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Savegames

Page Discussion
Revision as of 16:13, 2 December 2019 by Wwylele (talk | contribs) (→‎Tools: Bump my tools to the top. Sorry for self-promotion, but it is better than the old one in anyway now.)

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

Overview

Savegames are stored in DISA container format. Inside the DISA container, it forms a FAT filesystem. Please refer to these pages for how to fully extract save files. This page only describes additional encryption wear leveling on top of the DISA container. These layers only apply to gamecard save games. SD savegames and NAND savegames are DISA containers in plaintext after decrypting the common SD/NAND encryption layer.

Gamecard savegame Encryption

Repeating CTR Fail

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.

Savegame keyY

All gamecard and SD savegames are encrypted with AES-CTR. The base CTR for gamecard savegames is all-zero. The gamecard savegame keyslots' keyY(these savegame keyslots use the hardware key-generator) is unique for each region and for each game. The NCSD partition flags determine the method used to generate this keyY. When the save NCSD flags checked by the running NATIVE_FIRM are all-zero, the system will use the repeating CTR, otherwise a proper CTR which never repeats within the image is used.

The AES-CMAC (which uses a hardware key-generator keyslot, as mentioned above) at the the beginning of the savegame must match the calculated CMAC using the DISA/DIFF data, otherwise the savegame is considered corrupted(see below).

When all of the flags checked by the running NATIVE_FIRM are clear, the keyY(original keyY method used with saves where the CTR repeats within the image) is the following:

Offset Size Description
0x0 0x8 First 8-bytes from the plaintext CXI accessdesc signature.
0x8 0x4 u32 CardID0 from gamecard plaintext-mode command 0x90, Process9 reads this with the NTRCARD hw. The actual cmdID used by Process9 is different since Process9 reads it with the gamecard in encrypted-mode.
0xC 0x4 u32 CardID1 from gamecard plaintext-mode command 0xA0, Process9 reads this with the NTRCARD hw. The actual cmdID used by Process9 is different since Process9 reads it with the gamecard in encrypted-mode.

2.0.0-2 Hashed keyY and 2.2.0-4 Savegame Encryption

When certain NCSD partition flags are set, a SHA-256 hash is calculated over the data from the CXI(same data used with the original plain keyY), and the 0x40-bytes read from a gamecard command(this 0x40-byte data is also read by GetRomId, which is the gamecard-uniqueID). The first 0x10-bytes from this hash is used for the keyY. When flag[7] is set, the CTR will never repeat within the save image, unlike the original CTR-method. All games which had the retail NCSD image finalized after the 2.2.0-4 update(and contain 2.2.0-4+ in the System update partition), use this encryption method.

This keyY generation method was implemented with 2.0.0-2 via NCSD partition flag[3], however the proper CTR wasn't implemented for flag[7] until 2.2.0-4. The hashed keyY flag[3] implemented with 2.0.0-2 was likely never used with retail gamecards.

6.0.0-11 Savegame keyY

6.0.0-11 implemented support for generating the savegame keyY with a new method, this method is much more complex than previous keyY methods. This is enabled via new NCSD partition flags, all retail games which have the NCSD image finalized after the 6.0.0-11 release(and 6.0.0-11+ in the system update partition) will have these flags set for using this new method.

A SHA-256 hash is calculated over the same data used with the above hashed keyY method, after hashing the above data the following data is hashed: the CXI programID, and the ExeFS:/.code hash from the decrypted ExeFS header. An AES-CMAC (the keyslot used for this uses the hardware key-scrambler) is then calculated over this hash, the output CMAC is used for the savegame keyY.

The keyY used for calculating this AES-CMAC is initialized while NATIVE_FIRM is loading, this keyY is generated via the RSA engine. The RSA slot used here is slot0(key-data for slot0 is initialized by bootrom), this RSA slot0 key-data is overwritten during system boot. This RSA slot0 key-data gets overwritten with the RSA key-data used for verifying RSA signatures, every time Process9 verifies any RSA signatures except for NCCH accessdesc signatures. Starting with 7.0.0-13 this key-init function used at boot is also used to initialize a separate keyslot used for the new NCCH encryption method.

This Process9 key-init function first checks if a certain 0x10-byte block in the 0x01FF8000 region is all-zero. When all-zero it immediately returns, otherwise it clears that block then continues to do the key generation. This is likely for supporting launching a v6.0+ NATIVE_FIRM under this FIRM.

Gamecard wear leveling

The 3DS employs a wear leveling scheme on the savegame FLASH chips(only used for CARD1 gamecards). 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.

There are two versions of wear leveling have been observed. V1 is used for 128KB and 512 KB CARD1 flash chips. V2 is used for 1MB CARD1 flash chips (uncommon. Pokemon Sun/Moon is an exampe).

First, there are two 32-bit integers whose purposes are currently unknown. They generally increase the value as the savegame is written more times, so probably counter for how many times the journal became full and got flushed into the block map, and/or how many times alloc_cnt has wrapped around.

Then comes the actual blockmap. The block map contains entries of 10 bytes (V1) or 2 bytes (V2) with total number of (flash_size / 0x1000 - 1). The blockmap entry is simple: 

struct blockmap_entry_v1 {
        uint8_t phys_sec; // when bit7 is set, block is initialized and has checksums, otherwise checksums are all zero
        uint8_t alloc_cnt;
        uint8_t chksums[8];
} __attribute__((__packed__));

struct blockmap_entry_v2 {
        // Note that the phys_sec and alloc_cnt field are swapped in v2, 
        // but the initialized bit is still on the first byte
        uint8_t alloc_cnt; // when bit7 is set, block is initialized
        uint8_t phys_sec; 
        // v2 has no chksums
} __attribute__((__packed__));

There's one entry per 0x1000-byte sector, counting from physical sector 1 (sector 0 contains the blockmap/journal).

A 2-byte CRC16 follows the block map. For V1 it immediately follows the last block map entry. For V2 it is located at 0x3FE, and bytes before the CRC is padded with zero. The CRC16 checks all the bytes before it, including the two unknown integers, the block map, and the padding bytes for V2. The CRC standard used looks like CRC-16-IBM (modbus). Here is the code in Rust for it 

fn crc16(data: &[u8]) -> u16 {
    let poly = 0xA001;
    let mut crc = 0xFFFFu16;
    for byte in data {
        crc ^= <u16>::from(*byte);
        for _ in 0..8 {
            let b = crc & 1 != 0;
            crc >>= 1;
            if b {
                crc ^= poly;
            }
        }
    }
    crc
}

Then comes the journal. The journal contains entries that describes how sectors should be remapped. The rest bytes before 0x1000 after all journal entries are padded with 0xFF The journal entry structure is as follows: 

struct journal_entry_half {
        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];     // Unused & uninitialized for V2
} __attribute__((__packed__));

struct journal_entry{
        struct journal_entry_half entry;
        struct journal_entry_half dupe; // same data as `entry`. No idea what this is used fore
        uint32_t uninitialized;         // 0xFFFFFFFF in newer system
}__attribute__((__packed__));


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 (same CRC16 algorithm as above) is calculated, and the two bytes of the CRC16 are XORed together to produce the 8bit checksum

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

  • save3ds supports reading and modifying savegames, extdata and title database in FUSE filesystem or batch extracting/importing.
  • 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 CMAC must be used with /clean.sav, prior to writing output.sav to a gamecard. (This is an old tool that doesn't handle the savegame format correctly. --Wwylele (talk) 16:13, 2 December 2019 (CET))
  • 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).
  • wwylele's 3ds-save-tool supports extracting files from savegames and extdata. It properly reconstructs data from the DPFS tree and extracts files in directories hierarchy.

Japanese

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