AES Keys

The in each device have an  coprocessor with the GID-key and UID-key built in.

Running The Engine
Currently, there are several ways to run the hardware AES engine:
 * Patch iBoot to jump to aes_decrypt.
 * Use OpenIBoot.
 * Use the crypto bundle provided in XPwn to utilize it via userland. This method requires a kernel patch.
 * Use Greenpois0n console.

If you want to decrypt IMG3 files you need to use this. The GID-key currently has not been extracted from the phone, so the only way to use it is on the phone itself.

See Easier method of getting Img3 Key / IV for an iBoot patch.

Key 0x835
This is the UID-key.

Key 0x837
Generated by encrypting 345A2D6C5050D058780DA431F0710E15 with the S5L8900 GID-key, resulting in 188458A6D15034DFE386F23B61D43774.

It is used as the encryption key for IMG2 files. With the introduction of IMG3 in iOS 2.0, KBAGs are now used instead of the 0x837 key. Because iOS versions 1.x were used only on the iPhone and iPod touch (both use the S5L8900) the encrypted values for other processors don't matter.

Key 0x89A
For A4 devices: Generated by encrypting DB1F5B33606C5F1C1934AA66589C0661 with the UID-key, getting a device-specific key.

It is used as the public key for the SHSH blobs on the device.

Using greenpois0n to get the keys

 * Run steps 1 thru 5 from PwnStrap
 * Use 'xpwntool file.img3 /dev/null' to extract the KBAG hex string from file.img3
 * Start greenpois0n console: irecovery -s
 * Execute 'go aes dec _KBAG_STRING_' in irecovery console

Resources
Dev Team wiki