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IMG3 File Format
This is the replacement for the IMG2 File Format from firmware 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; // data_length+0xC 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.
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 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.