Caesar cipher

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']
def encrypt(original_text, shift_amount):
    original_text=original_text.lower()
    deco=""
    for letter in original_text:
        if letter not in alphabet:
            deco+=letter
        else:
            deco+= alphabet[(alphabet.index(letter) + shift_amount) % len(alphabet)]
    print(deco)
def decrypt(mensaje,salt):
    deco = ""
    for letter in mensaje:
        if letter not in alphabet:
            deco += letter
        elif (alphabet.index(letter) - salt) < 0:
            deco += alphabet[(alphabet.index(letter) - salt) + len(alphabet)]
        else:
            deco += alphabet[(alphabet.index(letter) - salt)]
    print(deco)
def caesar(direction):
    if direction == 'e':
        text = input("Type your message:\n").lower()
        shift = int(input("Type the shift number:\n"))
        encrypt(text, shift)
    else:
        text = input("Type your message:\n").lower()
        shift = int(input("Type the shift number:\n"))
        decrypt(text, shift)

u_choice = 'x'
while u_choice != 'e' and u_choice != 'd' :
    u_choice = input("Pulse 'd' para desencriptar o 'e' para encriptar: ").lower()
    if u_choice == 'e' or u_choice == 'd':
        caesar(u_choice)
        option=input("¿Quiere más? (y/n): ").lower()
        if option == 'y':
            u_choice='x'
    else:
        print("Opción incorrecta")