Python script for redshirt2 file encryption & decryption

redshirt2.py

#!/usr/bin/python3 -tt

# redshirt2 file encryption & decryption
# reads and writes file contents from/to stdin & stdout
# algorithm adapted from dx9's php implementation.
# http://forums.introversion.co.uk/viewtopic.php?f=29&t=13803&start=60#p485984
#
# usage: ./redshirt2.py < infile > outfile

import sys
import io

# enable binary IO
stdin = sys.stdin.buffer
stdout = sys.stdout.buffer

# the key used for encrypting and decrypting
# can be a byte array of any size
KEY = bytes.fromhex( '1f0709010b0205050311280c23161b02' )

# the size and content of an encrypted file's 'header'
HED = b'redshirt2'

# if we are encrypting or not
encrypting = False

# determine if file is encrypted by checking header content
for hb in HED:
	inb = ord( stdin.read( 1 ) )
	if inb != hb:
		encrypting = True
		# ensure input is seekable
		try: stdin.seek( 0 )
		except io.UnsupportedOperation as e:
			sys.stderr.write( str(e)+'\n' ); sys.exit()
		stdout.write( HED )
		break

ikey = 0

if encrypting: sys.stderr.write( 'Encrypting...\n' )
else: sys.stderr.write( 'Decrypting...\n' )

for ch in stdin.read():
	c = ch

	# keep whitespace
	if c >= 33:
		ikey = (ikey + 1) % len( KEY )
		shift = KEY[ikey]
		test0 = (shift - 33) if (shift > 33) else 0
		test1 = 95 if (shift > 33) else (128 - shift)

		if encrypting:
			if test1 <= c < 128: c -= 95
			c = (c + shift) % 256
			if c < 33: c = (c - 128 + test0 + 256) % 256
		else:
			c = (c - shift + 256) % 256
			if test1 <= c < 128:
				c = (c + 128 - test0 + 256) % 256
			if c < 33: c += 95

	stdout.write( c.to_bytes( 1, sys.byteorder ) )