IMG4 File Format

The IMG4 file format is a replacement for the IMG3 file format and is used on devices running a 64-bit processor (A7 or newer). It's also used across the entire Apple Watch series, in both the armv7k and arm64_32 processors. Just like the old IMG3 file format, it is used to contain files used by iOS' secure boot chain (im4p files), signatures of those files (im4m/ApImg4Ticket files) and some info used in the Restore Process ("IM4R" can be found in MobileDevice framework). A complete IMG4 file is the result of concatenating the payload, manifest and restore info (aka boot nonce) in one file and wrapping it with an IMG4 header.

Structure[edit]

Contrary to the IMG3 file format where a custom binary format was used, IMG4 files (and their payloads) are DER encoded ASN.1 objects. The reason for Apple's choice of ASN.1/DER encoding over their own binary Property List (PList) file format is unknown.

sequence [
   0: string "IMG4"
   1: payload   - IMG4 Payload, IM4P
   2: [0] (constructed) [
          manifest   - IMG4 Manifest, IM4M
      ]
]

IMG4 Payload[edit]

Payload contains the actual content of the file. IMG4 format allows to store payload in a separate file with im4p extension. im4p files are used more often than complete IMG4 files. Payload is structured as follows:

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

Item 0 in the KBAG sequences is 01 for the production KBAG and 02 for the development KBAG. Development KBAGs can only be decrypted with "development" devices (Apple-internal prototypes or demoted production devices), since they use a different GID. The result of that decryption is the same iv+key for either dev or prod KBAGs.

IMG4 Manifest[edit]

A.k.a. IM4M. Contains signature for one or multiple IMG4 payloads. Manifests are designed in a way that they can be stored separately from an IMG4 file or be a part of one. An example of an IMG4 manifest is ApImg4Ticket. Each IMG4 manifest is made of tags structured as follows (content = DER encoded object):

[type] (private) [   - DER tag's class is the type of the tag (4 bytes) encoded as a big-endian integer
   sequence [
      string type    - type of the tag, 4 bytes
      content    - content of the tag
   ]
]

Manifest structure (tag <type> = tag of type 'type'):

sequence [
   0: string "IM4M"
   1: integer version    - currently 0
   2: set [
          tag MANB [   - manifest body
              set [
                  tag MANP [   - manifest properties
                      set [
                          tag <manifest property> [
                              content
                          ]
                          ...   - tags, describing other properties
                      ]
                  ]
                  tag <type> [   - ibot, illb, sepi, krnl, NvMR, bbcl...
                      set [
                          tag <tag property> [
                              content
                          ]
                          ...
                      ]
                  ]
                  ...   - tags for other images
              ]
          ]
      ]
   3: octet string signature
   4: sequence [   - containing certificate chain (arbitrary number of certificates)
          certificates
      ]

External Links[edit]