Known internally as "XCI" (NX Card Image), this is the format used for storing the contents of a Nintendo Switch Gamecard.
This header is 0x200 bytes and is located at offset 0 in the Gamecard.
|0x0||0x100||RSA-2048 PKCS #1 signature over the header (data from 0x100 to 0x200)|
|0x104||0x4||Secure Area Start Address (in Media Units which are 0x200 bytes)|
|0x108||0x4||Backup Area Start Address (always 0xFFFFFFFF)|
|0x10C||0x1||TitleKeyDec Index (high nibble) and KEK Index (low nibble)|
|0x10E||0x1||Gamecard Header Version|
|0x110||0x8||Package ID (used for challenge–response authentication)|
|0x118||0x8||Valid Data End Address (in Media Units which are 0x200 bytes)|
|0x120||0x10||Gamecard Info IV (reversed)|
|0x130||0x8||HFS0 partition offset|
|0x138||0x8||HFS0 header size|
|0x140||0x20||SHA-256 hash of the HFS0 Header|
|0x160||0x20||SHA-256 hash of the Initial Data|
|0x180||0x4||Security Mode (0x01 = T1, 0x02 = T2)|
|0x184||0x4||T1 Key Index (always 2)|
|0x188||0x4||Key Index (always 0)|
|0x18C||0x4||Normal Area End Address (in Media Units which are 0x200 bytes)|
|0x190||0x70||Gamecard Info (AES-128-CBC encrypted)|
When decrypted, this 0x70 byte region is as follows:
|0x0||0x8||Firmware Version (0x00 = Development, 0x01 = Retail, [4.0.0+] 0x02 = Retail)|
|0x8||0x4||Access Control (0x00A10011 = 25MHz access, 0x00A10010 = 50MHz access)|
|0xC||0x4||Read Time Wait1 (always 0x1388)|
|0x10||0x4||Read Time Wait2 (always 0)|
|0x14||0x4||Write Time Wait1 (always 0)|
|0x18||0x4||Write Time Wait2 (always 0)|
|0x24||0x1||[9.0.0+] Compatibility Type (0x00 = Normal, 0x01 = Terra)|
|0x28||0x8||Update Partition Hash|
|0x30||0x8||CUP ID (always 0x0100000000000816, which is the title-listing data archive's title ID)|
This is the Gamecard's unique certificate and is located at offset 0x7000.
|0x0||0x100||RSA-2048 PKCS #1 signature over the data from 0x100 to 0x200|
The data between the Gamecard Certificate and the start of the HFS0 region is all 0xFF, except for one BOTW cart that has been found, inwhich it is 0x00.
This data is used for challenge–response authentication when changing to the Gamecard's secure mode.
|0x0||0x8||Package ID from Gamecard Header at offset 0x110|
|0x10||0x10||Challenge–response authentication data|
|0x20||0x10||Challenge–response authentication MAC|
|0x30||0xC||Challenge–response authentication Nonce|
|0x3C||0x1C4||Reserved (must be empty)|
This is the Gamecard file system which starts with magicnum "HFS0".
The "SHA-256 File System" or "HFS0" starts at offset 0xF000 in the Gamecard. The first 0x200 bytes act as a global header and represent the root partition which points to the other partitions ("normal", "logo", "update" and "secure).
A hash for this header is stored at offset 0x140 in the Gamecard Header.
The actual file system is as follows (also valid for the root partition):
|0x4||0x4||Number of files|
|0x8||0x4||Size of the string table|
|0x10||X||File Entry Table|
|0x10 + X||Y||String Table|
|0x10 + X + Y||Z||Raw File Data|
Where File Entry Table consists of Number of Files FileEntries:
|0x0||0x8||Offset of file in Data|
|0x8||0x8||Size of file in Data|
|0x10||0x4||Offset of filename in String Table|
|0x14||0x4||Size of Hashed region of file (for HFS0s, this is the size of the pre-filedata portion, for NCAs this is usually 0x200)|
|0x20||0x20||SHA-256 hash of the first (size of hashed region) bytes of filedata|
The string table is 00-padded to align the start of raw filedata with a sector/media unit boundary (usually?).
Observed gamecards contain three partitions: "update", "normal", and "secure".
The update partition (Gamecard partition 0 for fsp-srv cmd 31) contains .cnmt.nca + .nca files for the entire system update required to play the game. Launch day carts contain a full copy of 1.0 ncas, newer carts contain newer sysupdate NCAs etc.
The normal partition contains the .cnmt.nca and the game icondata nca. This is presumably for future compatibility so that if a future update changes the cryptographic protocol for the secure partition, Game icon data can still be shown in the home menu on old firmwares.
The secure partition contains an identical copy of the .cnmt.nca and game icondata nca, as well as all other ncas required for the game.
The entire rest of the Gamecard after the secure partition ends is all FF padding.
[4.0.0+] The "normal" partition is now empty and a new partition "logo" was added.