IMG4 File Format

There is a img4 file (sep-firmware.img4) found in IPSWFILE/Firmware/all_flash/all_flash.DEVICEIDENTIFIER.production on any A7 (64bit) devices. It has a IMG40 header, but also refers to IM4P.

The IMG40 structure is as follows:

sequence [
   0: string "IMG40"
   1: string encryption type - IM4P
   2: string type - sepi (Secure Enclave Processor Image), ...
   3: ...
]

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