import sys

# Encrypt bytes until this file offset
maxOffset = 20580

# Number of bytes to be skipped
# It will encrypt bytes at position: 0, 2049, 4098, 6147 and so on
inc = 2048

inFname = sys.argv[1]
dataEnc = open(inFname, "rb").read()

# Encrypt: NOT, XOR 0x31, NOT, XOR 0x6A
# Decrypt: XOR 0x6A, NOT, XOR 0x31, NOT

# In Python ~ operator works with signed integers.
# Need to implement the binary NOT operator
def bit_not(n, numbits=8):
	return (1 << numbits) - 1 - n

def decrypt(c):
	return bit_not(bit_not(ord(c) ^ 0x6A) ^ 0x31)
	
dataDec = [
	decrypt(dataEnc[idx]) 
	if  ((idx % inc == idx / inc) and (idx < maxOffset)) 
	else ord(dataEnc[idx]) 
	for idx in range(len(dataEnc))
]

with open(inFname + ".dec", "wb") as fOut:
	fOut.write("".join([chr(c) for c in dataDec]))