys for it. Once you manage to decrypt the ramdisk, you can run it through GenPass to decrypt the firmware key.
Certain files share the same key and IV per application processor (per build) provided the devices have the same pixel resolution:
- Update Ramdisk
- Restore Ramdisk
- SEP Firmware
The firmware version number for the Apple TV builds are the ones that the Apple TV reports (also known as the "marketing version").
All dates are relative to UTC.
GID AES is used by iBoot to decrypt firmware images. When iBoot loads the kernelcache, GID AES is disabled. This means in order to get firmware keys, you must gain code execution in a setting where GID AES is still enabled. In most cases, this means exploiting iBoot itself, before the kernelcache is loaded.
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. This list also contains a few firmwares for which there never was an IPSW (as far as can be told) such as 4.2.5 for the CDMA iPhone 4 (iPhone3,3). These few builds came preinstalled on the device, but are not available for download.