Savegames

This page describes the format and encryption of savegames contained in gamecards, SD/NAND, and SD/NAND extdata. You can find savegames from various 3DS games on the Games page.

Savegame Encryption

On the 3DS savegames are stored much like on the DS, that is on a FLASH chip in the gamecart. On the DS these savegames were stored in plain-text but on the 3DS a layer of encryption was added. This is AES-CTR, as the contents of several savegames exhibit the odd behavior that xor-ing certain parts of the savegame together will result in the plain-text appearing.

The reason this works is because the stream cipher used has a period of 512 bytes. That is to say, it will repeat the same keystream after 512 bytes. The way you encrypt with a stream cipher is you XOR your data with the keystream as it is produced. Unfortunately, if your streamcipher repeats and you are encrypting a known plain-text (in our case, zeros) you are basically giving away your valuable keystream.

So how do you use this to decrypt a savegame on a 3DS? First off, you chunk up the savegame into 512 byte chunks. Then, you bin these chunks by their contents, discarding any that contain only FF. Now look for the most common chunk. This is your keystream. Now XOR the keystream with your original savegame and you should have a fully decrypted savegame. XOR with the keystream again to produce an encrypted savegame.

All gamecard and SD savegames are encrypted with AES-CTR. The gamecard savegame keyslot keyY is unique for every region of each game. A flag stored in the NCSD determines the method used to generate this keyY. This same flag is also used for determining which CTR period is used as well. The keyY when the flag is clear is generated from data stored in the main CXI, and data retrieved from gamecard commands. When the flag is set, a hash is generated for the keyY over the data from the CXI, and an ID retrieved from a gamecard command. The base CTR for gamecard savegames is all-zero. SD savegames use a CTR where the base CTR is fixed per savegame, however the CTR doesn't repeat in the image.

With system version 2.2.0-4 the system can now use a different gamecard CTR method, which fixed the above flaw. With 2.2.0-4 the system checks a NCSD flag, when it's set the new CTR period is used, otherwise the 0x200-byte CTR period is used. All games released since 2.2.0-4 have this flag set. The new CTR-period is larger than 0x1000-bytes.

Wear leveling

The 3DS employs a wear leveling scheme on the savegame FLASH chips. This is done through the usage of blockmaps and a journal. The blockmap is located at offset 0 of the flash chip, and is immediately followed by the journal. The initial state is dictated by the blockmap, and the journal is then applied to that.

First, there are 8 bytes whose purposes are currently unknown. Then comes the actual blockmap. The blockmap structure is simple:

struct header_entry {
        uint8_t phys_sec; // when bit7 is set, block has checksums, otherwise checksums are all zero
        uint8_t alloc_cnt;
        uint8_t chksums[8];
} __attribute__((__packed__));

There's one entry per sector, counting from physical sector 1 (sector 0 contains the blockmap/journal).

The 2 bytes that follow the blockmap are the CRC16 (with starting value 0xFFFF (like modbus)) of the first 8 bytes and the blockmap.

Then comes the journal. The journal structure is as follows:

struct sector_entry {
        uint8_t virt_sec;       // Mapped to sector
        uint8_t prev_virt_sec;  // Physical sector previously mapped to
        uint8_t phys_sec;       // Mapped from sector
        uint8_t prev_phys_sec;  // Virtual sector previously mapped to
        uint8_t phys_realloc_cnt;       // Amount of times physical sector has been remapped
        uint8_t virt_realloc_cnt;       // Amount of times virtual sector has been remapped
        uint8_t chksums[8];
} __attribute__((__packed__));

struct long_sector_entry{
        struct sector_entry sector;
        struct sector_entry dupe;
        uint32_t magic;
}__attribute__((__packed__));

With magic being a constant 0x080d6ce0.

The checksums in the blockmap/journal entries work as follows:

• each byte is the checksum of an encrypted 0x200 bytes large block
• to calculate the checksum, a CRC16 of the block (with starting value 0xFFFF) is calculated, and the two bytes of the CRC16 are XORed together to produce the 8bit checksum

MAC header

Image offset	Length	Description
0x00	0x10	AES-CBC MAC over a 0x20-byte SHA256 hash
0x10	0xF0	Zero padding

This MAC is used to "sign" the DISA/DIFF header. Each time the savegame is updated the hash stored in the DISA/DIFF is updated, therefore the MAC must be updated each time the save is modified as well. SHA256_Update() is used to calculate the hash with the blocks described below.

Savegame Types

Type	Description
CTR-EXT0	SD/NAND Extdata
CTR-SYS0	System SaveData
CTR-NOR0	Gamecard Savegames
CTR-SAV0	Savegames
CTR-SIGN	SD Savegames
CTR-9DB0	?

Extdata SHA256 Blocks

Block Size	Description
0x8	Savegame type
0x8	First word is the hex ID from image filename, second word is the hex ID of the sub-dir under the <ExtdataIDLow> directory (all-zero for Quota.dat)
0x4	1 for Quota.dat, 0 otherwise
0x8	Same as the previous u64
0x100	DIFF header

System SaveData SHA256 Blocks

Block Size	Description
0x8	Savegame type
0x8	SaveID
0x100	DISA header

CTR-NOR0 SHA256 Blocks

Block Size	Description
0x8	Savegame type
0x100	DISA header

CTR-SAV0 SHA256 Blocks

Block Size	Description
0x8	Savegame type
	Input data, for gamecard savegames this is the output SHA-256 hash from CTR-NOR0.

For gamecard savegames the output hash from this is used with the MAC. This save-type is also used for SD savegames, for SD saves the input data is the 0x100-byte DISA header. For SD savegames, the calculated output hash is used with CTR-SIGN.

CTR-SIGN SHA256 Blocks

Block Size	Description
0x8	Savegame type
0x8	ProgramID/SaveID
0x20	SHA-256 hash from CTR-SAV0

This is used for SD savegames, the calculated hash from this is used with the MAC.

Partitions

There can be multiple partitions in the image. The partitions are represented by tables of DIFI blobs inside a DISA/DIFF structure. The order of the DIFI blobs is the order of the partitions in the image.

DISA

• This is located @ 0x100 in the image, following the MAC header.
• If the uint32 @ 0x68 in the DISA(the low 8-bits) is non-zero, then the secondary table is is used, otherwise the primary table is used.
• If the table has more then 1 DIFI then the uint32 @ 0x168 is the offset from the DATA partition to the file base (masked with 0xFFFFFFFE).

Start	Length	Description
0x00	4	Magic ("DISA")
0x04	4	Magic Number (0x40000)
0x08	8	Total partition entries in a table
0x10	8	Offset to secondary partition table
0x18	8	Offset to primary partition table
0x20	8	Partition table size
0x28	8	SAVE Partition entry offset in the partition table
0x30	8	SAVE Partition entry length in the partition table
0x38	8	DATA Partition entry offset in the partition table
0x40	8	DATA Partition entry length in the partition table
0x48	8	SAVE Partition offset
0x50	8	SAVE Partition length
0x58	8	DATA Partition offset
0x60	8	DATA Partition length
0x68	4	Active table (and the offset to the filebase)
0x6C	0x20	Hash from active table
0x8C	0x74	Reserved

• The hash in the DISA hashes the Active Table (starting from tables's offset to tables's offset + table length) with SHA256.
• The partition offsets are absolute offsets in the image.
• The SAVE partition offset is usually 0x1000. The SAVE/DATA partitions begins with the DPFS partitions, the relative offset for the IVFC partition data is specified by the DPFS header.

The DIFIs table at offset 0x200 in the image has 2 DIFIs when the DATA partition isn't used, 4 DIFIs otherwise. Each partition table contains the SAVE DIFI entry and optionally the DATA entry. The secondary partition table is located at offset 0x200 in the image, and the primary table follows the secondary table.

The non-active table is for backup.

DIFF

• This is the extdata equivalent of DISA, for extdata which use FS. DIFF is only used for extdata.
• When the active-table field low 8-bits is non-zero, the secondary partition is used. Otherwise, the primary partition is used.

Start	Length	Description
0x00	4	Magic ("DIFF")
0x04	4	Magic Number (0x30000)
0x08	8	Secondary partition table offset
0x10	8	Primary partition table offset
0x18	8	Partition table length
0x20	4	Active table (and the offset to the filebase)
0x24	0x20	Unknown
0x34	0x20	Hash of the active partition table
0x54	0xAC	Unknown

DIFI

These 0x12C-byte blobs describe the partitions. Following each partition is an unused 0xFFFFFFFF cleartext word in the raw image. Every DIFI blob describes a partition. Partitions are catted together, so after the end of one partition is the beginning of the next.

For most games there's only 1 partition (The SAVE partition) and some (like Asphalt 3D, Steel Diver & Lego Star Wars III) has 2 partitions.

• 2 Partitions means that the files inside the SAVE partition is on the DATA partition.
• The DISA/DIFF headers support a maximum of 2 partitions.

Start	Length	Description
0x00	4	Magic ("DIFI")
0x04	4	Magic Number (0x10000)
0x08	8	Offset to "IVFC" blob in DIFI (Always 0x44)
0x10	8	Size of "IVFC" blob
0x18	8	Offset to "DPFS" blob in DIFI (Always 0xBC)
0x20	8	Size of "DPFS" blob
0x28	8	Offset to the hash in DIFI (Always 0x10C)
0x30	8	Size of this hash
0x38	4	Flags (when this byte is non-zero, this is a DATA partition)
0x3C	8	File base offset (for DATA partitions)

IVFC

Start	Length	Description
0x00	4	Magic ("IVFC")
0x04	4	Magic Number (0x20000)
0x08	0x8	Master hash size
0x10	0x8	Level 1 relative offset
0x18	0x8	Level 1 hashdata size
0x20	0x4	Level 1 block size, in log2
0x24	0x4	Reserved
0x28	0x8	Level 2 relative offset
0x30	0x8	Level 2 hashdata size
0x38	0x4	Level 2 block size, in log2.
0x3C	0x4	Reserved
0x40	0x8	Level 3 relative offset
0x48	0x8	Level 3 hashdata size
0x50	0x4	Level 3 block size, in log2.
0x54	0x4	Reserved
0x58	8	Level 4 filesystem relative offset
0x60	8	Level 4 filesystem size
0x68	8	Level 4 filesystem block size, in log2.
0x70	8	Unknown (usually 0x78=120)

• This savegame IVFC is almost identical to the RomFS IVFC, except for the additional filesystem level. Exactly like RomFS, each level except level4 is a hash-table where each hash entry hashes the data in the next level, padded to the log2 block size.

DPFS

Start	Length	Description
0x00	4	Magic ("DPFS")
0x04	4	Magic Number (0x10000)
0x08	8	Offset to first table
0x10	8	First table length
0x18	8	First table block size (1<<value)
0x20	8	Offset to second table
0x28	8	Second table length
0x30	8	Second table block size (1<<value)
0x38	8	IVFC partition offset
0x40	8	IVFC partition size
0x48	8	IVFC partition block size (1<<value)

• Every block this table point to is written twice (concatenated). You can see that the offset to the next block is twice the length (except the data which always begin after 0x1000).
• The offsets contained in the DPFS and IVFC are relative to the partition offset in the DISA/DIFF. The offsets from the IVFC are additionally added with the IVFC partition offset from the DPFS.

The first partition's data usually starts at 0x2000. First comes the hashtable (usually start @ 0x40 into the partition) and then the filesystem.

The hashtable entries' size is 2^x where x is the 'Filesystem block size' from the IVFC block.

DIFI Hash

The last 0x20-bytes of the partition following the DIFI, IVFC and DPFS is a SHA256 hash. The offset to this hash is stored in the DIFI. This hashes the IVFC level 1, with the buffer which is hashed aligned to the IVFC level 1 log2 block-size.

Summary Drawing

The SAVE partition

• The SAVE filesystem works with a backup. There are two SAVE blocks inside the partition concatenated. Which SAVE block is the updated one is unknown yet.. (I'm guessing from experience that (image[0x100B] & 0x20) == 0x20 --> 1st SAVE --Elisherer 01:30, 18 October 2011 (CEST))

Finding the folders table:

• If DATA partition exists: At folder table exact offset from the SAVE struct (from the beginning of the struct).
• Otherwise: The 'folder table offset' * 'folder table media' (=0x200) from the 'filestore offset'. (usually 0 from filebase)

Finding the files table:

• If DATA partition exists: At file table exact offset from the SAVE struct (from the beginning of the struct).
• Otherwise: The 'file table offset' * 'file table media' (=0x200) from the 'filestore offset'.

Detemining the filestore base:

• If DATA partition exists: At file base from the DATA's DIFI struct into the DATA partition.
• Otherwise: At the 'filestore offset' from the beginning of the SAVE struct.

Folder's entry structure:

 struct folder_entry {
     u32 parent_folder_index;
     u8  filename[0x10];
     u32 folder_index;
     u32 unk1; 
     u32 last_file_index;
     u32 unk3; 
     u32 unk4;
 }

File's entry structure:

 struct file_entry {
     u32 parent_folder_index;
     u8  filename[0x10];
     u32 index;
     u32 unk1; // magic?
     u32 block_offset;
     u64 file_size;
     u32 unk2; // flags?
     u32 unk3;
 }

The first entry in both tables is the count of the table, the parent directory index will be the amount of table rows. The root includes itself, so there are the amount - 1 (minus one) folders in the root directory (or files). The entries that follow after the root are the actual folders/files.

Reading the files out is as simple as taking the file base offset and adding (block_offset * 0x200) to it.

Here's a follow-up example from the Legend of Zelda: Ocarina of Time 3D:

//FST entry = SAVE base + File base + (FST offset * 0x200) + (FST entry # * 0x30)
//0x2600    = 0x2000    + 0x400     + (0x1        * 0x200) + (0x0         * 0x30)

00002600: 03000000 09000000 00000000 00000000  ................
00002610: 00000000 00000000 00000000 00000000  ................
00002620: 00000000 00000000 00000000 00000000  ................
00002630: 01000000 73797374 656D2E64 61740000  ....system.dat..
00002640: 00000000 00000000 D57B1100 02000000  ........Õ{......
00002650: 22000000 00000000 E8121500 00000000  ".......è.......
00002660: 01000000 73617665 30302E62 696E0000  ....save00.bin..
00002670: 00000000 01000000 69921100 03000000  ........i’......
00002680: DC140000 00000000 04000000 00000000  Ü...............

Start	Length	Description
0x00	4	Magic ("SAVE")
0x04	4	Magic Number (0x40000)
0x08	8	Offset to data in this SAVE header(normally 0x20)
0x10	8	Partition Size [medias]
0x18	4	Partition Media Size
0x1C	8	Unknown
0x24	4	Unknown
0x28	8	FolderMap Offset
0x30	4	FolderMap Size
0x34	4	FolderMap Media Size
0x38	8	FileMap Offset
0x40	4	FileMap Size
0x44	4	FileMap Media Size
0x48	8	BlockMap Offset
0x50	4	BlockMap Size
0x54	4	BlockMap Media Size
0x58	8	File store offset (from SAVE)
0x60	4	File store length [medias]
0x64	4	File store media size
0x68	4/8	Folders Table offset (8 bytes in DATA)
0x6C	4	Folders Table Length (medias) (Only in no DATA)
0x70	4	Folders Table unknown
0x74	4	Folders Table Media size
0x78	4/8	Files Table offset (8 bytes in DATA)
0x7C	4	Files Table Length (medias) (Only in no DATA)
0x80	4	Files Table unknown
0x84	4	Files Table Media size

• The FolderMap and FileMap still unknown. They are tables of uint32.
• The BlockMap is a map of the blocks in the filestore. An entry in the BlockMap is 2 uint32: {uint32 start_block; uint32 end_block; }. This is still being researched. (You can use 3DSExplorer to see those maps.

Summary Drawing

Initialization

When a save FLASH contains all xFFFF blocks it's assumed uninitialized by the game cartridges and it initializes default data in place, without prompting the user. The 0xFFFFFFFF blocks are uninitialized data. When creating a non-gamecard savegame and other images/files, it's initially all 0xFFFFFFFF until it's formatted where some of the blocks are overwritten with encrypted data.

I got a new game SplinterCell3D-Pal and I downloaded the save and it was 128KB of 0xFF, except the first 0x10 bytes which were the letter 'Z' (uppercase) --Elisherer 22:41, 15 October 2011 (CEST)

Fun Facts

If you have facts that you found out by looking at the binary files please share them here:

• From one save to another the game backups the last files that were in the partition and the entire image header in "random" locations.. --Elisherer 22:41, 15 October 2011 (CEST)

Tools

• 3dsfuse supports reading and modifying savegames. In the mounted FUSE filesystem, the /output.sav is the raw FLASH save-image. When the save was modified, a separate tool to update the MAC must be used with /clean.sav, prior to writing output.sav to a gamecard.
• 3DSExplorer supports reading of savegames, it doesn't support reading the new encrypted savegames and maybe in the future it will support modifying (some of the modyfing code is already implemented).

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