Usage

only tested on windows

remember to put SomeKey.bin along with main.exe

g++ --std=c++20 -O2 main.cpp -o main.exe

./main.exe enc "[path to epk]"
./main.exe dec "[path to epk.epk_dec]"

# example
# ./main.exe dec root#data#locale#ck#epk#uistring.epk
# and find the dec file in the same folder: root#data#locale#ck#epk#uistring.epk_dec

# ./main.exe enc root#data#locale#ck#epk#uistring.epk_dec
# and find the enc file in the same folder: root#data#locale#ck#epk#uistring.epk_enc

Note

There may be something wrong in the readme, please refer to code, I wrote this at 2 am, and was quite unconscious.

struct EPK {
  char*  variantLengthBuf;
  char   paddingZero[0x10];
  word  bufSize;         // variantLengthBuf size, big endian
  char paddingZero[0x10-sizeof(word)]
  char md5[0x10];         // Auto calculate when enc in main.exe, calc based on bufSize
}

the md5 is calculated from buf + "8FE9D249BD2689BB4B70F5AE88A9E645"(ascii char, 32bytes)

the buffer is always zero padded to word, but bufSize is the actual size, and md5, enc/dec are based on the word aligned buffer (aka padded size).

Tips

SomeKey.bin dumps from 0x1409E6500

FDT file decrypt script can be found in scripts folder, (but badly written in python)

Someone on steam community successfully repacked the epk with modified content: https://steamcommunity.com/app/2396980/discussions/0/4513255384647123281/

The game load epks from C:\Users<username>\AppData\Local\typemoon\fsn2\data by default (with subdirs root/data/locale/ck/epk/uistring.epk for example), though I dont think creating such patch/mod and delivering to players is a fancy idea XD.

Bonus:

The game determin the %LOCALAPPDATA% by env $env.LOCALAPPDATA, so start the game with modified $env.LOCALAPPDATA will redirect all things to the desired location, see FSNr_bonus.7z

So you can ship a mod/patch without hooking or repacking the .bin/.dat