Vigenere

#!/usr/bin/env python
# -*- coding: UTF-8 -*-

import os, sys

ALPHA = range(256) # наш алфвит  -- вся ASCII-таблица.

KEY = 1 # 1 % 33

def get_symbol(filename):
    with open(filename, 'rb') as f:
        while True:
            b = f.read(1)
            if b:
                yield b
            else:
                break

def _key():
    while True:
        for k in ALPHA:
            yield k
        x = ALPHA.pop()  # ключ -- сдвиг алфавита на 1
        ALPHA.insert(0, x)

_KEY = _key()

def vigenere(symbol, is_decode):
    code = ord(symbol)
    if is_decode:
        new_symbol = (code - _KEY.next() + 256) % 256
    else:
        new_symbol = (code + _KEY.next()) % 256

    if new_symbol > 255:
        new_symbol = abs(255 - new_symbol)
    if new_symbol < 0:
        new_symbol = abs(255 + new_symbol)

    return chr(new_symbol)

if __name__ == '__main__':

    filename = os.path.abspath(sys.argv[1])
    decode = False
    if len(sys.argv) > 2:
        decode = sys.argv[2] == '-d'

    for ch in get_symbol(filename):
        sys.stdout.write(vigenere(ch, decode))