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Difference between revisions of "IMG4 File Format"
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− | IMG4 is a |
+ | The '''IMG4 file format''' is a replacement for the [[IMG3 File Format|IMG3 file format]] and is used on devices running a 64-bit processor (A7 or newer) and the [[Apple Watch]]. Just like the old IMG3 file format, it is used to contain files used by iOS' secure boot chain ([[#Payload|'''im4p''' files]]), signatures of those files (<code>ApImg4Ticket</code>) and likely some info used in the [[Restore_Process|Restore Process]] ("<code>IM4R</code>" can be found in MobileDevice framework). The only known place in iOS filesystem where a complete IMG4 file can be found is [[/usr/standalone/firmware/sep-firmware.img4]]. |
+ | == Structure == |
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− | IMG4 files are [http://en.wikipedia.org/wiki/Abstract_Syntax_Notation_One#Example_encoded_in_DER DER encoded ASN.1 objects] structured as follows: |
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+ | Contrary to the [[IMG3 File Format|IMG3 file format]] where a custom binary format was used, IMG4 files (and their payloads) are [[wikipedia:X.690#DER encoding|DER]] encoded [[wikipedia:Abstract Syntax Notation One|ASN.1]] objects. The reason for Apple's choice of ASN.1/DER encoding over their own [[PList File Format|Property List]] (PList) file format is currently unknown. |
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== External Links == |
== External Links == |
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− | * [https://twitter.com/i0n1c/status/501299082603020288 i0n1c's tweet] |
+ | * [https://twitter.com/i0n1c/status/501299082603020288 i0n1c's tweet]<!-- TODO: Work this into the article --> |
+ | * [https://www.itu.int/rec/T-REC-X.690/en X.690 - ASN.1 encoding rules: Specification of Basic Encoding Rules (BER), Canonical Encoding Rules (CER) and Distinguished Encoding Rules (DER)] |
Revision as of 00:54, 5 April 2016
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) and the Apple Watch. 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 (ApImg4Ticket
) and likely some info used in the Restore Process ("IM4R
" can be found in MobileDevice framework). The only known place in iOS filesystem where a complete IMG4 file can be found is /usr/standalone/firmware/sep-firmware.img4.
Structure
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 Property List (PList) file format is currently unknown.
sequence [ 0: string "IMG4" 1: payload - IMG4 Payload, IM4P 2: [0] (constructed) [ manifest - IMG4 Manifest, IM4M ] ]
IMG4 Payload
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 ] ] ]
IMG4 Manifest
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 ]