IM4P File Format

The IM4P file format is a replacement of the IMG3 File Format used on older devices. It was introduced with iOS 7.0.1 and appears to be exclusive to the iPhone 5s and newer, iPad Air and iPad mini 2.

The file is encoded as an DER encoded asn.1 object, structured like this:

sequence [ 0: string "IM4P" 1: string type  - ibot, rdsk, sepi, ... 2: string description  - 'iBoot-1940.1.75' 3: octetstring   - the encrypted data 4: octetstring  - containing asn1 encoded KBAG values sequence [ sequence [ 0: int: 01 1: octetstring: iv              2: octetstring: key ]          sequence [ 0: int: 02 1: octetstring: iv              2: octetstring: key ]       ] ]

each .im4p file is accompanied with a .plist file containing the sha1 of the .im4p file.