dectool.py

import sys
import os
from lib import enc_key, map_out, decrypt

in_file = len(sys.argv) > 1 and sys.argv[1]
while not in_file or not os.path.exists(in_file): 
    in_file = input("in: ")
in_file = os.path.relpath(in_file)

out_file, des_magic = map_out(in_file)
out_file = os.path.join(os.path.dirname(in_file), out_file)
print(f"out: {out_file}{os.path.exists(out_file) and ' (exists)' or ''}")
input()

decrypt(in_file, out_file, enc_key, des_magic)

dectoolbatch.py

import glob
import os
from lib import enc_key, map_out, decrypt

os.makedirs("out", exist_ok=True)
for in_file in glob.glob("../*.rpgmvo"):
    out_file, des_magic = map_out(in_file)
    out_file = os.path.join("out/", out_file)
    print(in_file, "->", out_file)
    decrypt(in_file, out_file, enc_key, des_magic)

lib.py

import os

ext_map = {
    ".rpgmvp": ".png",
    ".rpgmvo": ".ogg"
}
ext_magic = {
    ".png": [b"\x89PNG"],
    ".ogg": [b"OggS", b"ID3\x03", b"RIFF"]
}

with open("encryptionKey","rb") as enc_key_f:
    enc_key = bytearray(enc_key_f.read(16))

def map_out(in_file):
    in_file = os.path.basename(in_file)
    out_ext = ext_map.get(os.path.splitext(in_file)[1].lower(), ".dec")
    return os.path.splitext(in_file)[0] + out_ext, ext_magic.get(out_ext, None)

def decrypt(in_file, out_file, enc_key, des_magic):
    with open(in_file, "rb") as in_file_f, open(out_file, "wb") as out_file_f:
        if in_file_f.read(6) != b"RPGMV\x00":
            raise Exception("not RPG Maker MV-encrypted")
        in_file_f.read(10)

        hit_magic = -1
        for encb, keyb, counter in zip(in_file_f.read(16), enc_key, range(16)):
            decb = encb ^ keyb
            out_file_f.write(bytes([decb]))

            if counter < 4 and des_magic and hit_magic >= -1:
                if hit_magic < 0:
                    for magic, i in zip(des_magic, range(len(des_magic))):
                        if magic[counter] == decb:
                            hit_magic = i
                elif des_magic[hit_magic][counter] != decb:
                    hit_magic = -2

        out_file_f.write(in_file_f.read())

    if hit_magic < 0:
        print("WARNING: file doesn't line up with any known magic")