S5L File Formats

IMG2

This was the file format used prior to 2.0 firmware. Post 1.1.1, it was encrypted with Key 0x837. It can only be parsed by an iBoot in a firmware version less than beta 3, or the S5L8900 VROM. The S5L8720 has no support for it.

8900

This is the file format used by the S5L8900. Usually this wraps around an IMG2 file. It can only be parsed by an iBoot in a firmware version less than 2.0 beta 3, or the S5L8900 VROM. The S5L8720 has no support for it.

Header

typedef struct {
 uchar  magic[4];              // string "8900"
 uchar  version[3];            // string "1.0"
 uint8  format;                // plaintext format is 0x4, encrypted with GID-key format is 0x3, boot plaintext is 0x2, boot encrypted with UID-key is 0x1.
 uint32 unknown1;
 uint32 sizeOfData;            // size of data (ie, filesize - header(0x800) - footer signature(0x80) - footer certificate(0xC0A))
 uint32 footerSignatureOffset; // offset to footer signature 
 uint32 footerCertOffset;      // offset to footer certificate, from end of header (0x800)
 uint32 footerCertLen;
 uchar  salt[0x20];            // a seemingly random salt (an awfully big one though... needs more attention)
 uint16 unknown2;
 uint16 epoch;                 // the security epoch of the file
 uchar  headerSignature[0x10]; // encrypt(sha1(header[0:0x40])[0:0x10], key_0x837, zero_iv)
 uchar  padding[0x7B0];
} Apple8900Header;

Resources

The dev team's wiki page on the topic

IMG3

This is the replacement for the IMG2 File Format in iOS 2.0. The S5L8720 bootrom can understand this by default, but WTF 2.0 must be uploaded to the DFU mode of an S5L8900 that has code in it to parse IMG3 files, or the S5L8900 will not be able to understand them.

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
SDOM: Security Domain
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: Exclusive Chip ID

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, but the former remains unused. In the current method, iBoot will always use the first 16 bytes as the IV, then the remaining 16 (AES-128), 24 (AES-192, unused), or 32 (AES-256) bytes for the key.

Resources

cmw's IMG3 Unpacker