#!/usr/bin/env python
# Decrypts the Tom-Skype 3.6.x - 4.2.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 random
import struct
import sys

def decrypt(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 encrypt(plain, nonce=None):
    if nonce is None:
        nonce = random.randint(0, 0xff)
    crypt = [nonce]
    for p in bytearray(plain.encode('ascii')):
        crypt.append(((p + crypt[-1]) % 255) ^ 0x68)
    return ''.join('%.2X' % c for c in crypt)

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:
                plain = decrypt(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()