IMG3 File Format

This is the replacement for the IMG2 file format for iOS 2.0 and onward.

Header

struct Img3 {
    u32          magic;   // fourcc="IMG3"
    u32       fullSize;   // full size of fw image
    u32     sizeNoPack;   // size of fw image without header
    u32   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)
    u32           iden;   // identifier of image, used when bootrom is parsing images
                          // list to find LLB (illb), LLB parsing it to find iBoot (ibot),
                          // etc.
 
} typedef Img3;

Tag Format

u32          magic;
u32   total_length;
u32    data_length;

Tags

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 KEY and IV 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 with iPhone OS.
TYPE: Type of image, should contain the same string as 'iden' of the header
DATA: Real content of the file

Signature Check

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

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.