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Added some info about the FS used in the EEPROM savefiles
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.
Savefiles stored on the EEPROM are using a custom FS.
It is still unknown how to get correctly to the file table, but here is the file entry struct.
struct FileEntry {
u32 Unknown;
u8 FileName[0x10];
u32 EntryID;
u32 Unknown;
u32 Unknown;
u32 FileSize;
u32 Unknown;
u32 Unknown;
u32 Unknown;
}