The following text tries to document the structure of the CTR Executable Image (CXI) format.
Overview
A CXI file contains the 'application' program code, which runs on a single ARM11 core. It can communicate through SVC calls with the other ARM11 core running the 'system' program code. For reasons of clarity, the ARM11 cores will sometimes be called the 'appcore' and 'syscore' respectively.
The CXI file format contains application ARM11 code, the menu icon, the menu 3D model, and an embedded read-only (ROM) filesystem for external filestorage. In fact, the application ARM11 code, menu icon, and menu 3D model are embedded into its own filesystem, called the ExeFS.
More specifically, the CXI file format is structured in the following order:
- first an NCCH header,
- followed by an extended header,
- followed by a plain binary region,
- followed by an embedded executable filesystem (ExeFS),
- and finally followed by a read-only filesystem (RomFS).
The extended header contains additional information regarding access control. The plain binary region is an area specifically stored in plaintext, mostly containing SDK library strings for identification.
The extended header, the ExeFS and the RomFS are encrypted using AES CTR.
NCCH Header
| OFFSET | SIZE | DESCRIPTION |
|---|---|---|
| 0x000 | 0x100 | RSA-2048 signature of the NCCH header, using SHA-256. |
| 0x100 | 4 | Magic ID, always 'NCCH' |
| 0x104 | 4 | Content size, in media units (1 media unit = 0x200 bytes) |
| 0x108 | 8 | Partition ID |
| 0x110 | 2 | Maker code |
| 0x112 | 2 | Version |
| 0x114 | 4 | Reserved |
| 0x118 | 8 | Program ID |
| 0x120 | 1 | Temp flag |
| 0x121 | 0x2F | Reserved |
| 0x150 | 0x10 | Product code |
| 0x160 | 0x20 | Extended header hash |
| 0x180 | 4 | Extended header size |
| 0x184 | 4 | Reserved |
| 0x188 | 8 | Flags |
| 0x190 | 4 | Plain region offset, in media units |
| 0x194 | 4 | Plain region size, in media units |
| 0x198 | 8 | Reserved |
| 0x1A0 | 4 | ExeFS offset, in media units |
| 0x1A4 | 4 | ExeFS size, in media units |
| 0x1A8 | 4 | ExeFS hash region size, in media units |
| 0x1AC | 4 | Reserved |
| 0x1B0 | 4 | RomFS offset, in media units |
| 0x1B4 | 4 | RomFS size, in media units |
| 0x1B8 | 4 | RomFS hash region size, in media units |
| 0x1BC | 4 | Reserved |
| 0x1C0 | 0x20 | ExeFS superblock hash |
| 0x1E0 | 0x20 | RomFS superblock hash |
NCCH header example for Lego Starwars III
Magic: NCCH
Signature: 720FF8F83F2A1E998322A026D1434165
ED19642ABC1CB2722135AA202BEAD60A
80BCD21C768C597B8268FEF2C64EA710
4C9BA5E12CFFBD1D0C619F4EF7B42CA7
DD8482CB4EB26720AD66CDA57ABCBCFB
D63268A6E2896A59B3B744E39E45B88A
ABB4C0980ACC6210818DCE6DAC838A10
95D0F66B352474D4B3DA4B333F49912D
29AF7EA58BC8C890B18C70B7D540A9FB
EBE24A5312055617D3353B28C3EB1D17
61021BEFF6AD22C384835B40BD44DFAD
981F6350F9458B17BCB5F768C92ABC93
2BCE9888855A8998F4CDE40C9543514A
C57B84EB75A680E7C742632614620D1D
A253284DF3DC01091EB3800C36FD62EE
BA15340F1FD498FAB67C0302E9CDA397
Content size: 0x000e7f7a
Partition id: 0004000000038c00
Maker code: 3436
Version: 0002
Program id: 0004000000038c00
Temp flag: 00
Product code: CTR-P-ALGP
Extended header hash: 0C27E3C1DE7B2AE2D3114F32A4EEBF46
9AFD0CF352C11D4984C2A9F1D2144C63
Extended header size: 00000400
Flags: 0000030100000000
Plain region offset: 0x00004a00
Plain region size: 0x00000200
ExeFS offset: 0x00004c00
ExeFS size: 0x00143800
ExeFS hash region size: 0x00000200
RomFS offset: 0x00148400
RomFS size: 0x1ceab000
RomFS hash region size: 0x00000200
ExeFS Superblock Hash: 130C042615F647C4C63225EA9E67F8A2
7B15246B88FBC7A927257B84977B787B
RomFS Superblock Hash: A65BEE1060BB6A6821BBCEC600035B7E
64FB6EACA7F0960CFB1F5A37087728F7