IMG3 File Format

The IMG3 file format was introduced with iPhone OS 2.0b4 to replace the broken IMG2 format. This format is used on all devices with an A6X or older; A7 and newer devices use IM4P and IMG4 files.

Encryption

Apple got smarter this time, requiring the Hardware AES engine to be run per file. Decrypt the KBAG tag data with the GID Key to get the key and IV, and use those to decrypt the DATA section of the firmware file.

In the current method, iBoot will always use the first 16 bytes of the KBAG as the IV, then the remaining 16 (aes-128, used with S5L8900 and S5L8720), 24 (aes-192), or 32 (aes-256, used with S5L8920) bytes for the key.

Contents

All numbers are stored in little endian byte order; this includes the magic numbers (magic and ident).

typedef struct img3File {
    uint32_t magic;       // ASCII_LE("Img3")
    uint32_t fullSize;    // full size of fw image
    uint32_t sizeNoPack;  // size of fw image without header
    uint32_t sigCheckArea;// although that is just my name for it, this is the
                          // size of the start of the data section (the code) up to
                          // the start of the RSA signature (SHSH section)
    uint32_t ident;       // identifier of image, used when bootrom is parsing images
                          // list to find LLB (illb), LLB parsing it to find iBoot (ibot),
                          // etc. 
    img3Tag  tags[];      // continues until end of file
};

typedef struct img3Tag {
    uint32_t magic;            // see below
    uint32_t totalLength;      // length of tag including "magic" and these two length values
    uint32_t dataLength;       // length of tag data
    uint8_t  data[dataLength];
    uint8_t  pad[totalLength - dataLength - 12]; // Typically padded to 4 byte multiple
};

Tags

Due to being written in little-endian byte order, these tags are actually backwards when written to the file (e.g. VERS is stored as S R E V).

VERS: iBoot version of the image
SEPO: Security Epoch
SDOM: Security Domain
PROD: Production Mode
CHIP: Chip to be used with. example: 0x8900 for S5L8900.
BORD: Board to be used with
KBAG: Contains the IV and key required to decrypt; encrypted with the GID Key
SHSH: RSA encrypted SHA1 hash of the file
CERT: Certificate
ECID: Exclusive Chip ID unique to every device
TYPE: Type of image, should contain the same string as the header's ident
DATA: Real content of the file
NONC: Nonce used when file was signed.
CEPO: Chip epoch

Signature Check

Decryption is done using the modulus at cert + 0xA15
0xC to SHSH is SHAed