NCSD

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This page documents the format of NCSD.

Overview[edit]

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[edit]

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[edit]

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[edit]

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)[edit]

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[edit]

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[edit]

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 puts what many refer to as "private headers" here in place of actual development card information. This header is constituted by a cartridge-unique ID obtained from pxi:ps9::GetRomId and the title-unique cart ID (identical for all carts of the same title; can be retrieved using the NTR gamecard protocol command 0x90 or through the CTR protocol commands 0x90 or 0xA2).

InitialData[edit]

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[edit]

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[edit]

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

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