IMG4 File Format

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There is an img4 file (sep-firmware.img4) found in /usr/standalone/firmware on iOS 7.0.4 for the iPhone 5s. The IPSWFILE/Firmware/all_flash/all_flash.DEVICEIDENTIFIER.production for any A7 (64 bit) device and newer, contains an IM4P version of the same file. The file is an DER encoded ASN.1 object and structured as follows:

Header

30 - type tag indicating SEQUENCE
8  - ?
3  - 2,3,4 length in octets of value that follows (length of next value; size of file)
x octets - size of file from here
16 - indicating an [http://en.wikipedia.org/wiki/IA5String IA5String] to follow
04 - length in octets of IA5String that follows
49 4D 47 34 - IA5String, the magic "IMG4"
... - sequential data


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