A simple self-written cipherer and decipherer written in python 3.x

caesar_cipher.py

alphabet = ['a', 'b', 'c', 'd', 'e', 'f', 'g', 'h', 'i', 'j', 'k', 'l', 'm', 'n', 'o', 'p', 'q', 'r', 's', 't', 'u', 'v', 'w', 'x', 'y', 'z']

mode = input("What mode? (c)ipher / (d)ecipher: ")

if mode == "c":
    offset = input("Enter offset: ")

    if not offset.isdigit():
        print("\nOffset must be a positive Integer! Exiting...")
        exit()

    input = input("\nEnter text to offset with " + offset + ": ")

    input = input.lower()

    print("\nCiphering '" + input + "'....\n")

    output = ""
    for input_char in input:
        if not input_char == " ":
            x = (int(alphabet.index(input_char)) + int(offset)) % len(alphabet)
            output = output + alphabet[x]
        else:
            output = output + " "

    print("Your ciphered String is: \n" + output + "")
elif mode == "d":
    offset = input("Enter offset: ")

    if not offset.isdigit():
        print("\nOffset must be a positive Integer! Exiting...")
        exit()

    input = input("\nEnter text to offset with " + offset + ": ")

    input = input.lower()

    print("\nCiphering '" + input + "'....\n")

    output = ""
    for input_char in input:
        if not input_char == " ":
            x = (int(alphabet.index(input_char)) - int(offset)) % len(alphabet)
            output = output + alphabet[x]
        else:
            output = output + " "
            
    print("Your deciphered String is: \n" + output + "")