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 Grabbing IMG3 Keys for an iBoot patch.

Derived keys
Some derived keys are computed by the IOAESAccelerator kernel service at boot. These keys are generated by encrypting static values either with the UID key (0x7D0 identifier) or the GID key (0x3E8 identifier). The values defined in the iPhone 4 5.0 kernel are : __text:807E3000 keys_to_compute DCD 0x835,0x7D0,0x1010101,0x1010101,0x1010101,0x1010101 __text:807E3018                DCD 0x899,0x7D0,0xB5FCE8D1,0x8DBF3739,0xD14CC7EF,0xB0D4F1D0 __text:807E3030                DCD 0x89B,0x7D0,0x67993E18,0x543CB06B,0xF568A46F,0x49BD0C1C __text:807E3048                DCD 0x89A,0x7D0,0x335B1FDB,0x1C5F6C60,0x66AA3419,0x61069C58

Key 0x835
Generated by encrypting 01010101010101010101010101010101 with the UID-key. Used for data protection.

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 to encrypt the SHSH blobs on the device.

Key 0x89B
Generated by encrypting 183E99676BB03C546FA468F51C0CBD49 with the UID-key. It is used to encrypt the data partition key.

Key 0x899
Generated by encrypting D1E8FCB53937BF8DEFC74CD1D0F1D4B0 with the UID-key. Usage unknown.

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