Homework: Ceaser Cipher encoder and decoder

[Siylo]

Success!!! Thank you for the help. It was nice to see I wasn't too far from being correct. This is the most intricate program I've written so far. [edited] Ok, got a little ahead of myself. I tried the decrypter portion of the program and got a traceback error of

Error:
encrypted_message += chr(x + 65)
NameError: name 'encrypted_message' is not defined

should I remove the method == str("decrypt") and change the elif back to an else?

message = input("Enter the message you wish to encrypt or decrypt:")
shift_key = int(input("Enter the key value:"))
method = str(input("Would you like to encrypt or decrypt a message?"))
if method != str("encrypt") and method != str("decrypt"):
    method = input("Enter either encrypt or decrypt")
else:
    if method == str("encrypt"):
        encrypted_message = ""
        for character in message:
            if character.isalpha() == True:
                if character == character.lower():
                    x = ord(character) -97
                    x += int(shift_key)
                    x = x % 26
                    encrypted_message += chr(x + 97)
                else:
                    x = ord(character) -65
                    x += int(shift_key)
                    x = x % 26
                    encrypted_message += chr(x + 65)
            else:
                encrypted_message += character
        print(encrypted_message)
    elif method == str("decrypt"):
        decrypted_message = ""
        for character in message:
            if character.isalpha() == True:
                if character == character.lower():
                    x = ord(character) -97
                    x += int(-shift_key)
                    x = x % 26
                    encrypted_message += chr(x + 97)
                else:
                    x = ord(character) -65
                    x += int(-shift_key)
                    x = x % 26
                    encrypted_message += chr(x + 65)
            else:
                decrypted_message += character
        print(decrypted_message)