vigenere encoding and decoding in python

encoderanddecoder.py

#-----------------------------------------------------
# Uses vigenere.py by ilogik
# https://gist.github.com/ilogik/6f9431e4588015ecb194
#-----------------------------------------------------

import base64

def encode(key, string):
    encoded_chars = []
    for i in xrange(len(string)):
        key_c = key[i % len(key)]
        encoded_c = chr(ord(string[i]) + ord(key_c) % 256)
        encoded_chars.append(encoded_c)
    encoded_string = "".join(encoded_chars)
    return base64.urlsafe_b64encode(encoded_string)

def decode(key, string):
    decoded_chars = []
    string = base64.urlsafe_b64decode(string)
    for i in xrange(len(string)):
        key_c = key[i % len(key)]
        encoded_c = chr(abs(ord(string[i]) - ord(key_c) % 256))
        decoded_chars.append(encoded_c)
    decoded_string = "".join(decoded_chars)
    return decoded_string

while True:
    program = raw_input("encode or decode:")
    if program == "encode":
        string = raw_input("Thing to encode:")
        key = raw_input("key:")
        encrypted = encode(key, string)
        print(encrypted)
    if program == "decode":
        string = raw_input("Thing to decode:")
        key = raw_input("key:")
        decrypted = decode(key, string)
        print(decrypted)
    else:
        print("Unknown input, Please enter encode or decode")