Vigenere Cipher tool

vigenere.py

#!/usr/bin/env python
import sys, argparse, random

start = ord('.')
end = ord('Z')
modulo = (end - start) + 1

def alphabet():
   return reduce(lambda a, b: '%s%s' % (a,b), map(lambda x: chr(x), range(start, end+1)))

def print_alphabet():
   print alphabet()

def print_table():
   print '  %s' % alphabet()
   line = alphabet()
   for c in alphabet():
      print '%s %s' % (c, line)
      line = line[1:] + line[0]

def random_key():
   al = alphabet()
   return reduce(lambda a, b: '%s%s' % (a,b), map(lambda x: al[random.randint(0,len(al)-1)], range(0,11)))

def validate(text):
   for c in text:
      code = ord(c)
      if code < start or code > end:
         raise Exception("char '%s' in text '%s' not in vigenere table" % (c, text))

def extend_key(key, text):
   while(len(key) < len(text)):
      key = key + key
   return key[0:len(text)]

def encrypt(key, text):
   cipher = ''
   for i in range(0, len(text)):
      t = ord(text[i]) - start
      k = ord(key[i]) - start
      code = ((t + k) % (modulo)) + start
      ch = chr(code)
      cipher +=  chr(code)
   return cipher

def decrypt(key, cipher):
   text = ''
   for i in range(0, len(cipher)):
      t = ord(cipher[i]) - start
      k = ord(key[i]) - start
      code = ((t - k) % (modulo)) + start
      ch = chr(code)
      text +=  chr(code)
   return text

def process(processor, key, text):
   text = text.upper()
   key = extend_key(key.upper(), text)
   validate(key)
   validate(text)
   print "%s" % (processor(key, text))

if __name__ == '__main__':   
   parser = argparse.ArgumentParser(description='Vigenere Cipher')
   parser.add_argument('-A', '--alphabet', dest='print_alphabet', action="store_true", help='Print the alphabet')
   parser.add_argument('-T', '--table', dest='print_table', action="store_true", help='Print the Vigenere table')
   parser.add_argument('-d', '--decrypt', dest='processor', const=decrypt, default=encrypt, action="store_const", help='Decipher the text')
   parser.add_argument('-k', '--key', dest='key', help='Vigenere key')
   parser.add_argument('text', action='store', nargs="?", help='Text to (de)cipher. If not provided, read from stdin')
   
   args = parser.parse_args()
   
   if args.print_alphabet:
      print_alphabet()
      sys.exit(0)
   if args.print_table:
      print_table()
      sys.exit(0)
   
   if args.key: key = args.key
   else:
      key = random_key()
      sys.stderr.write("Missing -k key, using random '%s'\n" % key)
   
   if not args.text:
      for text in sys.stdin:
         if text[-1] == '\n': text = text[:-1]
         process(args.processor, key, text)
   else:
      process(args.processor, key, args.text)