NCSD

This page documents the format of NCSD.

Overview

There are two known specialisations of the NCSD container format. The CTR Cart Image (CCI) format and the 3DS' raw NAND format. CCI is the format of game ROM images.

CTR System Update (CSU) is a variant of CCI, where the only difference is in the file extension. This is used with developer System Updates and associated Tools.

NCSD images start with a NCSD header, followed by up to a maximum of 8 NCCH partitions.

For CCI images, the partitions are reserved as follows:

NCCH Index	Reserved Use
0	Executable Content (CXI)
1	E-Manual (CFA)
2	Download Play Child container (CFA)
6	New3DS Update Data (CFA)
7	Update Data (CFA)

The format of partitions can be determined from the partition FS flags (normally these are zero for CCI/CSU NCSD Images).

NCSD header

Offset	Size	Description
0x000	0x100	RSA-2048 SHA-256 signature of the NCSD header
0x100	4	Magic Number 'NCSD'
0x104	4	Size of the NCSD image, in media units (1 media unit = 0x200 bytes)
0x108	8	Media ID
0x110	8	Partitions FS type (0=None, 1=Normal, 3=FIRM, 4=AGB_FIRM save)
0x118	8	Partitions crypt type (each byte corresponds to a partition in the partition table)
0x120	0x40=(4+4)*8	Offset & Length partition table, in media units
0x160	0xA0	...

For carts,

Offset	Size	Description
0x160	0x20	Exheader SHA-256 hash
0x180	0x4	Additional header size
0x184	0x4	Sector zero offset
0x188	8	Partition Flags (See Below)
0x190	0x40=8*8	Partition ID table
0x1D0	0x20	Reserved
0x1F0	0xE	Reserved?
0x1FE	0x1	Support for this was implemented with 9.6.0-X FIRM. Bit0=1 enables using bits 1-2, it's unknown what these two bits are actually used for(the value of these two bits get compared with some other value during NCSD verification/loading). This appears to enable a new, likely hardware-based, antipiracy check on cartridges.
0x1FF	0x1	Support for this was implemented with 9.6.0-X FIRM, see below regarding save crypto.

For NAND,

Offset	Size	Description
0x160	0x5E	Unknown
0x1BE	0x42	Encrypted MBR partition-table, for the TWL partitions(key-data used for this keyslot is console-unique).

NCSD Signature

The RSA public key used for gamecard NCSD is stored in ITCM. The separate public key used for NAND NCSD is stored in Process9 .(ro)data instead of ITCM, and in boot ROM.

For the boot ROM check, sighax may be used to fake-sign NAND headers. Process9's check will fail, however, unless patched.

Partition Flags

Byte Index	Description
0	Backup Write Wait Time (The time to wait to write save to backup after the card is recognized (0-255 seconds)).NATIVE_FIRM loads this flag from the gamecard NCSD header starting with 6.0.0-11.
3	Media Card Device (1 = NOR Flash, 2 = None, 3 = BT) (SDK 3.X+)
4	Media Platform Index (1 = CTR)
5	Media Type Index (0 = Inner Device, 1 = Card1, 2 = Card2, 3 = Extended Device)
6	Media Unit Size i.e. u32 MediaUnitSize = 0x200*2^flags[6];
7	Media Card Device (1 = NOR Flash, 2 = None, 3 = BT) (Only SDK 2.X)

Partition Flags (In Terms of Save Crypto Determination)

Byte Index	Description
1	Starting with 6.0.0-11 NATIVE_FIRM will use this flag to determine the gamecard savegame keyY method, when flag[3] is set. 0 = 2.0.0-2 hashed keyY, 1 = new keyY method implemented with 6.0.0-11. 0x0A = implemented with 9.3.0-X. On Old3DS this is identical to the 2.2.0-4 crypto. On New3DS this is identical to the 2.2.0-4 crypto, except with New3DS-only gamecard savedata keyslots. Starting with 9.6.0-X FIRM, Process9 now sets <savecrypto_stateval> to partitionflag[1] + <the u8 value from NCSD+0x1FF>, instead of just setting it to partitionflag[1].
3	Support for this flag was implemented in NATIVE_FIRM with 2.0.0-2. When this flag is set the hashed gamecard savegame keyY method is used, this likely still uses the repeating-CTR however. With 6.0.0-11 the system will determine the gamecard savegame keyY method via flag[1], instead of just using the hashed keyY via this flag.
7	This flag enables using the hashed gamecard savegame keyY method, support for this flag was implemented in NATIVE_FIRM with 2.2.0-4. All games with the NCSD image finalized since 2.2.0-4(and contains 2.2.0-4+ in the system update partition) have this flag set, this flag also enables using new CTR method as well.

Starting with 9.6.0-X FIRM, Process9 will just write val0 to a state field then return 0, instead of returning an error when the save crypto type isn't recognized. This was the *only* actual functionality change in the Old3DS Process9 function for gamecard savedata crypto init.

Card Info Header

OFFSET	SIZE	DESCRIPTION
0x200	4	CARD2: Writable Address In Media Units (For 'On-Chip' Savedata). CARD1: Always 0xFFFFFFFF.
0x204	4	Card Info Bitmask
0x208	0xF8	Reserved
0x300	4	Filled size of cartridge
0x304	0xC	Reserved
0x310	2	Title version
0x312	2	Card revision
0x314	0xC	Reserved
0x320	8	Title ID of CVer in included update partition
0x328	2	Version number of CVer in included update partition
0x32A	0xCD6	Reserved

Development Card Info Header Extension

OFFSET	SIZE	DESCRIPTION
0x1000	0x200	InitialData
0x1200	0x200	CardDeviceReserved1
0x1400	0x10	TitleKeyData
0x1410	0x1BF0	CardDeviceReserved2
0x3000	0x1000	TestData

TitleKeyData contains the decrypted version of the title key found in the InitialData. This field appears to be what development--and maybe production?--cards read to know what card encryption seed to use in the CTR protocol.

The CardDeviceReserved areas have random-looking data whose purpose is unknown, other than perhaps to hide the TitleKey.

Note that a particular flashcard vendor, namely Gateway, puts what many refer to as "private headers" at CardDeviceReserved1 in place of actual development card information. This header consists of:

OFFSET	SIZE	DESCRIPTION
0x0	0x40	Unique cartridge ID; only the first 0x10 bytes are meaningful, the rest are 0xff; obtainable using encrypted 16-byte cartridge command 0xc6; the first 0x10 bytes can also be obtained in userland via pxi:ps9::GetRomId
0x40	0x4	Cartridge ID1; obtainable using 8-byte cartridge command 0x90 or 16-byte cartridge command 0xa2
0x44	0x4	Cartridge ID2; obtainable using 8-byte cartridge command 0xa0 or 16-byte cartridge command 0xa4
0x48	0x8	Padding (all-0xff)

The legitimacy of the unique cartridge ID can be validated by online services.

Some dumping tools, notably GodMode9 as of 2024-05-26, erroneously always write 0x00000000 into the position of the Cartridge ID2. This is presumably because the cartridge ID2 is always zero for retail carts.

InitialData

OFFSET	SIZE	DESCRIPTION
0x0	0x10	Seed (keyY used to decrypt the title key - keyX is keyslot 0x3B for production cards, or a key of all zeroes for development cards)
0x10	0x10	TitleKey (AES-CCM encrypted)
0x20	0x10	Mac
0x30	0xC	Nonce
0x3C	0xC4	Reserved
0x100	0x100	NcchHeader (copy of the first NCCH header, excluding the RSA signature)

TestData

The test data is the same one encountered in development DS/DSi cartridges. Its layout is as follows:

OFFSET	SIZE	DESCRIPTION
0x0	0x8	The bytes FF 00 FF 00 AA 55 AA 55.
0x8	0x1F8	An ascending byte sequence equal to the offset mod 256 (08 09 0A ... FE FF 00 01 ... FF).
0x200	0x200	A descending byte sequence equal to 255 minus the offset mod 256 (FF FE FD ... 00 FF DE ... 00).
0x400	0x200	Filled with 00 (0b00000000) bytes.
0x600	0x200	Filled with FF (0b11111111) bytes.
0x800	0x200	Filled with 0F (0b00001111) bytes.
0xA00	0x200	Filled with F0 (0b11110000) bytes.
0xC00	0x200	Filled with 55 (0b01010101) bytes.
0xE00	0x1FF	Filled with AA (0b10101010) bytes.
0xFFF	0x1	The final byte is 00 (0b00000000).

Production cards always return FF when attempting to read 0x1200-0x3FFF. They probably actually have the same data as development cards, but there's no way to read it.

Tools

ctrtool - (CMD)(Windows/Linux) Parsing NCSD files

3DSExplorer - (GUI)(Windows Only) Parsing NCSD files