Super Mario Run .stir file decrypter

unstir.py

# fun fact:
# this is internally referred to as 'SimpleEncryption'
# it is pretty damn simple, really
import sys

KEY = 0x5D
INITIAL_SPAN = 0x200
SPAN = 0x100

input_name = sys.argv[1]
if '.stir' in input_name:
	output_name = input_name.replace('.stir', '')
else:
	output_name = input_name + '_decrypted'

with open(input_name, 'rb') as f:
	work = bytearray(f.read())


first_block_size = min(INITIAL_SPAN, len(work))
value = KEY
for i in range(first_block_size):
	value ^= work[i]
	work[i] = value

offset = first_block_size
while offset < len(work):
	value ^= work[offset]
	work[offset] = value
	offset += SPAN


with open(output_name, 'wb') as f:
	f.write(work)