#!/usr/bin/env python
# Decrypts the Tom-Skype 5.5.x, 5.8.x, 5.9.x, and 5.10.x era keyfiles and
# outputs them in UTF-8
# Jeffrey Knockel jeffk <at> cs <dot> unm <dot> edu
# Usage: python decrypt.py < keyfile > plainfile

import binascii
import struct
import sys
from Crypto.Cipher import DES as Cipher

KEY = b'\x7a\xdd\xe7\xdc\x23\x25\x53\x75'

def decrypt(key, hexcrypt):
    crypt = binascii.a2b_hex(hexcrypt.encode('ascii'))
    cipher = Cipher.new(key, Cipher.MODE_ECB)
    return cipher.decrypt(crypt).split(b'\0')[0]

# This is the algorithm for Tom-Skype 3.6.x - 4.2.x era keyfiles that is
# sometimes mistakenly used by the Tom-Skype folks for encrypting new words in
# these keyfiles.  Although we can decrypt these words, newer versions of
# Tom-Skype can't, and so words encrypted with this algorithm won't actually
# trigger censorship or surveillance.
def decrypt_alt(hexcrypt):
    crypt = [int(hexcrypt[i:i + 2], 16) for i in range(0, len(hexcrypt), 2)]
    return b''.join(struct.pack('B', ((b ^ 0x68) - a) % 255) for a, b in zip(crypt, crypt[1:]))

def main():
    if hasattr(sys.stdout, 'buffer'):
        sys.stdout = sys.stdout.buffer
    for lineno, line in enumerate(sys.stdin):
        stripped = line.strip('\0\t\n\x0b\x0c\r ')
        if stripped:
            try:
                try:
                    plain = decrypt(KEY, stripped).decode('gbk').encode('utf-8')
                except Exception:
                    plain = decrypt_alt(stripped).decode('gbk').encode('utf-8')
            except Exception:
                sys.stderr.write('Error decrypting line %d\n' % (lineno + 1))
            else:
                sys.stdout.write(plain + b'\n')

if __name__ == '__main__':
    main()