Im trying to make an encryption program 

```
def intro():
    msg = input("Enter the message you wish to encrypt: ")
    return msg

def shift(msg):
    alpha = ['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']
    rotate = int(input("The level of the encryption?: "))
    text = ""
    for ch in msg:
        if ch == " " or ch == ".":
            pass
        elif msg == "$":
            print("nah")
        else:
            index = alpha.index(ch)
            newindex = index + rotate
            new= alpha[newindex]
            text += new
    return text

def start():
    msg = intro()
    text = shift(msg)
    print("Your encryptions is: " + text)

start()

```

I can't figure out a way to loop the list without getting an index out of range error. For example, if you put "z" it will shift to an "a". I also need for my program to loop till user inputs to end it. I just started coding in python a few months ago so any help will be appreciated!beginner