8900 File Format

This is the file format used by the S5L8900

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