decrypt.py

import sys
import struct
import binascii
from collections import Counter

def u16(b):
    return struct.unpack('<H', b)[0]
def p16(x):
    return struct.pack('<H', x)
def p32(x):
    return struct.pack('<I', x)

def xor_block(block, key):
    return b''.join(p16(u16(block[i:i+2])^key) for i in range(0,len(block),2))

def majority(block):
    c = Counter()
    for i in range(0, len(block), 2):
        c[u16(block[i:i+2])] += 1
    return c.most_common(1)[0][0]

def decrypt(s):
    decrypted = b''
    s = s[0x200:] # don't know what the first block is, but it doesn't matter
    # base = majority(s[:0x200]) ^ 0xFFFF # heuristic
    base = u16(s[:2]) ^ u16(b'\xFF\xFB') # better heuristic, thanks to foone!
    for i in range(0, len(s), 0x200):
        block = s[i:i+0x200]
        key = binascii.crc_hqx(p32(i//0x200), 0) ^ base
        decrypted += xor_block(block, key)
    return decrypted

def main():
    if len(sys.argv) < 3:
        print("usage: {} INFILE OUTFILE".format(sys.argv[0]))
        return
    with open(sys.argv[1], "rb") as f:
        encrypted = f.read()
    decrypted = decrypt(encrypted)
    with open(sys.argv[2], "wb") as f:
        f.write(decrypted)
    
if __name__ == "__main__":
    main()

🧊