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


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
      print "I don't know what you want to do..."

Try this for a hint

for ctr in range(len(alpha)):
    print alpha[ctr], key[ctr]

print alpha.find("F")