Caesar Cipher Encryption and Decryption

Here's my blog post to end my hiatus. I decided to make a program that encrypts and decrypts text given in the form of a Caesar cipher. So I did just that in 43 lines of Python (the main portion in 27 lines). On Pastebin like usual.

alphabet = "abcdefghijklmnopqrstuvwxyz"
def encrypt(s, shift):
    news = ""
    for i in s:
        if i.lower() in alphabet:
            if i.upper() == i:
                c = alphabet[(alphabet.index(i.lower()) + shift) % len(alphabet)] 
                news = news + c.upper()
            else:
                c = alphabet[(alphabet.index(i) + shift) % len(alphabet)] 
                news = news + c
        else:
            news = news + i
    print(news)
def decrypt(s, shift):
    news = ""
    for i in s:
        if i.lower() in alphabet:
            if i.upper() == i:
                c = alphabet[(alphabet.index(i.lower()) - shift) % len(alphabet)] 
                news = news + c.upper()
            else:
                c = alphabet[(alphabet.index(i) - shift) % len(alphabet)] 
                news = news + c
        else:
            news = news + i
    print(news)
def gettodo():
    a = input("What do you want to do? E for encrypt, D for decrypt: ").lower()
    if a[0] == "e":
        b = input("Enter a plaintext: ")
        c = int(input("Enter a shift: "))
        encrypt(b, c)
        gettodo()
    elif a[0] == "d":
        b = input("Enter a ciphertext: ")
        c = int(input("Enter a shift: "))
        decrypt(b, c)
        gettodo()
    else:
        print("Invalid input.")
        gettodo()
gettodo()