IMG3 File Format

This is the replacement for the IMG2 File Format in 2.0 firmware.

Header

struct Img3 {
    unsigned int      magic;   // fourcc="IMG3"
    unsigned int   fullSize;   // full size of fw image
    unsigned int sizeNoPack;   // size of fw image without header
    unsigned int 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)
    unsigned int       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

unsigned int magic;
unsigned int total_length;   //data_length+0xC
unsigned int data_length;

Tags

VERS: Version
SEPO: Unknown
PROD: Processor to be used with.
CHIP: Chip to be used with. "0x8900" for S5L8900 and "0x8720" for S5L8720. Instead of there being a check against some piece of hardware, whatever is verifying this (bootrom / iBoot / LLB / etc.) has this hardcoded in.
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: Electronic Chip ID unique to every iPhone 3GS.

Signature Check

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

Encryption

Apple got smarter this time, requiring the Hardware AES engine to be run per file. Decrypt the KBAG tag data (0x20 byte?) with the hardware AES engine and get the 0x10 byte IV and the 0x10 byte KEY.

iBoot has support for aes-192 and aes-256 also. In the current method, iBoot will always use the first 16 bytes as the IV, then the remaining 16 (aes-128, current method), 24 (aes-192), or 32 (aes-256) bytes for the key.

Resources

cmw's IMG3 Unpacker