Difference between revisions of "Savegames"

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(Correct info on the blockmap/journal. Also add a newly found CRC.)
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=== Encryption ===
+
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.
  
−
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 highly likely a streamcipher, 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.
+
== Overview ==
 +
Savegames are stored in [[DISA and DIFF|DISA container format]]. Inside the DISA container, it forms a [[Inner FAT|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 ==
 +
 
 +
Gamecard encryption is AES-CTR applied on top of DISA container, but below the wear leveling layer (if exists). The same key Y used for encryption is also used for DISA CMAC signing. Several versions of encryption scheme have been introduced over the time.
 +
 
 +
{| class="wikitable" border="1"
 +
|-
 +
!  FW Introduced
 +
!  Old3DS
 +
!  [[AES#Keyslot|AES Keyslots]] (Encryption / CMAC)
 +
!  KeyY generation method
 +
!  Repeating CTR
 +
|-
 +
| The initial version
 +
| style="background: #ccffbb" | Yes
 +
| 0x37 / 0x33
 +
| v1
 +
| style="background: #ccffbb" | Yes
 +
|-
 +
| [[2.0.0-2]]
 +
| style="background: #ccffbb" | Yes
 +
| 0x37 / 0x33
 +
| v2
 +
| style="background: #ccffbb" | Yes
 +
|-
 +
| [[2.2.0-4]]
 +
| style="background: #ccffbb" | Yes
 +
| 0x37 / 0x33
 +
| v2
 +
| style="background: #ffccbb" | No
 +
|-
 +
| [[6.0.0-11]]
 +
| style="background: #ccffbb" | Yes
 +
| 0x37 / 0x33
 +
| v3
 +
| style="background: #ffccbb" | No
 +
|-
 +
| [[9.6.0-24|9.6.0-X]]
 +
| style="background: #ffccbb" | No
 +
| 0x1A / 0x19
 +
| v2?
 +
| style="background: #ffccbb" | No
 +
|}
 +
 
 +
=== 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.
 
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.
Line 7: Line 54:
 
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.
 
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.
  
−
=== 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.
+
=== KeyY Generation method ===
 +
 
 +
The [[NCSD]] partition flags determine the method used to generate this keyY.
 +
 
 +
==== v1 ====
 +
 
 +
When all of the flags checked by the running NATIVE_FIRM are clear, the keyY is the following:
 +
{| class="wikitable" border="1"
 +
|-
 +
!  Offset
 +
!  Size
 +
!  Description
 +
|-
 +
| 0x0
 +
| 0x8
 +
| First 8-bytes from the plaintext [[NCCH#CXI|CXI]] accessdesc signature.
 +
|-
 +
| 0x8
 +
| 0x4
 +
| u32 CardID0 from [[Gamecards|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 [[Gamecards|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.
 +
|}
 +
 
 +
==== v2 ====
 +
 
 +
Key Y is the first 0x10 bytes of SHA-256 calculated over the following data
 +
 
 +
{| class="wikitable" border="1"
 +
|-
 +
!  Offset
 +
!  Size
 +
!  Description
 +
|-
 +
| 0x0
 +
| 0x8
 +
| First 8-bytes from the plaintext [[NCCH#CXI|CXI]] accessdesc signature.
 +
|-
 +
| 0x8
 +
| 0x40
 +
| read from a gamecard command(this 0x40-byte data is also read by [[Process_Services_PXI|GetRomId]], which is the gamecard-uniqueID)
 +
|}
 +
 
 +
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.
 +
 
 +
==== v3 ====
 +
 
 +
[[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.
 +
 
 +
First, a SHA-256 hash is calculated over the following data
 +
 
 +
{| class="wikitable" border="1"
 +
|-
 +
!  Offset
 +
!  Size
 +
!  Description
 +
|-
 +
| 0x0
 +
| 0x8
 +
| First 8-bytes from the plaintext [[NCCH#CXI|CXI]] accessdesc signature.
 +
|-
 +
| 0x8
 +
| 0x40
 +
| Same ID as [[Process_Services_PXI|GetRomId]]
 +
|-
 +
| 0x48
 +
| 0x8
 +
| CXI Program ID
 +
|-
 +
| 0x50
 +
| 0x20
 +
| ExeFS:/.code hash from the decrypted [[ExeFS]] header
 +
|}
 +
 
 +
Then an [[AES]]-CMAC is calculated over this hash. The output CMAC is used for keyY. The key slot for this CMAC is 0x2F.
 +
 
 +
The 0x2F keyY used for calculating this AES-CMAC (not to be confused with the final keyY for decrypting/signing savegames) 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|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 [[FIRM|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.
  
−
First, there are 8 bytes whose purposes are currently unknown. Then comes the blockmap.
+
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 example).
−
The blockmap structure is simple:
+
 
 +
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 <code>alloc_cnt</code> 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 <code>(flash_size / 0x1000 - 1)</code>.  
 +
The blockmap entry is simple:
 
<pre>
 
<pre>
−
struct header_entry {
+
struct blockmap_entry_v1 {
−
         uint8_t phys_sec; // when bit7 is set, block has checksums, otherwise checksums are all zero
+
         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 alloc_cnt;
 
         uint8_t chksums[8];
 
         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__));
 
} __attribute__((__packed__));
 
</pre>
 
</pre>
  
−
There's one entry per sector, counting from physical sector 1 (sector 0 contains the blockmap/journal).
+
There's one entry per 0x1000-byte sector, counting from physical sector 1 (sector 0 contains the blockmap/journal).
  
−
The 2 bytes that follow the blockmap are the CRC16 (modbus) of the first 8 bytes and the blockmap.
+
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
  
−
Then comes the journal.
 
−
The journal structure is as follows:
 
 
<pre>
 
<pre>
−
struct sector_entry {
+
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
 +
}
 +
</pre>
 +
 
 +
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:
 +
<pre>
 +
struct journal_entry_half {
 
         uint8_t virt_sec;      // Mapped to sector
 
         uint8_t virt_sec;      // Mapped to sector
 
         uint8_t prev_virt_sec;  // Physical sector previously mapped to
 
         uint8_t prev_virt_sec;  // Physical sector previously mapped to
Line 35: Line 195:
 
         uint8_t phys_realloc_cnt;      // Amount of times physical sector has been remapped
 
         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 virt_realloc_cnt;      // Amount of times virtual sector has been remapped
−
         uint8_t chksums[8];
+
         uint8_t chksums[8];     // Unused & uninitialized for V2
 
} __attribute__((__packed__));
 
} __attribute__((__packed__));
  
−
struct long_sector_entry{
+
struct journal_entry{
−
         struct sector_entry sector;
+
         struct journal_entry_half entry;
−
         struct sector_entry dupe;
+
         struct journal_entry_half dupe; // same data as `entry`. No idea what this is used fore
−
         uint32_t magic;
+
         uint32_t uninitialized;         // 0xFFFFFFFF in newer system
 
}__attribute__((__packed__));
 
}__attribute__((__packed__));
 
</pre>
 
</pre>
  
−
With magic being a constant 0x080d6ce0.
 
  
−
=== Partitions ===
+
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
  
−
There can be multiple partitions on the chip. For some games one is a backup partition, some other games seem to use only one partition, yet other games actually use multiple partitions. Partitions are defined at the start of the de-wearleveled blob. At offset 0x200 into the image, the DIFI blobs start. These 0x130 large blobs describe the partitions. Every DIFI blob describes a partition. In order to find the partitions, you will need the uint32_t at 0x9C into the DIFI block, and the uint32_t at 0xA4. The uint32_t at 0x9C describes the length of the hash table at the start of the partition, the uint32_t at 0xA4 is the length of the filesystem. Partitions are catted together, so the end of one partition is the beginning of the next.
+
== Initialization ==
  
−
The first partition starts at 0x2000. The hashtable at the start of the partitions describe each in-use block in the partition with a SHA256 of the 0x1000 sized block.
+
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.
  
−
* The exact location of the partition can vary in each save/game.
+
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) --[[User:Elisherer|Elisherer]] 22:41, 15 October 2011 (CEST)
−
* The first two hashes don't seem to be associated with any 0x1000 block.
 
−
* (edit) The last 0x20 bytes of the hash table, doesn't appear to change along with the rest of the data and repeats at the end of all other hash-tables, even when the hashes/data are different. (edit) The last 0x20 bytes of the hash table is NULL data, it is because the Hash table is only 0x1E0 in size and the XOR hash is 0x200 in size, so the 0x20 bytes you see at the end is actually 0x20 bytes of (FF) xor'd with the last 0x20 bytes of the key. Thus the data recurs. --[[User:Immortal|Immortal]] 09:14, 19 August 2011 (GMT)
 
  
−
The hash in the DISA blob hashes 300 bytes of the first DIFI blob.
+
== Fun Facts ==
−
 
 
−
* If the uint32 before the hash in the DISA is 0x01, the first DIFI blob is hashed, if it's 0x00 the second DIFI is hashed. The offsets and size for each DIFI can be found beneath the DISA tag (10h, 20h and 18h, 30h relative to the DISA location).
 
−
 
 
−
=== Filesystem ===
 
−
 
−
Savefiles are stored on the FLASH in a custom filesystem called SAVE. SAVE has a header which describes where the various bits of the filesystem live. The most important is the FST or filesystem table. You can find the FST by first finding the file base offset which is the offset to which all the entries in the FST are relative. The file base offset is a uint16_t at 0x58 from the filesystem start. The FST offset is a uint32_t at 0x6C and is in blocks (which are 0x200 bytes large).
 
−
 
 
−
Once you've found the FST, parsing it is fairly straightforward.
 
−
 
 
−
<pre>
 
−
struct fs_entry {
 
−
    u32 node_cnt;
 
−
    u8  filename[0x10];
 
−
    u32 index;
 
−
    u32 unk1; // magic?
 
−
    u32 block_offset;
 
−
    u32 file_size;
 
−
    u32 unk2;
 
−
    u32 unk3; // flags and/or date?
 
−
    u32 unk4;
 
−
}
 
−
</pre>
 
−
 
 
−
The first entry is the root directory, easily identifiable by the node_cnt being larger than 1. The node_cnt includes the root directory itself, so there are node_cnt - 1 files in the root directory. The entries that follow after the root directory are the actual files. Reading them out is as simple as taking the file base offset and adding (block_offset * 0x200) to it.
 
−
 
 
−
Example from Super MonkeyBall 3D:
 
−
<pre>
 
−
0003800: 04000000 21000000 00000000 00000000  ....!...........
 
−
0003810: 00000000 00000000 00000000 00000000  ................
 
−
0003820: 00000000 00000000 00000000 00000000  ................
 
−
0003830: 01000000 736d6233 64732e64 61740000  ....smb3ds.dat..
 
−
0003840: 00000000 00000000 d57b1100 05000000  .........{......
 
−
0003850: e4060000 00000000 c8cf0008 00000000  ................
 
−
0003860: 01000000 6d677265 706c6179 30302e64  ....mgreplay00.d
 
−
0003870: 61740000 01000000 d57b1100 09000000  at.......{......
 
−
0003880: 1c210000 00000000 cd331000 00000000  .!.......3......
 
−
0003890: 01000000 6d677265 706c6179 30312e64  ....mgreplay01.d
 
−
00038a0: 61740000 02000000 d57b1100 1a000000  at.......{......
 
−
00038b0: 1c210000 00000000 00000000 00000000  .!..............
 
−
</pre>
 
  
−
=== Initialization ===
+
If you have facts that you found out by looking at the binary files please share them here:
  
−
When a save EEPROM contains all xFFFF blocks it's assumed uninitialized by the game cartridges and it initializes default data in place, without prompting the user.
+
* From one save to another the game backups the last files that were in the partition and the entire image header in "random" locations.. --[[User:Elisherer|Elisherer]] 22:41, 15 October 2011 (CEST)
  
 +
== Tools ==
  
 +
* [https://github.com/wwylele/save3ds save3ds] supports reading and modifying savegames, extdata and title database in FUSE filesystem or batch extracting/importing.
 +
* [https://github.com/3dshax/3ds/tree/master/3dsfuse 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. --[[User:Wwylele|Wwylele]] ([[User talk: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).
 +
* [https://github.com/wwylele/3ds-save-tool 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.
  
 
[[セーブデータ|Japanese]]
 
[[セーブデータ|Japanese]]

Latest revision as of 15:15, 3 September 2021

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[edit]

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[edit]

Gamecard encryption is AES-CTR applied on top of DISA container, but below the wear leveling layer (if exists). The same key Y used for encryption is also used for DISA CMAC signing. Several versions of encryption scheme have been introduced over the time.

FW Introduced Old3DS AES Keyslots (Encryption / CMAC) KeyY generation method Repeating CTR
The initial version Yes 0x37 / 0x33 v1 Yes
2.0.0-2 Yes 0x37 / 0x33 v2 Yes
2.2.0-4 Yes 0x37 / 0x33 v2 No
6.0.0-11 Yes 0x37 / 0x33 v3 No
9.6.0-X No 0x1A / 0x19 v2? No

Repeating CTR Fail[edit]

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.


KeyY Generation method[edit]

The NCSD partition flags determine the method used to generate this keyY.

v1[edit]

When all of the flags checked by the running NATIVE_FIRM are clear, the keyY 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.

v2[edit]

Key Y is the first 0x10 bytes of SHA-256 calculated over the following data

Offset Size Description
0x0 0x8 First 8-bytes from the plaintext CXI accessdesc signature.
0x8 0x40 read from a gamecard command(this 0x40-byte data is also read by GetRomId, which is the gamecard-uniqueID)

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.

v3[edit]

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.

First, a SHA-256 hash is calculated over the following data

Offset Size Description
0x0 0x8 First 8-bytes from the plaintext CXI accessdesc signature.
0x8 0x40 Same ID as GetRomId
0x48 0x8 CXI Program ID
0x50 0x20 ExeFS:/.code hash from the decrypted ExeFS header

Then an AES-CMAC is calculated over this hash. The output CMAC is used for keyY. The key slot for this CMAC is 0x2F.

The 0x2F keyY used for calculating this AES-CMAC (not to be confused with the final keyY for decrypting/signing savegames) 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[edit]

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 example).

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[edit]

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[edit]

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[edit]

  • 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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