Ken Ji - Cipher Project

choice = int(input("Enter 1 to encrypt a message ; Enter 2 to decrypt a message"))

letters = ['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']

while choice != 1 and choice != 2:
    choice = int(input("That is not a valid method.Enter 1 to encrypt a message ; Enter 2 to decrypt a message"))

if choice == 1:
    text = str(input("Please enter the message that you wish to encrypt(no capital letters plz):"))
    encode = []
    for i in text:
        if i in letters:
            encode.append(str(letters.index(i)-1))
        else:
            encode.append("=")
    print("`".join(encode))

elif choice == 2:
    num = input("Please enter the message that you wish to decrypt: ").split("`")
    decode = []

    for i in range(0,len(num)):
        if num[i] != "=":
            num[i] = str(letters[int(num[i])+1])
            decode.append(num[i])
        else:
            num[i] = " "
            decode.append(num[i])

    print("".join(decode))