String manipulation in python..NEED HELP!!!!

[qbailey at ihets.org]

I need help with a program i am doing. it is a cryptography program. i am given a regular alphabet and a key. i need to use the user input and use the regular alphabet and use the corresponding letter in the key and that becomes the new letter. i have the basic code but need help with how to mainpulate the string to do the encryption/decryption. please help

here is my code so far:


""" crypto.py
    Implements a simple substitution cypher
"""

alpha = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
key =   "XPMGTDHLYONZBWEARKJUFSCIQV"

def main():
  keepGoing = True
  while keepGoing:
    response = menu()
    if response == "1":
      plain = raw_input("text to be encoded: ")
      print encode(plain)
    elif response == "2":
      coded = raw_input("code to be decyphered: ")
      print decode(coded)
    elif response == "0":
      print "Thanks for doing secret spy stuff with me."
      keepGoing = False
    else:
      print "I don't know what you want to do..."




i really need help on how to encrypt it im not sure how to go about doing that please help.