IMG3 File Format

This is the replacement for the IMG2 file format for iOS 2.0 and onward. iOS 7.0.1 introduced IM4P File Format, exclusive to the iPhone 5c and iPhone 5s.

Header
typedef struct Img3_t { uint32_t magic;      // ASCII("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 iden;       // identifier of image, used when bootrom is parsing images // list to find LLB (illb), LLB parsing it to find iBoot (ibot), // etc. };

Tag Format
typedef struct Img3Tag_t { uint32_t magic; uint32_t total_length; uint32_t 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.